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<pre><a name="line-1"></a><span class='hs-comment'>{-# OPTIONS_GHC -fno-warn-unused-imports #-}</span>
<a name="line-2"></a>
<a name="line-3"></a><span class='hs-comment'>{-| Conventional Haskell stream programming forces you to choose only two of the
<a name="line-4"></a> following three features:
<a name="line-5"></a>
<a name="line-6"></a> * Effects
<a name="line-7"></a>
<a name="line-8"></a> * Streaming
<a name="line-9"></a>
<a name="line-10"></a> * Composability
<a name="line-11"></a>
<a name="line-12"></a> If you sacrifice /Effects/ you get Haskell's pure and lazy lists, which you
<a name="line-13"></a> can transform using composable functions in constant space, but without
<a name="line-14"></a> interleaving effects.
<a name="line-15"></a>
<a name="line-16"></a> If you sacrifice /Streaming/ you get 'mapM', 'forM' and
<a name="line-17"></a> \"ListT done wrong\", which are composable and effectful, but do not return
<a name="line-18"></a> a single result until the whole list has first been processed and loaded
<a name="line-19"></a> into memory.
<a name="line-20"></a>
<a name="line-21"></a> If you sacrifice /Composability/ you write a tightly coupled read,
<a name="line-22"></a> transform, and write loop in 'IO', which is streaming and effectful, but is
<a name="line-23"></a> not modular or separable.
<a name="line-24"></a>
<a name="line-25"></a> @pipes@ gives you all three features: effectful, streaming, and composable
<a name="line-26"></a> programming. @pipes@ also provides a wide variety of stream programming
<a name="line-27"></a> abstractions which are all subsets of a single unified machinery:
<a name="line-28"></a>
<a name="line-29"></a> * effectful 'Producer's (like generators),
<a name="line-30"></a>
<a name="line-31"></a> * effectful 'Consumer's (like iteratees),
<a name="line-32"></a>
<a name="line-33"></a> * effectful 'Pipe's (like Unix pipes), and:
<a name="line-34"></a>
<a name="line-35"></a> * 'ListT' done right.
<a name="line-36"></a>
<a name="line-37"></a> All of these are connectable and you can combine them together in clever and
<a name="line-38"></a> unexpected ways because they all share the same underlying type.
<a name="line-39"></a>
<a name="line-40"></a> @pipes@ requires a basic understanding of monad transformers, which you can
<a name="line-41"></a> learn about by reading either:
<a name="line-42"></a>
<a name="line-43"></a> * the paper \"Monad Transformers - Step by Step\",
<a name="line-44"></a>
<a name="line-45"></a> * chapter 18 of \"Real World Haskell\" on monad transformers, or:
<a name="line-46"></a>
<a name="line-47"></a> * the documentation of the @transformers@ library.
<a name="line-48"></a>
<a name="line-49"></a> If you want a Quick Start guide to @pipes@, read the documentation in
<a name="line-50"></a> "Pipes.Prelude" from top to bottom.
<a name="line-51"></a>
<a name="line-52"></a> This tutorial is more extensive and explains the @pipes@ API in greater
<a name="line-53"></a> detail and illustrates several idioms.
<a name="line-54"></a>-}</span>
<a name="line-55"></a>
<a name="line-56"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Pipes</span><span class='hs-varop'>.</span><span class='hs-conid'>Tutorial</span> <span class='hs-layout'>(</span>
<a name="line-57"></a> <span class='hs-comment'>-- * Introduction</span>
<a name="line-58"></a> <span class='hs-comment'>-- $introduction</span>
<a name="line-59"></a>
<a name="line-60"></a> <span class='hs-comment'>-- * Producers</span>
<a name="line-61"></a> <span class='hs-comment'>-- $producers</span>
<a name="line-62"></a>
<a name="line-63"></a> <span class='hs-comment'>-- * Composability</span>
<a name="line-64"></a> <span class='hs-comment'>-- $composability</span>
<a name="line-65"></a>
<a name="line-66"></a> <span class='hs-comment'>-- * Consumers</span>
<a name="line-67"></a> <span class='hs-comment'>-- $consumers</span>
<a name="line-68"></a>
<a name="line-69"></a> <span class='hs-comment'>-- * Pipes</span>
<a name="line-70"></a> <span class='hs-comment'>-- $pipes</span>
<a name="line-71"></a>
<a name="line-72"></a> <span class='hs-comment'>-- * ListT</span>
<a name="line-73"></a> <span class='hs-comment'>-- $listT</span>
<a name="line-74"></a>
<a name="line-75"></a> <span class='hs-comment'>-- * Tricks</span>
<a name="line-76"></a> <span class='hs-comment'>-- $tricks</span>
<a name="line-77"></a>
<a name="line-78"></a> <span class='hs-comment'>-- * Conclusion</span>
<a name="line-79"></a> <span class='hs-comment'>-- $conclusion</span>
<a name="line-80"></a>
<a name="line-81"></a> <span class='hs-comment'>-- * Appendix: Types</span>
<a name="line-82"></a> <span class='hs-comment'>-- $types</span>
<a name="line-83"></a>
<a name="line-84"></a> <span class='hs-comment'>-- * Appendix: Time Complexity</span>
<a name="line-85"></a> <span class='hs-comment'>-- $timecomplexity</span>
<a name="line-86"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-87"></a>
<a name="line-88"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Category</span>
<a name="line-89"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-90"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Error</span>
<a name="line-91"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Writer</span><span class='hs-varop'>.</span><span class='hs-conid'>Strict</span>
<a name="line-92"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Pipes</span>
<a name="line-93"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Pipes</span><span class='hs-varop'>.</span><span class='hs-conid'>Lift</span>
<a name="line-94"></a><span class='hs-keyword'>import</span> <span class='hs-keyword'>qualified</span> <span class='hs-conid'>Pipes</span><span class='hs-varop'>.</span><span class='hs-conid'>Prelude</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>P</span>
<a name="line-95"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Prelude</span> <span class='hs-varid'>hiding</span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varop'>.</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-varid'>id</span><span class='hs-layout'>)</span>
<a name="line-96"></a>
<a name="line-97"></a><span class='hs-comment'>{- $introduction
<a name="line-98"></a> The @pipes@ library decouples stream processing stages from each other so
<a name="line-99"></a> that you can mix and match diverse stages to produce useful streaming
<a name="line-100"></a> programs. If you are a library writer, @pipes@ lets you package up
<a name="line-101"></a> streaming components into a reusable interface. If you are an application
<a name="line-102"></a> writer, @pipes@ lets you connect pre-made streaming components with minimal
<a name="line-103"></a> effort to produce a highly-efficient program that streams data in constant
<a name="line-104"></a> memory.
<a name="line-105"></a>
<a name="line-106"></a> To enforce loose coupling, components can only communicate using two
<a name="line-107"></a> commands:
<a name="line-108"></a>
<a name="line-109"></a> * 'yield': Send output data
<a name="line-110"></a>
<a name="line-111"></a> * 'await': Receive input data
<a name="line-112"></a>
<a name="line-113"></a> @pipes@ has four types of components built around these two commands:
<a name="line-114"></a>
<a name="line-115"></a> * 'Producer's can only 'yield' values and they model streaming sources
<a name="line-116"></a>
<a name="line-117"></a> * 'Consumer's can only 'await' values and they model streaming sinks
<a name="line-118"></a>
<a name="line-119"></a> * 'Pipe's can both 'yield' and 'await' values and they model stream
<a name="line-120"></a> transformations
<a name="line-121"></a>
<a name="line-122"></a> * 'Effect's can neither 'yield' nor 'await' and they model non-streaming
<a name="line-123"></a> components
<a name="line-124"></a>
<a name="line-125"></a> You can connect these components together in four separate ways which
<a name="line-126"></a> parallel the four above types:
<a name="line-127"></a>
<a name="line-128"></a> * 'for' handles 'yield's
<a name="line-129"></a>
<a name="line-130"></a> * ('>~') handles 'await's
<a name="line-131"></a>
<a name="line-132"></a> * ('>->') handles both 'yield's and 'await's
<a name="line-133"></a>
<a name="line-134"></a> * ('>>=') handles return values
<a name="line-135"></a>
<a name="line-136"></a> As you connect components their types will change to reflect inputs and
<a name="line-137"></a> outputs that you've fused away. You know that you're done connecting things
<a name="line-138"></a> when you get an 'Effect', meaning that you have handled all inputs and
<a name="line-139"></a> outputs. You run this final 'Effect' to begin streaming.
<a name="line-140"></a>-}</span>
<a name="line-141"></a>
<a name="line-142"></a><span class='hs-comment'>{- $producers
<a name="line-143"></a> 'Producer's are effectful streams of input. Specifically, a 'Producer' is a
<a name="line-144"></a> monad transformer that extends any base monad with a new 'yield' command.
<a name="line-145"></a> This 'yield' command lets you send output downstream to an anonymous
<a name="line-146"></a> handler, decoupling how you generate values from how you consume them.
<a name="line-147"></a>
<a name="line-148"></a> The following @stdinLn@ 'Producer' shows how to incrementally read in
<a name="line-149"></a> 'String's from standard input and 'yield' them downstream, terminating
<a name="line-150"></a> gracefully when reaching the end of the input:
<a name="line-151"></a>
<a name="line-152"></a>> -- echo.hs
<a name="line-153"></a>>
<a name="line-154"></a>> import Control.Monad (unless)
<a name="line-155"></a>> import Pipes
<a name="line-156"></a>> import System.IO (isEOF)
<a name="line-157"></a>>
<a name="line-158"></a>> -- +--------+-- A 'Producer' that yields 'String's
<a name="line-159"></a>> -- | |
<a name="line-160"></a>> -- | | +-- Every monad transformer has a base monad.
<a name="line-161"></a>> -- | | | This time the base monad is 'IO'.
<a name="line-162"></a>> -- | | |
<a name="line-163"></a>> -- | | | +-- Every monadic action has a return value.
