/usr/lib/hugs/packages/HaXml/Text/ParserCombinators/Poly.hs is in libhugs-haxml-bundled 98.200609.21-5.3.
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( -- * A Parser datatype parameterised on arbitrary token type
Parser(P) -- datatype, instance of: Functor, Monad
, runParser -- :: Parser t a -> [t] -> (Either String a, [t])
, failBad -- :: String -> Parser t a
, commit -- :: Parser t a -> Parser t a
-- * Combinators
-- ** primitives
, next -- :: Parser t t
, satisfy -- :: (t->Bool) -> Parser t t
, apply -- :: Parser t (a->b) -> Parser t a -> Parser t b
, discard -- :: Parser t a -> Parser t b -> Parser t a
-- ** error-handling
, adjustErr -- :: Parser t a -> (String->String) -> Parser t a
, adjustErrBad-- :: Parser t a -> (String->String) -> Parser t a
, indent -- :: Int -> String -> String
-- ** choices
, onFail -- :: Parser t a -> Parser t a -> Parser t a
, oneOf -- :: Show t => [Parser t a] -> Parser t a
, oneOf' -- :: [(String,Parser t a)] -> Parser t a
, optional -- :: Parser t a -> Parser t (Maybe a)
-- ** sequences
, many -- :: Parser t a -> Parser t [a]
, many1 -- :: Parser t a -> Parser t [a]
, sepBy -- :: Parser t a -> Parser t sep -> Parser t [a]
, sepBy1 -- :: Parser t a -> Parser t sep -> Parser t [a]
, bracketSep -- :: Parser t bra -> Parser t sep -> Parser t ket
-- -> Parser t a -> Parser t [a]
, bracket -- :: Parser t bra -> Parser t ket -> Parser t a
-- -> Parser t a
, manyFinally -- :: Parser t a -> Parser t z -> Parser t [a]
-- ** re-parsing
, reparse -- :: [t] -> Parser t ()
) where
-- | The @Parser@ datatype is a fairly generic parsing monad with error
-- reporting. It can be used for arbitrary token types, not just
-- String input. (If you require a running state, use module PolyState
-- instead)
newtype Parser t a = P ([t] -> (EitherE String a, [t]))
-- A return type like Either, that distinguishes not only between
-- right and wrong answers, but also had gradations of wrongness.
type EitherE a b = Either (Bool,a) b
-- | Apply a parser to an input token sequence.
runParser :: Parser t a -> [t] -> (Either String a, [t])
runParser (P p) =
(\ (e,ts)-> (case e of {Left (_,m)->Left m; Right m->Right m}, ts) )
. p
instance Functor (Parser t) where
fmap f (P p) = P (\ts-> case p ts of
(Left msg, ts') -> (Left msg, ts')
(Right x, ts') -> (Right (f x), ts'))
instance Monad (Parser t) where
return x = P (\ts-> (Right x, ts))
(P f) >>= g = P (\ts-> case f ts of
(Left msg, ts') -> (Left msg, ts')
(Right x, ts') -> let (P g') = g x in g' ts')
fail e = P (\ts-> (Left (False,e), ts))
-- | When a simple fail is not strong enough, use failBad for emphasis.
-- An emphasised (severe) error can propagate out through choice operators.
failBad :: String -> Parser t a
failBad msg = P (\ts-> (Left (True,msg), ts))
-- | Commit is a way of raising the severity of any errors found within
-- its argument. Used in the middle of a parser definition, it means that
-- any operations prior to commitment fail softly, but after commitment,
-- they fail hard.
commit :: Parser t a -> Parser t a
commit (P p) = P (\ts-> case p ts of
(Left (_,e), ts') -> (Left (True,e), ts')
right -> right )
-- Combinators
-- | One token
next :: Parser t t
next = P (\ts-> case ts of
[] -> (Left (False,"Ran out of input (EOF)"), [])
(t:ts') -> (Right t, ts') )
-- | One token satifying a predicate
satisfy :: (t->Bool) -> Parser t t
satisfy p = do{ x <- next
; if p x then return x else fail "Parse.satisfy: failed"
}
infixl 3 `apply`
-- | Apply a parsed function to a parsed value
apply :: Parser t (a->b) -> Parser t a -> Parser t b
pf `apply` px = do { f <- pf; x <- px; return (f x) }
infixl 3 `discard`
-- | @x `discard` y@ parses both x and y, but discards the result of y
discard :: Parser t a -> Parser t b -> Parser t a
px `discard` py = do { x <- px; _ <- py; return x }
-- | @p `adjustErr` f@ applies the transformation @f@ to any error message
-- generated in @p@, having no effect if @p@ succeeds.
adjustErr :: Parser t a -> (String->String) -> Parser t a
(P p) `adjustErr` f = P (\ts-> case p ts of
(Left (b,msg), ts') -> (Left (b,(f msg)), ts')
right -> right )
-- | @adjustErrBad@ is just like @adjustErr@ except it also raises the
-- severity of the error.
adjustErrBad :: Parser t a -> (String->String) -> Parser t a
p `adjustErrBad` f = commit (p `adjustErr` f)
infixl 6 `onFail` -- not sure about precedence 6?
