From: simonpj <unknown>
Date: Tue, 6 May 2003 14:48:12 +0000 (+0000)
Subject: [project @ 2003-05-06 14:48:12 by simonpj]
X-Git-Tag: Approx_11550_changesets_converted~925
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=0773c6f70a7b46b243a6a47fe371fe1654a8e1b2;p=ghc-hetmet.git

[project @ 2003-05-06 14:48:12 by simonpj]
Note about context in data type decls
---

diff --git a/ghc/docs/users_guide/bugs.sgml b/ghc/docs/users_guide/bugs.sgml
index de507e9..60cadd5 100644
--- a/ghc/docs/users_guide/bugs.sgml
+++ b/ghc/docs/users_guide/bugs.sgml
@@ -283,6 +283,23 @@ main = print (array (1,1) [(1,2), (1,3)])</programlisting>
       </listitem>
     
       <listitem>
+	<para>GHC does not allow you to have a data type with a context 
+	   that mentions type variables that are not data type parameters.
+	  For example:
+<programlisting>
+  data C a b => T a = MkT a
+</programlisting>
+	  so that <literal>MkT</literal>'s type is
+<programlisting>
+  MkT :: forall a b. C a b => a -> T a
+</programlisting>
+        In principle, with a suitable class declaration with a functional dependency,
+	 it's possible that this type is not ambiguous; but GHC nevertheless rejects
+	  it.  The type variables mentioned in the context of the data type declaration must
+	be among the type parameters of the data type.</para>
+      </listitem>
+
+      <listitem>
 	<para>GHCi does not respect the <literal>default</literal>
         declaration in the module whose scope you are in.  Instead,
         for expressions typed at the command line, you always get the