<a name="line-164"></a>> -- | | | | This action returns '()' when finished
<a name="line-165"></a>> -- v v v v
<a name="line-166"></a>> stdinLn :: Producer String IO ()
<a name="line-167"></a>> stdinLn = do
<a name="line-168"></a>> eof <- lift isEOF -- 'lift' an 'IO' action from the base monad
<a name="line-169"></a>> unless eof $ do
<a name="line-170"></a>> str <- lift getLine
<a name="line-171"></a>> yield str -- 'yield' the 'String'
<a name="line-172"></a>> stdinLn -- Loop
<a name="line-173"></a>
<a name="line-174"></a> 'yield' emits a value, suspending the current 'Producer' until the value is
<a name="line-175"></a> consumed. If nobody consumes the value (which is possible) then 'yield'
<a name="line-176"></a> never returns. You can think of 'yield' as having the following type:
<a name="line-177"></a>
<a name="line-178"></a>@
<a name="line-179"></a> 'yield' :: 'Monad' m => a -> 'Producer' a m ()
<a name="line-180"></a>@
<a name="line-181"></a>
<a name="line-182"></a> The true type of 'yield' is actually more general and powerful. Throughout
<a name="line-183"></a> the tutorial I will present type signatures like this that are simplified at
<a name="line-184"></a> first and then later reveal more general versions. So read the above type
<a name="line-185"></a> signature as simply saying: \"You can use 'yield' within a 'Producer', but
<a name="line-186"></a> you may be able to use 'yield' in other contexts, too.\"
<a name="line-187"></a>
<a name="line-188"></a> Click the link to 'yield' to navigate to its documentation. There you will
<a name="line-189"></a> see that 'yield' actually uses the 'Producer'' (with an apostrophe) type
<a name="line-190"></a> synonym which hides a lot of polymorphism behind a simple veneer. The
<a name="line-191"></a> documentation for 'yield' says that you can also use 'yield' within a
<a name="line-192"></a> 'Pipe', too, because of this polymorphism:
<a name="line-193"></a>
<a name="line-194"></a>@
<a name="line-195"></a> 'yield' :: 'Monad' m => a -> 'Pipe' x a m ()
<a name="line-196"></a>@
<a name="line-197"></a>
<a name="line-198"></a> Use simpler types like these to guide you until you understand the fully
<a name="line-199"></a> general type.
<a name="line-200"></a>
<a name="line-201"></a> 'for' loops are the simplest way to consume a 'Producer' like @stdinLn@.
<a name="line-202"></a> 'for' has the following type:
<a name="line-203"></a>
<a name="line-204"></a>@
<a name="line-205"></a> \-\- +-- Producer +-- The body of the +-- Result
<a name="line-206"></a> \-\- | to loop | loop |
<a name="line-207"></a> \-\- v over v v
<a name="line-208"></a> \-\- -------------- ------------------ ----------
<a name="line-209"></a> 'for' :: 'Monad' m => 'Producer' a m r -> (a -> 'Effect' m ()) -> 'Effect' m r
<a name="line-210"></a>@
<a name="line-211"></a>
<a name="line-212"></a> @(for producer body)@ loops over @(producer)@, substituting each 'yield' in
<a name="line-213"></a> @(producer)@ with @(body)@.
<a name="line-214"></a>
<a name="line-215"></a> You can also deduce that behavior purely from the type signature:
<a name="line-216"></a>
<a name="line-217"></a> * The body of the loop takes exactly one argument of type @(a)@, which is
<a name="line-218"></a> the same as the output type of the 'Producer'. Therefore, the body of the
<a name="line-219"></a> loop must get its input from that 'Producer' and nowhere else.
<a name="line-220"></a>
<a name="line-221"></a> * The return value of the input 'Producer' matches the return value of the
<a name="line-222"></a> result, therefore 'for' must loop over the entire 'Producer' and not skip
<a name="line-223"></a> anything.
<a name="line-224"></a>
<a name="line-225"></a> The above type signature is not the true type of 'for', which is actually
<a name="line-226"></a> more general. Think of the above type signature as saying: \"If the first
<a name="line-227"></a> argument of 'for' is a 'Producer' and the second argument returns an
<a name="line-228"></a> 'Effect', then the final result must be an 'Effect'.\"
<a name="line-229"></a>
<a name="line-230"></a> Click the link to 'for' to navigate to its documentation. There you will
<a name="line-231"></a> see the fully general type and underneath you will see equivalent simpler
<a name="line-232"></a> types. One of these says that if the body of the loop is a 'Producer', then
<a name="line-233"></a> the result is a 'Producer', too:
<a name="line-234"></a>
<a name="line-235"></a>@
<a name="line-236"></a> 'for' :: 'Monad' m => 'Producer' a m r -> (a -> 'Producer' b m ()) -> 'Producer' b m r
<a name="line-237"></a>@
<a name="line-238"></a>
<a name="line-239"></a> The first type signature I showed for 'for' was a special case of this
<a name="line-240"></a> slightly more general signature because a 'Producer' that never 'yield's is
<a name="line-241"></a> also an 'Effect':
<a name="line-242"></a>
<a name="line-243"></a>@
<a name="line-244"></a> data 'X' -- The uninhabited type
<a name="line-245"></a>
<a name="line-246"></a>\ type 'Effect' m r = 'Producer' 'X' m r
<a name="line-247"></a>@
<a name="line-248"></a>
<a name="line-249"></a> This is why 'for' permits two different type signatures. The first type
<a name="line-250"></a> signature is just a special case of the second one:
<a name="line-251"></a>
<a name="line-252"></a>@
<a name="line-253"></a> 'for' :: 'Monad' m => 'Producer' a m r -> (a -> 'Producer' b m ()) -> 'Producer' b m r
<a name="line-254"></a>
<a name="line-255"></a>\ -- Specialize \'b\' to \'X\'
<a name="line-256"></a> 'for' :: 'Monad' m => 'Producer' a m r -> (a -> 'Producer' 'X' m ()) -> 'Producer' 'X' m r
<a name="line-257"></a>
<a name="line-258"></a>\ -- Producer X = Effect
<a name="line-259"></a> 'for' :: 'Monad' m => 'Producer' a m r -> (a -> 'Effect' m ()) -> 'Effect' m r
<a name="line-260"></a>@
<a name="line-261"></a>
<a name="line-262"></a> This is the same trick that all @pipes@ functions use to work with various
<a name="line-263"></a> combinations of 'Producer's, 'Consumer's, 'Pipe's, and 'Effect's. Each
<a name="line-264"></a> function really has just one general type, which you can then simplify down
<a name="line-265"></a> to multiple useful alternative types.
<a name="line-266"></a>
<a name="line-267"></a> Here's an example use of a 'for' @loop@, where the second argument (the
<a name="line-268"></a> loop body) is an 'Effect':
<a name="line-269"></a>
<a name="line-270"></a>> -- echo.hs
<a name="line-271"></a>>
<a name="line-272"></a>> loop :: Effect IO ()
<a name="line-273"></a>> loop = for stdinLn $ \str -> do -- Read this like: "for str in stdinLn"
<a name="line-274"></a>> lift $ putStrLn str -- The body of the 'for' loop
<a name="line-275"></a>>
<a name="line-276"></a>> -- more concise: loop = for stdinLn (lift . putStrLn)
<a name="line-277"></a>
<a name="line-278"></a> In this example, 'for' loops over @stdinLn@ and replaces every 'yield' in
<a name="line-279"></a> @stdinLn@ with the body of the loop, printing each line. This is exactly
<a name="line-280"></a> equivalent to the following code, which I've placed side-by-side with the
<a name="line-281"></a> original definition of @stdinLn@ for comparison:
<a name="line-282"></a>
<a name="line-283"></a>> loop = do | stdinLn = do
<a name="line-284"></a>> eof <- lift isEOF | eof <- lift isEOF
<a name="line-285"></a>> unless eof $ do | unless eof $ do
<a name="line-286"></a>> str <- lift getLine | str <- lift getLine
<a name="line-287"></a>> (lift . putStrLn) str | yield str
<a name="line-288"></a>> loop | stdinLn
<a name="line-289"></a>
<a name="line-290"></a> You can think of 'yield' as creating a hole and a 'for' loop is one way to
<a name="line-291"></a> fill that hole.
<a name="line-292"></a>
<a name="line-293"></a> Notice how the final @loop@ only 'lift's actions from the base monad and
<a name="line-294"></a> does nothing else. This property is true for all 'Effect's, which are just
<a name="line-295"></a> glorified wrappers around actions in the base monad. This means we can run
<a name="line-296"></a> these 'Effect's to remove their 'lift's and lower them back to the
<a name="line-297"></a> equivalent computation in the base monad:
<a name="line-298"></a>
<a name="line-299"></a>@
<a name="line-300"></a> 'runEffect' :: 'Monad' m => 'Effect' m r -> m r
<a name="line-301"></a>@
<a name="line-302"></a>
<a name="line-303"></a> This is the real type signature of 'runEffect', which refuses to accept
<a name="line-304"></a> anything other than an 'Effect'. This ensures that we handle all inputs and
<a name="line-305"></a> outputs before streaming data:
<a name="line-306"></a>
<a name="line-307"></a>> -- echo.hs
<a name="line-308"></a>>
<a name="line-309"></a>> main :: IO ()
<a name="line-310"></a>> main = runEffect loop
<a name="line-311"></a>
<a name="line-312"></a> ... or you could inline the entire @loop@ into the following one-liner:
<a name="line-313"></a>
<a name="line-314"></a>> main = runEffect $ for stdinLn (lift . putStrLn)
<a name="line-315"></a>
<a name="line-316"></a> Our final program loops over standard input and echoes every line to
<a name="line-317"></a> standard output until we hit @Ctrl-D@ to end the input stream:
<a name="line-318"></a>
<a name="line-319"></a>> $ ghc -O2 echo.hs
<a name="line-320"></a>> $ ./echo
<a name="line-321"></a>> Test<Enter>
<a name="line-322"></a>> Test
<a name="line-323"></a>> ABC<Enter>
<a name="line-324"></a>> ABC
<a name="line-325"></a>> <Ctrl-D>
<a name="line-326"></a>> $
<a name="line-327"></a>
<a name="line-328"></a> The final behavior is indistinguishable from just removing all the 'lift's
<a name="line-329"></a> from @loop@:
<a name="line-330"></a>
<a name="line-331"></a>> main = do | loop = do
<a name="line-332"></a>> eof <- isEof | eof <- lift isEof
<a name="line-333"></a>> unless eof $ do | unless eof $ do
<a name="line-334"></a>> str <- getLine | str <- lift getLine
<a name="line-335"></a>> putStrLn str | (lift . putStrLn) str
<a name="line-336"></a>> main | loop
<a name="line-337"></a>
<a name="line-338"></a> This @main@ is what we might have written by hand if we were not using
<a name="line-339"></a> @pipes@, but with @pipes@ we can decouple the input and output logic from
<a name="line-340"></a> each other. When we connect them back together, we still produce streaming
<a name="line-341"></a> code equivalent to what a sufficiently careful Haskell programmer would
<a name="line-342"></a> have written.
