<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Type annotations in patterns</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REL="HOME"
TITLE="The Hugs 98 User's Guide"
HREF="index.html"><LINK
REL="UP"
TITLE="Language extensions supported by Hugs and GHC"
HREF="hugs-ghc.html"><LINK
REL="PREVIOUS"
TITLE="Quantified types"
HREF="quantified-types.html"><LINK
REL="NEXT"
TITLE="Implicit parameters"
HREF="implicit-parameters.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="hugs-ug.css"></HEAD
><BODY
CLASS="SECT1"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>The Hugs 98 User's Guide</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="quantified-types.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 6. Language extensions supported by Hugs and GHC</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="implicit-parameters.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><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
></LI
></UL
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="quantified-types.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="implicit-parameters.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Quantified types</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="hugs-ghc.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Implicit parameters</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