-- | @p `onFail` q@ means parse p unless p fails in which case parse q instead.
-- Can be chained together to give multiple attempts to parse something.
-- (Note that q could itself be a failing parser, e.g. to change the error
-- message from that defined in p to something different.)
-- However, a *severe* failure in p cannot be ignored.
onFail :: Parser t a -> Parser t a -> Parser t a
(P p) `onFail` (P q) = P (\ts-> case p ts of
r@(Left (True,_), _) -> r
(Left _, _) -> q ts
right -> right )
-- | Parse the first alternative in the list that succeeds.
oneOf :: [Parser t a] -> Parser t a
oneOf [] = do { n <- next
; fail ("failed to parse any of the possible choices")
}
--oneOf :: Show t => [Parser t a] -> Parser t a
--oneOf [] = do { n <- next
-- ; fail ("failed to parse any of the possible choices"
-- ++"\n next token is "++show n)
-- }
oneOf (p:ps) = p `onFail` oneOf ps
-- | Parse the first alternative that succeeds, but if none succeed,
-- report only the severe errors, and if none of those, then report
-- all the soft errors.
oneOf' :: [(String, Parser t a)] -> Parser t a
oneOf' = accum []
where accum errs [] =
case filter isBad errs of
[] -> fail ("failed to parse any of the possible choices:\n"
++indent 2 (concatMap showErr (reverse errs)))
[(_,(_,e))] -> failBad e
es -> failBad ("one of the following failures occurred:\n"
++indent 2 (concatMap showErr (reverse es)))
accum errs ((e,P p):ps) =
P (\ts-> case p ts of
(Left err,_) -> let (P p) = accum ((e,err):errs) ps
in p ts
right -> right )
showErr (name,(_,err)) = name++":\n"++indent 2 err
isBad (_,(b,_)) = b
-- | Helper for formatting error messages: indents all lines by a fixed amount.
indent :: Int -> String -> String
indent n = unlines . map (replicate n ' ' ++) . lines
-- | 'optional' indicates whether the parser succeeded through the Maybe type.
optional :: Parser t a -> Parser t (Maybe a)
optional p = fmap Just p `onFail` return Nothing
-- | 'many p' parses a list of elements with individual parser p.
-- Cannot fail, since an empty list is a valid return value.
many :: Parser t a -> Parser t [a]
many p = many1 p `onFail` return []
-- | Parse a non-empty list of items.
many1 :: Parser t a -> Parser t [a]
many1 p = do { x <- p `adjustErr` (("In a sequence:\n"++). indent 2)
; xs <- many p
; return (x:xs)
}
-- `adjustErr` ("When looking for a non-empty sequence:\n\t"++)
-- | Parse a list of items separated by discarded junk.
sepBy :: Parser t a -> Parser t sep -> Parser t [a]
sepBy p sep = do sepBy1 p sep `onFail` return []
-- | Parse a non-empty list of items separated by discarded junk.
sepBy1 :: Parser t a -> Parser t sep -> Parser t [a]
sepBy1 p sep = do { x <- p
; xs <- many (do {sep; p})
; return (x:xs)
}
`adjustErr` ("When looking for a non-empty sequence with separators:\n\t"++)
-- | Parse a list of items, discarding the start, end, and separator
-- items.
bracketSep :: Parser t bra -> Parser t sep -> Parser t ket
-> Parser t a -> Parser t [a]
bracketSep open sep close p =
do { open; close; return [] }
`onFail`
do { open `adjustErr` ("Missing opening bracket:\n\t"++)
; x <- p `adjustErr` ("After first bracket in a group:\n\t"++)
; xs <- many (do {sep; p})
; close `adjustErrBad` ("When looking for closing bracket:\n\t"++)
; return (x:xs)
}
-- | Parse a bracketed item, discarding the brackets.
bracket :: Parser t bra -> Parser t ket -> Parser t a -> Parser t a
bracket open close p = do
do { open `adjustErr` ("Missing opening bracket:\n\t"++)
; x <- p
; close `adjustErrBad` ("Missing closing bracket:\n\t"++)
; return x
}
-- | 'manyFinally e t' parses a possibly-empty sequence of e's,
-- terminated by a t. Any parse failures could be due either to
-- a badly-formed terminator or a badly-formed element, so raise
-- both possible errors.
manyFinally :: Parser t a -> Parser t z -> Parser t [a]
manyFinally p t =
do { xs <- many p
; oneOf' [ ("sequence terminator", do { t; return () } )
, ("item in a sequence", do { p; return () } )
]
; return xs
}
------------------------------------------------------------------------
-- | Push some tokens back onto the front of the input stream and reparse.
-- This is useful e.g. for recursively expanding macros. When the
-- user-parser recognises a macro use, it can lookup the macro
-- expansion from the parse state, lex it, and then stuff the
-- lexed expansion back down into the parser.
reparse :: [t] -> Parser t ()
reparse ts = P (\inp-> (Right (), ts++inp))
------------------------------------------------------------------------
|