<a name="line-343"></a>
<a name="line-344"></a> You can also use 'for' to loop over lists, too. To do so, convert the list
<a name="line-345"></a> to a 'Producer' using 'each', which is exported by default from "Pipes":
<a name="line-346"></a>
<a name="line-347"></a>> each :: Monad m => [a] -> Producer a m ()
<a name="line-348"></a>> each as = mapM_ yield as
<a name="line-349"></a>
<a name="line-350"></a> Combine 'for' and 'each' to iterate over lists using a \"foreach\" loop:
<a name="line-351"></a>
<a name="line-352"></a>>>> runEffect $ for (each [1..4]) (lift . print)
<a name="line-353"></a>1
<a name="line-354"></a>2
<a name="line-355"></a>3
<a name="line-356"></a>4
<a name="line-357"></a>
<a name="line-358"></a> 'each' is actually more general and works for any 'Foldable':
<a name="line-359"></a>
<a name="line-360"></a>@
<a name="line-361"></a> 'each' :: ('Monad' m, 'Foldable' f) => f a -> 'Producer' a m ()
<a name="line-362"></a>@
<a name="line-363"></a>
<a name="line-364"></a> So you can loop over any 'Foldable' container or even a 'Maybe':
<a name="line-365"></a>
<a name="line-366"></a>>>> runEffect $ for (each (Just 1)) (lift . print)
<a name="line-367"></a>1
<a name="line-368"></a>
<a name="line-369"></a>-}</span>
<a name="line-370"></a>
<a name="line-371"></a><span class='hs-comment'>{- $composability
<a name="line-372"></a> You might wonder why the body of a 'for' loop can be a 'Producer'. Let's
<a name="line-373"></a> test out this feature by defining a new loop body that @duplicate@s every
<a name="line-374"></a> value:
<a name="line-375"></a>
<a name="line-376"></a>> -- nested.hs
<a name="line-377"></a>>
<a name="line-378"></a>> import Pipes
<a name="line-379"></a>> import qualified Pipes.Prelude as P -- Pipes.Prelude already has 'stdinLn'
<a name="line-380"></a>>
<a name="line-381"></a>> duplicate :: Monad m => a -> Producer a m ()
<a name="line-382"></a>> duplicate x = do
<a name="line-383"></a>> yield x
<a name="line-384"></a>> yield x
<a name="line-385"></a>>
<a name="line-386"></a>> loop :: Producer String IO ()
<a name="line-387"></a>> loop = for P.stdinLn duplicate
<a name="line-388"></a>>
<a name="line-389"></a>> -- This is the exact same as:
<a name="line-390"></a>> --
<a name="line-391"></a>> -- loop = for P.stdinLn $ \x -> do
<a name="line-392"></a>> -- yield x
<a name="line-393"></a>> -- yield x
<a name="line-394"></a>
<a name="line-395"></a> This time our @loop@ is a 'Producer' that outputs 'String's, specifically
<a name="line-396"></a> two copies of each line that we read from standard input. Since @loop@ is a
<a name="line-397"></a> 'Producer' we cannot run it because there is still unhandled output.
<a name="line-398"></a> However, we can use yet another 'for' to handle this new duplicated stream:
<a name="line-399"></a>
<a name="line-400"></a>> -- nested.hs
<a name="line-401"></a>>
<a name="line-402"></a>> main = runEffect $ for loop (lift . putStrLn)
<a name="line-403"></a>
<a name="line-404"></a> This creates a program which echoes every line from standard input to
<a name="line-405"></a> standard output twice:
<a name="line-406"></a>
<a name="line-407"></a>> $ ./nested
<a name="line-408"></a>> Test<Enter>
<a name="line-409"></a>> Test
<a name="line-410"></a>> Test
<a name="line-411"></a>> ABC<Enter>
<a name="line-412"></a>> ABC
<a name="line-413"></a>> ABC
<a name="line-414"></a>> <Ctrl-D>
<a name="line-415"></a>> $
<a name="line-416"></a>
<a name="line-417"></a> But is this really necessary? Couldn't we have instead written this using a
<a name="line-418"></a> nested for loop?
<a name="line-419"></a>
<a name="line-420"></a>> main = runEffect $
<a name="line-421"></a>> for P.stdinLn $ \str1 ->
<a name="line-422"></a>> for (duplicate str1) $ \str2 ->
<a name="line-423"></a>> lift $ putStrLn str2
<a name="line-424"></a>
<a name="line-425"></a> Yes, we could have! In fact, this is a special case of the following
<a name="line-426"></a> equality, which always holds no matter what:
<a name="line-427"></a>
<a name="line-428"></a>@
<a name="line-429"></a> \-\- s :: Monad m => 'Producer' a m () -- i.e. \'P.stdinLn\'
<a name="line-430"></a> \-\- f :: Monad m => a -> 'Producer' b m () -- i.e. \'duplicate\'
<a name="line-431"></a> \-\- g :: Monad m => b -> 'Producer' c m () -- i.e. \'(lift . putStrLn)\'
<a name="line-432"></a>
<a name="line-433"></a>\ for (for s f) g = for s (\\x -> for (f x) g)
<a name="line-434"></a>@
<a name="line-435"></a>
<a name="line-436"></a> We can understand the rationale behind this equality if we first define the
<a name="line-437"></a> following operator that is the point-free counterpart to 'for':
<a name="line-438"></a>
<a name="line-439"></a>@
<a name="line-440"></a> (~>) :: Monad m
<a name="line-441"></a> => (a -> 'Producer' b m r)
<a name="line-442"></a> -> (b -> 'Producer' c m r)
<a name="line-443"></a> -> (a -> 'Producer' c m r)
<a name="line-444"></a> (f ~> g) x = for (f x) g
<a name="line-445"></a>@
<a name="line-446"></a>
<a name="line-447"></a> Using ('~>') (pronounced \"into\"), we can transform our original equality
<a name="line-448"></a> into the following more symmetric equation:
<a name="line-449"></a>
<a name="line-450"></a>@
<a name="line-451"></a> f :: Monad m => a -> 'Producer' b m r
<a name="line-452"></a> g :: Monad m => b -> 'Producer' c m r
<a name="line-453"></a> h :: Monad m => c -> 'Producer' d m r
<a name="line-454"></a>
<a name="line-455"></a>\ \-\- Associativity
<a name="line-456"></a> (f ~> g) ~> h = f ~> (g ~> h)
<a name="line-457"></a>@
<a name="line-458"></a>
<a name="line-459"></a> This looks just like an associativity law. In fact, ('~>') has another nice
<a name="line-460"></a> property, which is that 'yield' is its left and right identity:
<a name="line-461"></a>
<a name="line-462"></a>> -- Left Identity
<a name="line-463"></a>> yield ~> f = f
<a name="line-464"></a>
<a name="line-465"></a>> -- Right Identity
<a name="line-466"></a>> f ~> yield = f
<a name="line-467"></a>
<a name="line-468"></a> In other words, 'yield' and ('~>') form a 'Category', specifically the
<a name="line-469"></a> generator category, where ('~>') plays the role of the composition operator
<a name="line-470"></a> and 'yield' is the identity. If you don't know what a 'Category' is, that's
<a name="line-471"></a> okay, and category theory is not a prerequisite for using @pipes@. All you
<a name="line-472"></a> really need to know is that @pipes@ uses some simple category theory to keep
<a name="line-473"></a> the API intuitive and easy to use.
<a name="line-474"></a>
<a name="line-475"></a> Notice that if we translate the left identity law to use 'for' instead of
<a name="line-476"></a> ('~>') we get:
<a name="line-477"></a>
<a name="line-478"></a>> for (yield x) f = f x
<a name="line-479"></a>
<a name="line-480"></a> This just says that if you iterate over a pure single-element 'Producer',
<a name="line-481"></a> then you could instead cut out the middle man and directly apply the body of
<a name="line-482"></a> the loop to that single element.
<a name="line-483"></a>
<a name="line-484"></a> If we translate the right identity law to use 'for' instead of ('~>') we
<a name="line-485"></a> get:
<a name="line-486"></a>
<a name="line-487"></a>> for s yield = s
<a name="line-488"></a>
<a name="line-489"></a> This just says that if the only thing you do is re-'yield' every element of
<a name="line-490"></a> a stream, you get back your original stream.
<a name="line-491"></a>
<a name="line-492"></a> These three \"for loop\" laws summarize our intuition for how 'for' loops
<a name="line-493"></a> should behave and because these are 'Category' laws in disguise that means
<a name="line-494"></a> that 'Producer's are composable in a rigorous sense of the word.
<a name="line-495"></a>
<a name="line-496"></a> In fact, we get more out of this than just a bunch of equations. We also
<a name="line-497"></a> get a useful operator: ('~>'). We can use this operator to condense
<a name="line-498"></a> our original code into the following more succinct form that composes two
<a name="line-499"></a> transformations:
<a name="line-500"></a>
<a name="line-501"></a>> main = runEffect $ for P.stdinLn (duplicate ~> lift . putStrLn)
<a name="line-502"></a>
<a name="line-503"></a> This means that we can also choose to program in a more functional style and
<a name="line-504"></a> think of stream processing in terms of composing transformations using
<a name="line-505"></a> ('~>') instead of nesting a bunch of 'for' loops.
<a name="line-506"></a>
<a name="line-507"></a> The above example is a microcosm of the design philosophy behind the @pipes@
<a name="line-508"></a> library:
<a name="line-509"></a>
<a name="line-510"></a> * Define the API in terms of categories
<a name="line-511"></a>
<a name="line-512"></a> * Specify expected behavior in terms of category laws
<a name="line-513"></a>
<a name="line-514"></a> * Think compositionally instead of sequentially
<a name="line-515"></a>-}</span>
<a name="line-516"></a>
<a name="line-517"></a><span class='hs-comment'>{- $consumers
<a name="line-518"></a> Sometimes you don't want to use a 'for' loop because you don't want to consume
<a name="line-519"></a> every element of a 'Producer' or because you don't want to process every
<a name="line-520"></a> value of a 'Producer' the exact same way.
<a name="line-521"></a>
<a name="line-522"></a> The most general solution is to externally iterate over the 'Producer' using
<a name="line-523"></a> the 'next' command:
<a name="line-524"></a>
<a name="line-525"></a>@
<a name="line-526"></a> 'next' :: 'Monad' m => 'Producer' a m r -> m ('Either' r (a, 'Producer' a m r))
<a name="line-527"></a>@
<a name="line-528"></a>
<a name="line-529"></a> Think of 'next' as pattern matching on the head of the 'Producer'. This
<a name="line-530"></a> 'Either' returns a 'Left' if the 'Producer' is done or it returns a 'Right'
<a name="line-531"></a> containing the next value, @a@, along with the remainder of the 'Producer'.
<a name="line-532"></a>
<a name="line-533"></a> However, sometimes we can get away with something a little more simple and
<a name="line-534"></a> elegant, like a 'Consumer', which represents an effectful sink of values. A
<a name="line-535"></a> 'Consumer' is a monad transformer that extends the base monad with a new
<a name="line-536"></a> 'await' command. This 'await' command lets you receive input from an
<a name="line-537"></a> anonymous upstream source.
