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><A
NAME="TYPE-ANNOTATIONS"
>6.4. Type annotations in patterns</A
></H1
><P
>Haskell 98 allows expressions to be annotated with type signatures.
With the <CODE
CLASS="OPTION"
>-98</CODE
> option,
these annotations are also allowed on patterns:
<PRE
CLASS="PROGRAMLISTING"
>f (x::Int) = fromIntegral x :: Double</PRE
>
Moreover type variables in pattern annotations are treated specially:
unless the type variable is already bound (by another pattern annotation),
it is universally quantified over the pattern and its scope, e.g.
<PRE
CLASS="PROGRAMLISTING"
>snoc (xs::[a]) (x::a) = xs++[x] :: [a]</PRE
>
Occurrences of the type variable in type signatures within this scope
are bound to this type variable.
In the above example the second and third occurrences of <TT
CLASS="LITERAL"
>a</TT
>
are bound by the first.
This permits locally defined variables to be given signatures in situations
where it would be impossible in Haskell 98:
<PRE
CLASS="PROGRAMLISTING"
>sortImage :: Ord b => (a -> b) -> [a] -> [a]
sortImage (f::a->b) = sortBy cmp
where
cmp :: a -> a -> Ordering
cmp x y = compare (f x) (f y)</PRE
>
Note that the relationship between signature declarations and pattern
annotations is asymmetrical: pattern annotations may capture type variables
in signature declarations, but not vice versa.
There is no connection between the type variables in the type signature
of <CODE
CLASS="FUNCTION"
>sortImage</CODE
> and those in its definition,
but the occurrence of <TT
CLASS="LITERAL"
>a</TT
> in the signature of
<CODE
CLASS="FUNCTION"
>cmp</CODE
> is bound by the pattern <TT
CLASS="LITERAL"
>(f::a->b)</TT
>.</P
><P
>There are some differences with GHC's scoped type variables:</P
><P
></P
><UL
><LI
><P
>In GHC, type variables bound by pattern annotations are existentially
quantified, and so may be instantiated.
Thus the following is accepted by GHC but not Hugs:
<PRE
CLASS="PROGRAMLISTING"
>g (xs::[a]) = xs ++ "\n"</PRE
></P
></LI
><LI
><P
>In GHC, type variables bound in the head of a class or instance declaration
are bound in method definitions in the <TT
CLASS="LITERAL"
>where</TT
> part,
but this is not the case in Hugs.</P
></LI
><LI
><P
>GHC also allows <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>result type signatures</I
></SPAN
>,
where a type signature is attached to the left side of a function definition,
but Hugs does not.</P
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