From 04783b794ebe6f1b8edde5db931c46d9a0a2a46c Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Thu, 29 Apr 1999 12:21:50 +0000
Subject: [PATCH] [project @ 1999-04-29 12:21:50 by simonpj] Document Olaf
Chitils point about pattern matching against a polymoprhic argument
---
ghc/docs/users_guide/glasgow_exts.vsgml | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/ghc/docs/users_guide/glasgow_exts.vsgml b/ghc/docs/users_guide/glasgow_exts.vsgml
index 9d0afcdfcf19..99eaf5dc3594 100644
--- a/ghc/docs/users_guide/glasgow_exts.vsgml
+++ b/ghc/docs/users_guide/glasgow_exts.vsgml
@@ -1,5 +1,5 @@
%
-% $Id: glasgow_exts.vsgml,v 1.8 1999/03/30 11:26:24 sof Exp $
+% $Id: glasgow_exts.vsgml,v 1.9 1999/04/29 12:21:50 simonpj Exp $
%
% GHC Language Extensions.
%
@@ -1198,6 +1198,26 @@ and <tt>bind</tt> to extract the polymorphic bind and return functions
from the <tt>MonadT</tt> data structure, rather than using pattern
matching.
+You cannot pattern-match against an argument that is polymorphic.
+For example:
+<tscreen><verb>
+ newtype TIM s a = TIM (ST s (Maybe a))
+
+ runTIM :: (forall s. TIM s a) -> Maybe a
+ runTIM (TIM m) = runST m
+</verb></tscreen>
+
+Here the pattern-match fails, because you can't pattern-match against
+an argument of type <tt>(forall s. TIM s a)</tt>. Instead you
+must bind the variable and pattern match in the right hand side:
+<tscreen><verb>
+ runTIM :: (forall s. TIM s a) -> Maybe a
+ runTIM tm = case tm of { TIM m -> runST m }
+</verb></tscreen>
+The <tt>tm</tt> on the right hand side is (invisibly) instantiated, like
+any polymorphic value at its occurrence site, and now you can pattern-match
+against it.
+
<sect2>The partial-application restriction
<p>
--
GitLab