<a name="line-538"></a>
<a name="line-539"></a> The following @stdoutLn@ 'Consumer' shows how to incrementally 'await'
<a name="line-540"></a> 'String's and print them to standard output, terminating gracefully when
<a name="line-541"></a> receiving a broken pipe error:
<a name="line-542"></a>
<a name="line-543"></a>> import Control.Monad (unless)
<a name="line-544"></a>> import Control.Exception (try, throwIO)
<a name="line-545"></a>> import qualified GHC.IO.Exception as G
<a name="line-546"></a>> import Pipes
<a name="line-547"></a>>
<a name="line-548"></a>> -- +--------+-- A 'Consumer' that awaits 'String's
<a name="line-549"></a>> -- | |
<a name="line-550"></a>> -- v v
<a name="line-551"></a>> stdoutLn :: Consumer String IO ()
<a name="line-552"></a>> stdoutLn = do
<a name="line-553"></a>> str <- await -- 'await' a 'String'
<a name="line-554"></a>> x <- lift $ try $ putStrLn str
<a name="line-555"></a>> case x of
<a name="line-556"></a>> -- Gracefully terminate if we got a broken pipe error
<a name="line-557"></a>> Left e@(G.IOError { G.ioe_type = t}) ->
<a name="line-558"></a>> lift $ unless (t == G.ResourceVanished) $ throwIO e
<a name="line-559"></a>> -- Otherwise loop
<a name="line-560"></a>> Right () -> stdoutLn
<a name="line-561"></a>
<a name="line-562"></a> 'await' is the dual of 'yield': we suspend our 'Consumer' until we receive a
<a name="line-563"></a> new value. If nobody provides a value (which is possible) then 'await'
<a name="line-564"></a> never returns. You can think of 'await' as having the following type:
<a name="line-565"></a>
<a name="line-566"></a>@
<a name="line-567"></a> 'await' :: 'Monad' m => 'Consumer' a m a
<a name="line-568"></a>@
<a name="line-569"></a>
<a name="line-570"></a> One way to feed a 'Consumer' is to repeatedly feed the same input using
<a name="line-571"></a> using ('>~') (pronounced \"feed\"):
<a name="line-572"></a>
<a name="line-573"></a>@
<a name="line-574"></a> \-\- +- Feed +- Consumer to +- Returns new
<a name="line-575"></a> \-\- | action | feed | Effect
<a name="line-576"></a> \-\- v v v
<a name="line-577"></a> \-\- ---------- -------------- ----------
<a name="line-578"></a> ('>~') :: 'Monad' m => 'Effect' m b -> 'Consumer' b m c -> 'Effect' m c
<a name="line-579"></a>@
<a name="line-580"></a>
<a name="line-581"></a> @(draw >~ consumer)@ loops over @(consumer)@, substituting each 'await' in
<a name="line-582"></a> @(consumer)@ with @(draw)@.
<a name="line-583"></a>
<a name="line-584"></a> So the following code replaces every 'await' in 'P.stdoutLn' with
<a name="line-585"></a> @(lift getLine)@ and then removes all the 'lift's:
<a name="line-586"></a>
<a name="line-587"></a>>>> runEffect $ lift getLine >~ stdoutLn
<a name="line-588"></a>Test<Enter>
<a name="line-589"></a>Test
<a name="line-590"></a>ABC<Enter>
<a name="line-591"></a>ABC
<a name="line-592"></a>42<Enter>
<a name="line-593"></a>42
<a name="line-594"></a>...
<a name="line-595"></a>
<a name="line-596"></a> You might wonder why ('>~') uses an 'Effect' instead of a raw action in the
<a name="line-597"></a> base monad. The reason why is that ('>~') actually permits the following
<a name="line-598"></a> more general type:
<a name="line-599"></a>
<a name="line-600"></a>@
<a name="line-601"></a> ('>~') :: 'Monad' m => 'Consumer' a m b -> 'Consumer' b m c -> 'Consumer' a m c
<a name="line-602"></a>@
<a name="line-603"></a>
<a name="line-604"></a> ('>~') is the dual of ('~>'), composing 'Consumer's instead of 'Producer's.
<a name="line-605"></a>
<a name="line-606"></a> This means that you can feed a 'Consumer' with yet another 'Consumer' so
<a name="line-607"></a> that you can 'await' while you 'await'. For example, we could define the
<a name="line-608"></a> following intermediate 'Consumer' that requests two 'String's and returns
<a name="line-609"></a> them concatenated:
<a name="line-610"></a>
<a name="line-611"></a>> doubleUp :: Monad m => Consumer String m String
<a name="line-612"></a>> doubleUp = do
<a name="line-613"></a>> str1 <- await
<a name="line-614"></a>> str2 <- await
<a name="line-615"></a>> return (str1 ++ str2)
<a name="line-616"></a>>
<a name="line-617"></a>> -- more concise: doubleUp = (++) <$> await <*> await
<a name="line-618"></a>
<a name="line-619"></a> We can now insert this in between @(lift getLine)@ and @stdoutLn@ and see
<a name="line-620"></a> what happens:
<a name="line-621"></a>
<a name="line-622"></a>>>> runEffect $ lift getLine >~ doubleUp >~ stdoutLn
<a name="line-623"></a>Test<Enter>
<a name="line-624"></a>ing<Enter>
<a name="line-625"></a>Testing
<a name="line-626"></a>ABC<Enter>
<a name="line-627"></a>DEF<Enter>
<a name="line-628"></a>ABCDEF
<a name="line-629"></a>42<Enter>
<a name="line-630"></a>000<Enter>
<a name="line-631"></a>42000
<a name="line-632"></a>...
<a name="line-633"></a>
<a name="line-634"></a> 'doubleUp' splits every request from 'stdoutLn' into two separate requests
<a name="line-635"></a> and
<a name="line-636"></a> returns back the concatenated result.
<a name="line-637"></a>
<a name="line-638"></a> We didn't need to parenthesize the above chain of ('>~') operators, because
<a name="line-639"></a> ('>~') is associative:
<a name="line-640"></a>
<a name="line-641"></a>> -- Associativity
<a name="line-642"></a>> (f >~ g) >~ h = f >~ (g >~ h)
<a name="line-643"></a>
<a name="line-644"></a> ... so we can always omit the parentheses since the meaning is unambiguous:
<a name="line-645"></a>
<a name="line-646"></a>> f >~ g >~ h
<a name="line-647"></a>
<a name="line-648"></a> Also, ('>~') has an identity, which is 'await'!
<a name="line-649"></a>
<a name="line-650"></a>> -- Left identity
<a name="line-651"></a>> await >~ f = f
<a name="line-652"></a>>
<a name="line-653"></a>> -- Right Identity
<a name="line-654"></a>> f >~ await = f
<a name="line-655"></a>
<a name="line-656"></a> In other words, ('>~') and 'await' form a 'Category', too, specifically the
<a name="line-657"></a> iteratee category, and 'Consumer's are also composable.
<a name="line-658"></a>-}</span>
<a name="line-659"></a>
<a name="line-660"></a><span class='hs-comment'>{- $pipes
<a name="line-661"></a> Our previous programs were unsatisfactory because they were biased either
<a name="line-662"></a> towards the 'Producer' end or the 'Consumer' end. As a result, we had to
<a name="line-663"></a> choose between gracefully handling end of input (using 'P.stdinLn') or
<a name="line-664"></a> gracefully handling end of output (using 'P.stdoutLn'), but not both at the
<a name="line-665"></a> same time.
<a name="line-666"></a>
<a name="line-667"></a> However, we don't need to restrict ourselves to using 'Producer's
<a name="line-668"></a> exclusively or 'Consumer's exclusively. We can connect 'Producer's and
<a name="line-669"></a> 'Consumer's directly together using ('>->') (pronounced \"pipe\"):
<a name="line-670"></a>
<a name="line-671"></a>@
<a name="line-672"></a> ('>->') :: 'Monad' m => 'Producer' a m r -> 'Consumer' a m r -> 'Effect' m r
<a name="line-673"></a>@
<a name="line-674"></a>
<a name="line-675"></a> This returns an 'Effect' which we can run:
<a name="line-676"></a>
<a name="line-677"></a>> -- echo2.hs
<a name="line-678"></a>>
<a name="line-679"></a>> import Pipes
<a name="line-680"></a>> import qualified Pipes.Prelude as P -- Pipes.Prelude also provides 'stdoutLn'
<a name="line-681"></a>>
<a name="line-682"></a>> main = runEffect $ P.stdinLn >-> P.stdoutLn
<a name="line-683"></a>
<a name="line-684"></a> This program is more declarative of our intent: we want to stream values
<a name="line-685"></a> from 'P.stdinLn' to 'P.stdoutLn'. The above \"pipeline\" not only echoes
<a name="line-686"></a> standard input to standard output, but also handles both end of input and
<a name="line-687"></a> broken pipe errors:
<a name="line-688"></a>
<a name="line-689"></a>> $ ./echo2
<a name="line-690"></a>> Test<Enter>
<a name="line-691"></a>> Test
<a name="line-692"></a>> ABC<Enter>
<a name="line-693"></a>> ABC
<a name="line-694"></a>> 42<Enter>
<a name="line-695"></a>> 42
<a name="line-696"></a>> <Ctrl-D>
<a name="line-697"></a>> $
<a name="line-698"></a>
<a name="line-699"></a> ('>->') is \"pull-based\" meaning that control flow begins at the most
<a name="line-700"></a> downstream component (i.e. 'P.stdoutLn' in the above example). Any time a
<a name="line-701"></a> component 'await's a value it blocks and transfers control upstream and
<a name="line-702"></a> every time a component 'yield's a value it blocks and restores control back
<a name="line-703"></a> downstream, satisfying the 'await'. So in the above example, ('>->')
<a name="line-704"></a> matches every 'await' from 'P.stdoutLn' with a 'yield' from 'P.stdinLn'.
<a name="line-705"></a>
<a name="line-706"></a> Streaming stops when either 'P.stdinLn' terminates (i.e. end of input) or
<a name="line-707"></a> 'P.stdoutLn' terminates (i.e. broken pipe). This is why ('>->') requires
<a name="line-708"></a> that both the 'Producer' and 'Consumer' share the same type of return value:
<a name="line-709"></a> whichever one terminates first provides the return value for the entire
<a name="line-710"></a> 'Effect'.
<a name="line-711"></a>
<a name="line-712"></a> Let's test this by modifying our 'Producer' and 'Consumer' to each return a
<a name="line-713"></a> diagnostic 'String':
<a name="line-714"></a>
<a name="line-715"></a>> -- echo3.hs
<a name="line-716"></a>>
<a name="line-717"></a>> import Control.Applicative ((<$)) -- (<$) modifies return values
<a name="line-718"></a>> import Pipes
<a name="line-719"></a>> import qualified Pipes.Prelude as P
<a name="line-720"></a>> import System.IO
<a name="line-721"></a>>
<a name="line-722"></a>> main = do
<a name="line-723"></a>> hSetBuffering stdout NoBuffering
<a name="line-724"></a>> str <- runEffect $
<a name="line-725"></a>> ("End of input!" <$ P.stdinLn) >-> ("Broken pipe!" <$ P.stdoutLn)
<a name="line-726"></a>> hPutStrLn stderr str
<a name="line-727"></a>
<a name="line-728"></a> This lets us diagnose whether the 'Producer' or 'Consumer' terminated first:
<a name="line-729"></a>
<a name="line-730"></a>> $ ./echo3
<a name="line-731"></a>> Test<Enter>
<a name="line-732"></a>> Test
<a name="line-733"></a>> <Ctrl-D>
<a name="line-734"></a>> End of input!
<a name="line-735"></a>> $ ./echo3 | perl -e 'close STDIN'
<a name="line-736"></a>> Test<Enter>
<a name="line-737"></a>> Broken pipe!
<a name="line-738"></a>> $
<a name="line-739"></a>
<a name="line-740"></a> You might wonder why ('>->') returns an 'Effect' that we have to run instead
<a name="line-741"></a> of directly returning an action in the base monad. This is because you can
<a name="line-742"></a> connect things other than 'Producer's and 'Consumer's, like 'Pipe's, which
<a name="line-743"></a> are effectful stream transformations.
<a name="line-744"></a>
<a name="line-745"></a> A 'Pipe' is a monad transformer that is a mix between a 'Producer' and
<a name="line-746"></a> 'Consumer', because a 'Pipe' can both 'await' and 'yield'. The following
<a name="line-747"></a> example 'Pipe' is analagous to the Prelude's 'take', only allowing a fixed
<a name="line-748"></a> number of values to flow through:
<a name="line-749"></a>
<a name="line-750"></a>> -- take.hs
<a name="line-751"></a>>
<a name="line-752"></a>> import Control.Monad (replicateM_)
<a name="line-753"></a>> import Pipes
<a name="line-754"></a>> import Prelude hiding (take)
<a name="line-755"></a>>
<a name="line-756"></a>> -- +--------- A 'Pipe' that
<a name="line-757"></a>> -- | +---- 'await's 'a's and
<a name="line-758"></a>> -- | | +-- 'yield's 'a's
<a name="line-759"></a>> -- | | |
<a name="line-760"></a>> -- v v v
<a name="line-761"></a>> take :: Int -> Pipe a a IO ()
<a name="line-762"></a>> take n = do
<a name="line-763"></a>> replicateM_ n $ do -- Repeat this block 'n' times
<a name="line-764"></a>> x <- await -- 'await' a value of type 'a'
<a name="line-765"></a>> yield x -- 'yield' a value of type 'a'
<a name="line-766"></a>> lift $ putStrLn "You shall not pass!" -- Fly, you fools!
<a name="line-767"></a>
<a name="line-768"></a> You can use 'Pipe's to transform 'Producer's, 'Consumer's, or even other
<a name="line-769"></a> 'Pipe's using the same ('>->') operator:
<a name="line-770"></a>
<a name="line-771"></a>@
<a name="line-772"></a> ('>->') :: 'Monad' m => 'Producer' a m r -> 'Pipe' a b m r -> 'Producer' b m r
<a name="line-773"></a> ('>->') :: 'Monad' m => 'Pipe' a b m r -> 'Consumer' b m r -> 'Consumer' a m r
<a name="line-774"></a> ('>->') :: 'Monad' m => 'Pipe' a b m r -> 'Pipe' b c m r -> 'Pipe' a c m r
<a name="line-775"></a>@
<a name="line-776"></a>
<a name="line-777"></a> For example, you can compose 'P.take' after 'P.stdinLn' to limit the number
<a name="line-778"></a> of lines drawn from standard input:
<a name="line-779"></a>
<a name="line-780"></a>> maxInput :: Int -> Producer String IO ()
<a name="line-781"></a>> maxInput n = P.stdinLn >-> take n
<a name="line-782"></a>
<a name="line-783"></a>>>> runEffect $ maxInput 3 >-> P.stdoutLn
<a name="line-784"></a>Test<Enter>
<a name="line-785"></a>Test
<a name="line-786"></a>ABC<Enter>
<a name="line-787"></a>ABC
<a name="line-788"></a>42<Enter>
<a name="line-789"></a>42
<a name="line-790"></a>You shall not pass!
<a name="line-791"></a>>>>
<a name="line-792"></a>
<a name="line-793"></a> ... or you can pre-compose 'P.take' before 'P.stdoutLn' to limit the number
<a name="line-794"></a> of lines written to standard output:
<a name="line-795"></a>
<a name="line-796"></a>> maxOutput :: Int -> Consumer String IO ()
<a name="line-797"></a>> maxOutput n = take n >-> P.stdoutLn
<a name="line-798"></a>
<a name="line-799"></a>>>> runEffect $ P.stdinLn >-> maxOutput 3
<a name="line-800"></a><Exact same behavior>
<a name="line-801"></a>
<a name="line-802"></a> Those both gave the same behavior because ('>->') is associative:
<a name="line-803"></a>
<a name="line-804"></a>> (p1 >-> p2) >-> p3 = p1 >-> (p2 >-> p3)
<a name="line-805"></a>
<a name="line-806"></a> Therefore we can just leave out the parentheses:
<a name="line-807"></a>
<a name="line-808"></a>>>> runEffect $ P.stdinLn >-> take 3 >-> P.stdoutLn
<a name="line-809"></a><Exact same behavior>
<a name="line-810"></a>
<a name="line-811"></a> ('>->') is designed to behave like the Unix pipe operator, except with less
<a name="line-812"></a> quirks. In fact, we can continue the analogy to Unix by defining 'cat'
<a name="line-813"></a> (named after the Unix @cat@ utility), which reforwards elements endlessly:
<a name="line-814"></a>
<a name="line-815"></a>> cat :: Monad m => Pipe a a m r
<a name="line-816"></a>> cat = forever $ do
<a name="line-817"></a>> x <- await
<a name="line-818"></a>> yield x
<a name="line-819"></a>
<a name="line-820"></a> 'cat' is the identity of ('>->'), meaning that 'cat' satisfies the
<a name="line-821"></a> following two laws:
<a name="line-822"></a>
<a name="line-823"></a>> -- Useless use of 'cat'
<a name="line-824"></a>> cat >-> p = p
<a name="line-825"></a>>
<a name="line-826"></a>> -- Forwarding output to 'cat' does nothing
<a name="line-827"></a>> p >-> cat = p
<a name="line-828"></a>
<a name="line-829"></a> Therefore, ('>->') and 'cat' form a 'Category', specifically the category of
<a name="line-830"></a> Unix pipes, and 'Pipe's are also composable.
<a name="line-831"></a>
<a name="line-832"></a> A lot of Unix tools have very simple definitions when written using @pipes@:
<a name="line-833"></a>
<a name="line-834"></a>> -- unix.hs
<a name="line-835"></a>>
<a name="line-836"></a>> import Control.Monad (forever)
<a name="line-837"></a>> import Pipes
<a name="line-838"></a>> import qualified Pipes.Prelude as P -- Pipes.Prelude provides 'take', too
<a name="line-839"></a>> import Prelude hiding (head)
<a name="line-840"></a>>
<a name="line-841"></a>> head :: Monad m => Int -> Pipe a a m ()
<a name="line-842"></a>> head = P.take
<a name="line-843"></a>>
<a name="line-844"></a>> yes :: Monad m => Producer String m r
<a name="line-845"></a>> yes = forever $ yield "y"
<a name="line-846"></a>>
<a name="line-847"></a>> main = runEffect $ yes >-> head 3 >-> P.stdoutLn
<a name="line-848"></a>
<a name="line-849"></a> This prints out 3 \'@y@\'s, just like the equivalent Unix pipeline:
<a name="line-850"></a>
<a name="line-851"></a>> $ ./unix
<a name="line-852"></a>> y
<a name="line-853"></a>> y
<a name="line-854"></a>> y
<a name="line-855"></a>> $ yes | head -3
<a name="line-856"></a>> y
<a name="line-857"></a>> y
<a name="line-858"></a>> y
<a name="line-859"></a>> $
<a name="line-860"></a>
<a name="line-861"></a> This lets us write \"Haskell pipes\" instead of Unix pipes. These are much
<a name="line-862"></a> easier to build than Unix pipes and we can connect them directly within
<a name="line-863"></a> Haskell for interoperability with the Haskell language and ecosystem.
<a name="line-864"></a>-}</span>
<a name="line-865"></a>
<a name="line-866"></a><span class='hs-comment'>{- $listT
<a name="line-867"></a> @pipes@ also provides a \"ListT done right\" implementation. This differs
<a name="line-868"></a> from the implementation in @transformers@ because this 'ListT':
<a name="line-869"></a>
<a name="line-870"></a> * obeys the monad laws, and
<a name="line-871"></a>
<a name="line-872"></a> * streams data immediately instead of collecting all results into memory.
<a name="line-873"></a>
<a name="line-874"></a> The latter property is actually an elegant consequence of obeying the monad
<a name="line-875"></a> laws.
<a name="line-876"></a>
<a name="line-877"></a> To bind a list within a 'ListT' computation, combine 'Select' and 'each':
<a name="line-878"></a>
<a name="line-879"></a>> import Pipes
<a name="line-880"></a>>
<a name="line-881"></a>> pair :: ListT IO (Int, Int)
<a name="line-882"></a>> pair = do
<a name="line-883"></a>> x <- Select $ each [1, 2]
<a name="line-884"></a>> lift $ putStrLn $ "x = " ++ show x
<a name="line-885"></a>> y <- Select $ each [3, 4]
<a name="line-886"></a>> lift $ putStrLn $ "y = " ++ show y
<a name="line-887"></a>> return (x, y)
<a name="line-888"></a>
<a name="line-889"></a> You can then loop over a 'ListT' by using 'every':
<a name="line-890"></a>
<a name="line-891"></a>@
<a name="line-892"></a> 'every' :: 'Monad' m => 'ListT' m a -> 'Producer' a m ()
<a name="line-893"></a>@
<a name="line-894"></a>
<a name="line-895"></a> So you can use your 'ListT' within a 'for' loop:
<a name="line-896"></a>
<a name="line-897"></a>>>> runEffect $ for (every pair) (lift . print)
<a name="line-898"></a>x = 1
<a name="line-899"></a>y = 3
<a name="line-900"></a>(1,3)
<a name="line-901"></a>y = 4
<a name="line-902"></a>(1,4)
<a name="line-903"></a>x = 2
<a name="line-904"></a>y = 3
<a name="line-905"></a>(2,3)
<a name="line-906"></a>y = 4
<a name="line-907"></a>(2,4)
<a name="line-908"></a>
<a name="line-909"></a> ... or a pipeline:
<a name="line-910"></a>
<a name="line-911"></a>>>> import qualified Pipes.Prelude as P
<a name="line-912"></a>>>> runEffect $ every pair >-> P.print
<a name="line-913"></a><Exact same behavior>
<a name="line-914"></a>
<a name="line-915"></a> Note that 'ListT' is lazy and only produces as many elements as we request:
<a name="line-916"></a>
<a name="line-917"></a>>>> runEffect $ for (every pair >-> P.take 2) (lift . print)
<a name="line-918"></a>x = 1
<a name="line-919"></a>y = 3
<a name="line-920"></a>(1,3)
<a name="line-921"></a>y = 4
<a name="line-922"></a>(1,4)
<a name="line-923"></a>
<a name="line-924"></a> You can also go the other way, binding 'Producer's directly within a
<a name="line-925"></a> 'ListT'. In fact, this is actually what 'Select' was already doing:
<a name="line-926"></a>
<a name="line-927"></a>@
<a name="line-928"></a> 'Select' :: 'Producer' a m () -> 'ListT' m a
<a name="line-929"></a>@
<a name="line-930"></a>
<a name="line-931"></a> This lets you write crazy code like:
<a name="line-932"></a>
<a name="line-933"></a>> import Pipes
<a name="line-934"></a>> import qualified Pipes.Prelude as P
<a name="line-935"></a>>
<a name="line-936"></a>> input :: Producer String IO ()
<a name="line-937"></a>> input = P.stdinLn >-> P.takeWhile (/= "quit")
<a name="line-938"></a>>
<a name="line-939"></a>> name :: ListT IO String
<a name="line-940"></a>> name = do
<a name="line-941"></a>> firstName <- Select input
<a name="line-942"></a>> lastName <- Select input
<a name="line-943"></a>> return (firstName ++ " " ++ lastName)
<a name="line-944"></a>
<a name="line-945"></a> Here we're binding standard input non-deterministically (twice) as if it
<a name="line-946"></a> were an effectful list:
<a name="line-947"></a>
<a name="line-948"></a>>>> runEffect $ every name >-> P.stdoutLn
<a name="line-949"></a>Daniel<Enter>
<a name="line-950"></a>Fischer<Enter>
<a name="line-951"></a>Daniel Fischer
<a name="line-952"></a>Wagner<Enter>
<a name="line-953"></a>Daniel Wagner
<a name="line-954"></a>quit<Enter>
<a name="line-955"></a>Donald<Enter>
<a name="line-956"></a>Stewart<Enter>
<a name="line-957"></a>Donald Stewart
<a name="line-958"></a>Duck<Enter>
<a name="line-959"></a>Donald Duck
<a name="line-960"></a>quit<Enter>
<a name="line-961"></a>quit<Enter>
<a name="line-962"></a>>>>
<a name="line-963"></a>
<a name="line-964"></a> Notice how this streams out values immediately as they are generated, rather
<a name="line-965"></a> than building up a large intermediate result and then printing all the
<a name="line-966"></a> values in one batch at the end.
<a name="line-967"></a>-}</span>
<a name="line-968"></a>
<a name="line-969"></a><span class='hs-comment'>{- $tricks
<a name="line-970"></a> @pipes@ is more powerful than meets the eye so this section presents some
<a name="line-971"></a> non-obvious tricks you may find useful.
<a name="line-972"></a>
<a name="line-973"></a> Many pipe combinators will work on unusual pipe types and the next few
<a name="line-974"></a> examples will use the 'cat' pipe to demonstrate this.
<a name="line-975"></a>
<a name="line-976"></a> For example, you can loop over the output of a 'Pipe' using 'for', which is
<a name="line-977"></a> how 'P.map' is defined:
<a name="line-978"></a>
<a name="line-979"></a>> map :: Monad m => (a -> b) -> Pipe a b m r
<a name="line-980"></a>> map f = for cat $ \x -> yield (f x)
<a name="line-981"></a>>
<a name="line-982"></a>> -- Read this as: For all values flowing downstream, apply 'f'
<a name="line-983"></a>
<a name="line-984"></a> This is equivalent to:
<a name="line-985"></a>
<a name="line-986"></a>> map f = forever $ do
<a name="line-987"></a>> x <- await
<a name="line-988"></a>> yield (f x)
<a name="line-989"></a>
<a name="line-990"></a> You can also feed a 'Pipe' input using ('>~'). This means we could have
<a name="line-991"></a> instead defined the @yes@ pipe like this:
<a name="line-992"></a>
<a name="line-993"></a>> yes :: Monad m => Producer String m r
<a name="line-994"></a>> yes = return "y" >~ cat
<a name="line-995"></a>>
<a name="line-996"></a>> -- Read this as: Keep feeding "y" downstream
<a name="line-997"></a>
<a name="line-998"></a> This is equivalent to:
<a name="line-999"></a>
<a name="line-1000"></a>> yes = forever $ yield "y"
<a name="line-1001"></a>
<a name="line-1002"></a> You can also sequence two 'Pipe's together. This is how 'P.drop' is
<a name="line-1003"></a> defined:
<a name="line-1004"></a>
<a name="line-1005"></a>> drop :: Monad m => Int -> Pipe a a m r
<a name="line-1006"></a>> drop n = do
<a name="line-1007"></a>> replicateM_ n await
<a name="line-1008"></a>> cat
<a name="line-1009"></a>
<a name="line-1010"></a> This is equivalent to:
<a name="line-1011"></a>
<a name="line-1012"></a>> drop n = do
<a name="line-1013"></a>> replicateM_ n await
<a name="line-1014"></a>> forever $ do
<a name="line-1015"></a>> x <- await
<a name="line-1016"></a>> yield x
<a name="line-1017"></a>
<a name="line-1018"></a> You can even compose pipes inside of another pipe:
<a name="line-1019"></a>
<a name="line-1020"></a>> customerService :: Producer String IO ()
<a name="line-1021"></a>> customerService = do
<a name="line-1022"></a>> each [ "Hello, how can I help you?" -- Begin with a script
<a name="line-1023"></a>> , "Hold for one second."
<a name="line-1024"></a>> ]
<a name="line-1025"></a>> P.stdinLn >-> P.takeWhile (/= "Goodbye!") -- Now continue with a human
<a name="line-1026"></a>
<a name="line-1027"></a> Also, you can often use 'each' in conjunction with ('~>') to traverse nested
<a name="line-1028"></a> data structures. For example, you can print all non-'Nothing' elements
<a name="line-1029"></a> from a doubly-nested list:
<a name="line-1030"></a>
<a name="line-1031"></a>>>> runEffect $ (each ~> each ~> each ~> lift . print) [[Just 1, Nothing], [Just 2, Just 3]]
<a name="line-1032"></a>1
<a name="line-1033"></a>2
<a name="line-1034"></a>3
<a name="line-1035"></a>
<a name="line-1036"></a> Another neat thing to know is that 'every' has a more general type:
<a name="line-1037"></a>
<a name="line-1038"></a>@
<a name="line-1039"></a> 'every' :: ('Monad' m, 'Enumerable' t) => t m a -> 'Producer' a m ()
<a name="line-1040"></a>@
<a name="line-1041"></a>
<a name="line-1042"></a> 'Enumerable' generalizes 'Foldable' and if you have an effectful container
<a name="line-1043"></a> of your own that you want others to traverse using @pipes@, just have your
<a name="line-1044"></a> container implement the 'toListT' method of the 'Enumerable' class:
<a name="line-1045"></a>
<a name="line-1046"></a>> class Enumerable t where
<a name="line-1047"></a>> toListT :: Monad m => t m a -> ListT m a
<a name="line-1048"></a>
<a name="line-1049"></a> You can even use 'Enumerable' to traverse effectful types that are not even
<a name="line-1050"></a> proper containers, like 'Control.Monad.Trans.Maybe.MaybeT':
<a name="line-1051"></a>
<a name="line-1052"></a>> input :: MaybeT IO Int
<a name="line-1053"></a>> input = do
<a name="line-1054"></a>> str <- lift getLine
<a name="line-1055"></a>> guard (str /= "Fail")
<a name="line-1056"></a>
<a name="line-1057"></a>>>> runEffect $ every input >-> P.stdoutLn
<a name="line-1058"></a>Test<Enter>
<a name="line-1059"></a>Test
<a name="line-1060"></a>>>> runEffect $ every input >-> P.stdoutLn
<a name="line-1061"></a>Fail<Enter>
<a name="line-1062"></a>>>>
<a name="line-1063"></a>
<a name="line-1064"></a>-}</span>
<a name="line-1065"></a>
<a name="line-1066"></a><span class='hs-comment'>{- $conclusion
<a name="line-1067"></a> This tutorial covers the concepts of connecting, building, and reading
<a name="line-1068"></a> @pipes@ code. However, this library is only the core component in an
<a name="line-1069"></a> ecosystem of streaming components. Derived libraries that build immediately
<a name="line-1070"></a> upon @pipes@ include:
<a name="line-1071"></a>
<a name="line-1072"></a> * @pipes-concurrency@: Concurrent reactive programming and message passing
<a name="line-1073"></a>
<a name="line-1074"></a> * @pipes-parse@: Minimal utilities for stream parsing
<a name="line-1075"></a>
<a name="line-1076"></a> * @pipes-safe@: Resource management and exception safety for @pipes@
<a name="line-1077"></a>
<a name="line-1078"></a> These libraries provide functionality specialized to common streaming
<a name="line-1079"></a> domains. Additionally, there are several libraries on Hackage that provide
<a name="line-1080"></a> even higher-level functionality, which you can find by searching under the
<a name="line-1081"></a> \"Pipes\" category or by looking for packages with a @pipes-@ prefix in
<a name="line-1082"></a> their name. Current examples include:
<a name="line-1083"></a>
<a name="line-1084"></a> * @pipes-network@/@pipes-network-tls@: Networking
<a name="line-1085"></a>
<a name="line-1086"></a> * @pipes-zlib@: Compression and decompression
<a name="line-1087"></a>
<a name="line-1088"></a> * @pipes-binary@: Binary serialization
<a name="line-1089"></a>
<a name="line-1090"></a> * @pipes-attoparsec@: High-performance parsing
<a name="line-1091"></a>
<a name="line-1092"></a> * @pipes-aeson@: JSON serialization and deserialization
<a name="line-1093"></a>
<a name="line-1094"></a> Even these derived packages still do not explore the full potential of
<a name="line-1095"></a> @pipes@ functionality, which actually permits bidirectional communication.
<a name="line-1096"></a> Advanced @pipes@ users can explore this library in greater detail by
<a name="line-1097"></a> studying the documentation in the "Pipes.Core" module to learn about the
<a name="line-1098"></a> symmetry of the underlying 'Proxy' type and operators.
<a name="line-1099"></a>
<a name="line-1100"></a> To learn more about @pipes@, ask questions, or follow @pipes@ development,
<a name="line-1101"></a> you can subscribe to the @haskell-pipes@ mailing list at:
<a name="line-1102"></a>
<a name="line-1103"></a> <https://groups.google.com/forum/#!forum/haskell-pipes>
<a name="line-1104"></a>
<a name="line-1105"></a> ... or you can mail the list directly at:
<a name="line-1106"></a>
<a name="line-1107"></a> <mailto:haskell-pipes@googlegroups.com>
<a name="line-1108"></a>
<a name="line-1109"></a> Additionally, for questions regarding types or type errors, you might find
<a name="line-1110"></a> the following appendix on types very useful.
<a name="line-1111"></a>-}</span>
<a name="line-1112"></a>
<a name="line-1113"></a><span class='hs-comment'>{- $types
<a name="line-1114"></a> @pipes@ uses parametric polymorphism (i.e. generics) to overload all
<a name="line-1115"></a> operations. You've probably noticed this overloading already::
<a name="line-1116"></a>
<a name="line-1117"></a> * 'yield' works within both 'Producer's and 'Pipe's
<a name="line-1118"></a>
<a name="line-1119"></a> * 'await' works within both 'Consumer's and 'Pipe's
<a name="line-1120"></a>
<a name="line-1121"></a> * ('>->') connects 'Producer's, 'Consumer's, and 'Pipe's in varying ways
<a name="line-1122"></a>
<a name="line-1123"></a> This overloading is great when it works, but when connections fail they
<a name="line-1124"></a> produce type errors that appear intimidating at first. This section
<a name="line-1125"></a> explains the underlying types so that you can work through type errors
<a name="line-1126"></a> intelligently.
<a name="line-1127"></a>
<a name="line-1128"></a> 'Producer's, 'Consumer's, 'Pipe's, and 'Effect's are all special cases of a
<a name="line-1129"></a> single underlying type: a 'Proxy'. This overarching type permits fully
<a name="line-1130"></a> bidirectional communication on both an upstream and downstream interface.
<a name="line-1131"></a> You can think of it as having the following shape:
<a name="line-1132"></a>
<a name="line-1133"></a>> Proxy a' a b' b m r
<a name="line-1134"></a>>
<a name="line-1135"></a>> Upstream | Downstream
<a name="line-1136"></a>> +---------+
<a name="line-1137"></a>> | |
<a name="line-1138"></a>> a' <== <== b' -- Information flowing upstream
<a name="line-1139"></a>> | |
<a name="line-1140"></a>> a ==> ==> b -- Information flowing downstream
<a name="line-1141"></a>> | | |
<a name="line-1142"></a>> +----|----+
<a name="line-1143"></a>> v
<a name="line-1144"></a>> r
<a name="line-1145"></a>
<a name="line-1146"></a> The four core types do not use the upstream flow of information. This means
<a name="line-1147"></a> that the @a'@ and @b'@ in the above diagram go unused unless you use the
<a name="line-1148"></a> more advanced features provided in "Pipes.Core".
<a name="line-1149"></a>
<a name="line-1150"></a> @pipes@ uses type synonyms to hide unused inputs or outputs and clean up
<a name="line-1151"></a> type signatures. These type synonyms come in two flavors:
<a name="line-1152"></a>
<a name="line-1153"></a> * Concrete type synonyms that explicitly close unused inputs and outputs of
<a name="line-1154"></a> the 'Proxy' type
<a name="line-1155"></a>
<a name="line-1156"></a> * Polymorphic type synonyms that don't explicitly close unused inputs or
<a name="line-1157"></a> outputs
<a name="line-1158"></a>
<a name="line-1159"></a> The concrete type synonyms use @()@ to close unused inputs and 'X' (the
<a name="line-1160"></a> uninhabited type) to close unused outputs:
<a name="line-1161"></a>
<a name="line-1162"></a> * 'Effect': explicitly closes both ends, forbidding 'await's and 'yield's
<a name="line-1163"></a>
<a name="line-1164"></a>> type Effect = Proxy X () () X
<a name="line-1165"></a>>
<a name="line-1166"></a>> Upstream | Downstream
<a name="line-1167"></a>> +---------+
<a name="line-1168"></a>> | |
<a name="line-1169"></a>> X <== <== ()
<a name="line-1170"></a>> | |
<a name="line-1171"></a>> () ==> ==> X
<a name="line-1172"></a>> | | |
<a name="line-1173"></a>> +----|----+
<a name="line-1174"></a>> v
<a name="line-1175"></a>> r
<a name="line-1176"></a>
<a name="line-1177"></a> * 'Producer': explicitly closes the upstream end, forbidding 'await's
<a name="line-1178"></a>
<a name="line-1179"></a>> type Producer b = Proxy X () () b
<a name="line-1180"></a>>
<a name="line-1181"></a>> Upstream | Downstream
<a name="line-1182"></a>> +---------+
<a name="line-1183"></a>> | |
<a name="line-1184"></a>> X <== <== ()
<a name="line-1185"></a>> | |
<a name="line-1186"></a>> () ==> ==> b
<a name="line-1187"></a>> | | |
<a name="line-1188"></a>> +----|----+
<a name="line-1189"></a>> v
<a name="line-1190"></a>> r
<a name="line-1191"></a>
<a name="line-1192"></a> * 'Consumer': explicitly closes the downstream end, forbidding 'yield's
<a name="line-1193"></a>
<a name="line-1194"></a>> type Consumer a = Proxy () a () X
<a name="line-1195"></a>>
<a name="line-1196"></a>> Upstream | Downstream
<a name="line-1197"></a>> +---------+
<a name="line-1198"></a>> | |
<a name="line-1199"></a>> () <== <== ()
<a name="line-1200"></a>> | |
<a name="line-1201"></a>> a ==> ==> X
<a name="line-1202"></a>> | | |
<a name="line-1203"></a>> +----|----+
<a name="line-1204"></a>> v
<a name="line-1205"></a>> r
<a name="line-1206"></a>
<a name="line-1207"></a> * 'Pipe': marks both ends open, allowing both 'await's and 'yield's
<a name="line-1208"></a>
<a name="line-1209"></a>> type Pipe a b = Proxy () a () b
<a name="line-1210"></a>>
<a name="line-1211"></a>> Upstream | Downstream
<a name="line-1212"></a>> +---------+
<a name="line-1213"></a>> | |
<a name="line-1214"></a>> () <== <== ()
<a name="line-1215"></a>> | |
<a name="line-1216"></a>> a ==> ==> b
<a name="line-1217"></a>> | | |
<a name="line-1218"></a>> +----|----+
<a name="line-1219"></a>> v
<a name="line-1220"></a>> r
<a name="line-1221"></a>
<a name="line-1222"></a> When you compose 'Proxy's using ('>->') all you are doing is placing them
<a name="line-1223"></a> side by side and fusing them laterally. For example, when you compose a
<a name="line-1224"></a> 'Producer', 'Pipe', and a 'Consumer', you can think of information flowing
<a name="line-1225"></a> like this:
<a name="line-1226"></a>
<a name="line-1227"></a>> Producer Pipe Consumer
<a name="line-1228"></a>> +-----------+ +----------+ +------------+
<a name="line-1229"></a>> | | | | | |
<a name="line-1230"></a>> X <== <== () <== <== () <== <== ()
<a name="line-1231"></a>> | stdinLn | | take 3 | | stdoutLn |
<a name="line-1232"></a>> () ==> ==> String ==> ==> String ==> ==> X
<a name="line-1233"></a>> | | | | | | | | |
<a name="line-1234"></a>> +-----|-----+ +----|-----+ +------|-----+
<a name="line-1235"></a>> v v v
<a name="line-1236"></a>> () () ()
<a name="line-1237"></a>
<a name="line-1238"></a> Composition fuses away the intermediate interfaces, leaving behind an
<a name="line-1239"></a> 'Effect':
<a name="line-1240"></a>
<a name="line-1241"></a>> Effect
<a name="line-1242"></a>> +-----------------------------------+
<a name="line-1243"></a>> | |
<a name="line-1244"></a>> X <== <== ()
<a name="line-1245"></a>> | stdinLn >-> take 3 >-> stdoutLn |
<a name="line-1246"></a>> () ==> ==> X
<a name="line-1247"></a>> | |
<a name="line-1248"></a>> +----------------|------------------+
<a name="line-1249"></a>> v
<a name="line-1250"></a>> ()
<a name="line-1251"></a>
<a name="line-1252"></a> @pipes@ also provides polymorphic type synonyms with apostrophes at the end
<a name="line-1253"></a> of their names. These use universal quantification to leave open any unused
<a name="line-1254"></a> input or output ends (which I mark using @*@):
<a name="line-1255"></a>
<a name="line-1256"></a> * 'Producer'': marks the upstream end unused but still open
<a name="line-1257"></a>
<a name="line-1258"></a>> type Producer' b m r = forall x' x . Proxy x' x () b m r
<a name="line-1259"></a>>
<a name="line-1260"></a>> Upstream | Downstream
<a name="line-1261"></a>> +---------+
<a name="line-1262"></a>> | |
<a name="line-1263"></a>> * <== <== ()
<a name="line-1264"></a>> | |
<a name="line-1265"></a>> * ==> ==> b
<a name="line-1266"></a>> | | |
<a name="line-1267"></a>> +----|----+
<a name="line-1268"></a>> v
<a name="line-1269"></a>> r
<a name="line-1270"></a>
<a name="line-1271"></a> * 'Consumer'': marks the downstream end unused but still open
<a name="line-1272"></a>
<a name="line-1273"></a>> type Consumer' a m r = forall y' y . Proxy () a y' y m r
<a name="line-1274"></a>>
<a name="line-1275"></a>> Upstream | Downstream
<a name="line-1276"></a>> +---------+
<a name="line-1277"></a>> | |
<a name="line-1278"></a>> () <== <== *
<a name="line-1279"></a>> | |
<a name="line-1280"></a>> a ==> ==> *
<a name="line-1281"></a>> | | |
<a name="line-1282"></a>> +----|----+
<a name="line-1283"></a>> v
<a name="line-1284"></a>> r
<a name="line-1285"></a>
<a name="line-1286"></a> * 'Effect'': marks both ends unused but still open
<a name="line-1287"></a>
<a name="line-1288"></a>> type Effect' m r = forall x' x y' y . Proxy x' x y' y m r
<a name="line-1289"></a>>
<a name="line-1290"></a>> Upstream | Downstream
<a name="line-1291"></a>> +---------+
<a name="line-1292"></a>> | |
<a name="line-1293"></a>> * <== <== *
<a name="line-1294"></a>> | |
<a name="line-1295"></a>> * ==> ==> *
<a name="line-1296"></a>> | | |
<a name="line-1297"></a>> +----|----+
<a name="line-1298"></a>> v
<a name="line-1299"></a>> r
<a name="line-1300"></a>
<a name="line-1301"></a> Note that there is no polymorphic generalization of a 'Pipe'.
<a name="line-1302"></a>
<a name="line-1303"></a> Like before, if you compose a 'Producer'', a 'Pipe', and a 'Consumer'':
<a name="line-1304"></a>
<a name="line-1305"></a>> Producer' Pipe Consumer'
<a name="line-1306"></a>> +-----------+ +----------+ +------------+
<a name="line-1307"></a>> | | | | | |
<a name="line-1308"></a>> * <== <== () <== <== () <== <== *
<a name="line-1309"></a>> | stdinLn | | take 3 | | stdoutLn |
<a name="line-1310"></a>> * ==> ==> String ==> ==> String ==> ==> *
<a name="line-1311"></a>> | | | | | | | | |
<a name="line-1312"></a>> +-----|-----+ +-----|----+ +------|-----+
<a name="line-1313"></a>> v v v
<a name="line-1314"></a>> () () ()
<a name="line-1315"></a>
<a name="line-1316"></a> ... they fuse into an 'Effect'':
<a name="line-1317"></a>
<a name="line-1318"></a>> Effect'
<a name="line-1319"></a>> +-----------------------------------+
<a name="line-1320"></a>> | |
<a name="line-1321"></a>> * <== <== *
<a name="line-1322"></a>> | stdinLn >-> take 3 >-> stdoutLn |
<a name="line-1323"></a>> * ==> ==> *
<a name="line-1324"></a>> | |
<a name="line-1325"></a>> +----------------|------------------+
<a name="line-1326"></a>> v
<a name="line-1327"></a>> ()
<a name="line-1328"></a>
<a name="line-1329"></a> Polymorphic type synonyms come in handy when you want to keep the type as
<a name="line-1330"></a> general as possible. For example, the type signature for 'yield' uses
<a name="line-1331"></a> 'Producer'' to keep the type signature simple while still leaving the
<a name="line-1332"></a> upstream input end open:
<a name="line-1333"></a>
<a name="line-1334"></a>@
<a name="line-1335"></a> 'yield' :: 'Monad' m => a -> 'Producer'' a m ()
<a name="line-1336"></a>@
<a name="line-1337"></a>
<a name="line-1338"></a> This type signature lets us use 'yield' within a 'Pipe', too, because the
<a name="line-1339"></a> 'Pipe' type synonym is a special case of the polymorphic 'Producer'' type
<a name="line-1340"></a> synonym:
<a name="line-1341"></a>
<a name="line-1342"></a>@
<a name="line-1343"></a> type 'Producer'' b m r = forall x' x . 'Proxy' x' x () b m r
<a name="line-1344"></a> type 'Pipe' a b m r = 'Proxy' () a () b m r
<a name="line-1345"></a>@
<a name="line-1346"></a>
<a name="line-1347"></a> The same is true for 'await', which uses the polymorphic 'Consumer'' type
<a name="line-1348"></a> synonym:
<a name="line-1349"></a>
<a name="line-1350"></a>@
<a name="line-1351"></a> 'await' :: 'Monad' m => 'Consumer'' a m a
<a name="line-1352"></a>@
<a name="line-1353"></a>
<a name="line-1354"></a> We can use 'await' within a 'Pipe' because a 'Pipe' is a special case of the
<a name="line-1355"></a> polymorphic 'Consumer'' type synonym:
<a name="line-1356"></a>
<a name="line-1357"></a>@
<a name="line-1358"></a> type 'Consumer'' a m r = forall y' y . 'Proxy' () a y' y m r
<a name="line-1359"></a> type 'Pipe' a b m r = 'Proxy' () a () b m r
<a name="line-1360"></a>@
<a name="line-1361"></a>
<a name="line-1362"></a> However, polymorphic type synonyms cause problems in many other cases:
<a name="line-1363"></a>
<a name="line-1364"></a> * They usually give the wrong behavior when used as the argument of a
<a name="line-1365"></a> function (known as the \"negative\" or \"contravariant\" position) like
<a name="line-1366"></a> this:
<a name="line-1367"></a>
<a name="line-1368"></a>> f :: Producer' a m r -> ... -- Wrong
<a name="line-1369"></a>>
<a name="line-1370"></a>> f :: Producer a m r -> ... -- Right
<a name="line-1371"></a>
<a name="line-1372"></a> The former function only accepts polymorphic 'Producer's as arguments.
<a name="line-1373"></a> The latter function accepts both polymorphic and concrete 'Producer's,
<a name="line-1374"></a> which is probably what you want.
<a name="line-1375"></a>
<a name="line-1376"></a> * Even when you desire a polymorphic argument, this induces a higher-ranked
<a name="line-1377"></a> type, because it translates to a @forall@ which you cannot factor out to
<a name="line-1378"></a> the top-level to simplify the type signature:
<a name="line-1379"></a>
<a name="line-1380"></a>> f :: (forall x' x y' . Proxy x' x y' m r) -> ...
<a name="line-1381"></a>
<a name="line-1382"></a> These kinds of type signatures require the @RankNTypes@ extension.
<a name="line-1383"></a>
<a name="line-1384"></a> * Even when you have polymorphic type synonyms as the result of a function
<a name="line-1385"></a> (i.e. the \"positive\" or \"covariant\" position), recent versions of
<a name="line-1386"></a> @ghc@ such still require the @RankNTypes@ extension. For example, the
<a name="line-1387"></a> 'Pipes.Prelude.fromHandle' function from "Pipes.Prelude" requires
<a name="line-1388"></a> @RankNTypes@ to compile correctly on @ghc-7.6.3@:
<a name="line-1389"></a>
<a name="line-1390"></a>> fromHandle :: MonadIO m => Handle -> Producer' String m ()
<a name="line-1391"></a>
<a name="line-1392"></a> * You can't use polymorphic type synonyms inside other type constructors
<a name="line-1393"></a> without the @ImpredicativeTypes@ extension:
<a name="line-1394"></a>
<a name="line-1395"></a>> io :: IO (Producer' a m r) -- Type error without ImpredicativeTypes
<a name="line-1396"></a>
<a name="line-1397"></a> * You can't partially apply polymorphic type synonyms:
<a name="line-1398"></a>
<a name="line-1399"></a>> stack :: MaybeT (Producer' a m) r -- Type error
<a name="line-1400"></a>
<a name="line-1401"></a> In these scenarios you should fall back on the concrete type synonyms, which
<a name="line-1402"></a> are better behaved. If concrete type synonyms are unsatisfactory, then ask
<a name="line-1403"></a> @ghc@ to infer the most general type signature and use that.
<a name="line-1404"></a>
<a name="line-1405"></a> For the purposes of debugging type errors you can just remember that:
<a name="line-1406"></a>
<a name="line-1407"></a>> Input --+ +-- Output
<a name="line-1408"></a>> | |
<a name="line-1409"></a>> v v
<a name="line-1410"></a>> Proxy a' a b' b m r
<a name="line-1411"></a>> ^ ^
<a name="line-1412"></a>> | |
<a name="line-1413"></a>> +----+-- Ignore these
<a name="line-1414"></a>
<a name="line-1415"></a> For example, let's say that you try to run the 'P.stdinLn' 'Producer'. This
<a name="line-1416"></a> produces the following type error:
<a name="line-1417"></a>
<a name="line-1418"></a>>>> runEffect P.stdinLn
<a name="line-1419"></a><interactive>:4:5:
<a name="line-1420"></a> Couldn't match expected type `X' with actual type `String'
<a name="line-1421"></a> Expected type: Effect m0 r0
<a name="line-1422"></a> Actual type: Proxy X () () String IO ()
<a name="line-1423"></a> In the first argument of `runEffect', namely `P.stdinLn'
<a name="line-1424"></a> In the expression: runEffect P.stdinLn
<a name="line-1425"></a>
<a name="line-1426"></a> 'runEffect' expects an 'Effect', which is equivalent to the following type:
<a name="line-1427"></a>
<a name="line-1428"></a>> Effect IO () = Proxy X () () X IO ()
<a name="line-1429"></a>
<a name="line-1430"></a> ... but 'P.stdinLn' type-checks as a 'Producer', which has the following
<a name="line-1431"></a> type:
<a name="line-1432"></a>
<a name="line-1433"></a>> Producer String IO () = Proxy X () () String IO ()
<a name="line-1434"></a>
<a name="line-1435"></a> The fourth type variable (the output) does not match. For an 'Effect' this
<a name="line-1436"></a> type variable should be closed (i.e. 'X'), but 'P.stdinLn' has a 'String'
<a name="line-1437"></a> output, thus the type error:
<a name="line-1438"></a>
<a name="line-1439"></a>> Couldn't match expected type `X' with actual type `String'
<a name="line-1440"></a>
<a name="line-1441"></a> Any time you get type errors like these you can work through them by
<a name="line-1442"></a> expanding out the type synonyms and seeing which type variables do not
<a name="line-1443"></a> match.
<a name="line-1444"></a>
<a name="line-1445"></a> You may also consult this table of type synonyms to more easily compare
<a name="line-1446"></a> them:
<a name="line-1447"></a>
<a name="line-1448"></a>> type Effect = Proxy X () () X
<a name="line-1449"></a>> type Producer b = Proxy X () () b
<a name="line-1450"></a>> type Consumer a = Proxy () a () X
<a name="line-1451"></a>> type Pipe a b = Proxy () a () b
<a name="line-1452"></a>>
<a name="line-1453"></a>> type Server b' b = Proxy X () b' b
<a name="line-1454"></a>> type Client a' a = Proxy a' a () X
<a name="line-1455"></a>>
<a name="line-1456"></a>> type Effect' m r = forall x' x y' y . Proxy x' x y' y m r
<a name="line-1457"></a>> type Producer' b m r = forall x' x . Proxy x' x () b m r
<a name="line-1458"></a>> type Consumer' a m r = forall y' y . Proxy () a y' y m r
<a name="line-1459"></a>>
<a name="line-1460"></a>> type Server' b' b m r = forall x' x . Proxy x' x b' b m r
<a name="line-1461"></a>> type Client' a' a m r = forall y' y . Proxy a' a y' y m r
<a name="line-1462"></a>
<a name="line-1463"></a>-}</span>
<a name="line-1464"></a>
<a name="line-1465"></a><span class='hs-comment'>{- $timecomplexity
<a name="line-1466"></a> There are three functions that give quadratic time complexity when used in
<a name="line-1467"></a> within @pipes@:
<a name="line-1468"></a>
<a name="line-1469"></a> * 'sequence'
<a name="line-1470"></a>
<a name="line-1471"></a> * 'replicateM'
<a name="line-1472"></a>
<a name="line-1473"></a> * 'mapM'
<a name="line-1474"></a>
<a name="line-1475"></a> For example, the time complexity of this code segment scales quadratically
<a name="line-1476"></a> with `n`:
<a name="line-1477"></a>
<a name="line-1478"></a>> import Control.Monad (replicateM)
<a name="line-1479"></a>> import Pipes
<a name="line-1480"></a>>
<a name="line-1481"></a>> quadratic :: Int -> Consumer a m [a]
<a name="line-1482"></a>> quadratic n = replicateM n await
<a name="line-1483"></a>
<a name="line-1484"></a> These three functions are generally bad practice to use, because all three
<a name="line-1485"></a> of them correspond to \"ListT done wrong\", building a list in memory
<a name="line-1486"></a> instead of streaming results.
<a name="line-1487"></a>
<a name="line-1488"></a> However, sometimes situations arise where one deliberately intends to build
<a name="line-1489"></a> a list in memory. The solution is to use the \"codensity transformation\"
<a name="line-1490"></a> to transform the code to run with linear time complexity. This involves:
<a name="line-1491"></a>
<a name="line-1492"></a> * wrapping the code in the @Codensity@ monad transformer (from
<a name="line-1493"></a> @Control.Monad.Codensity@ module of the @kan-extensions@ package) using
<a name="line-1494"></a> 'lift'
<a name="line-1495"></a>
<a name="line-1496"></a> * applying 'sequence' \/ 'replicateM' \/ 'mapM'
<a name="line-1497"></a>
<a name="line-1498"></a> * unwrapping the code using @lowerCodensity@
<a name="line-1499"></a>
<a name="line-1500"></a> To illustrate this, we'd transform the above example to:
<a name="line-1501"></a>
<a name="line-1502"></a>> import Control.Monad.Codensity (lowerCodensity)
<a name="line-1503"></a>>
<a name="line-1504"></a>> linear :: Monad m => Int -> Consumer a m [a]
<a name="line-1505"></a>> linear n = lowerCodensity $ replicateM n $ lift await
<a name="line-1506"></a>
<a name="line-1507"></a> This will produce the exact same result, but in linear time.
<a name="line-1508"></a>-}</span>
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