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Document eta expansion under GHC bugs and infelicities

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simonpj committed Apr 30, 2012
1 parent 081ee17 commit 2822e00d3e126e30c3ed2ca8dc90a075180e42ec
Showing with 20 additions and 6 deletions.
  1. +20 −6 docs/users_guide/bugs.xml
@@ -107,17 +107,31 @@ main = do args <- getArgs
<sect3 id="infelicities-exprs-pats">
<title>Expressions and patterns</title>
- <para>None known.</para>
+ <para>In its default mode, GHC makes some programs sligtly more defined
+ than they should be. For example, consider
+ <programlisting>
+f :: [a] -> b -> b
+f [] = error "urk"
+f (x:xs) = \v -> v
+main = print (f [] `seq` True)
+ </programlisting>
+This should call <literal>error</literal> but actually prints <literal>True</literal>.
+Reason: GHC eta-expands <literal>f</literal> to
+ <programlisting>
+f :: [a] -> b -> b
+f [] v = error "urk"
+f (x:xs) v = v
+ </programlisting>
+This improves efficiency slightly but significantly for most programs, and
+is bad for only a few. To suppress this bogus "optimisation" use <option>-fpedantic-bottoms</option>.
<sect3 id="infelicities-decls">
<title>Declarations and bindings</title>
- <para>GHC's typechecker makes all pattern bindings monomorphic
- by default; this behaviour can be disabled with
- <option>-XNoMonoPatBinds</option>. See <xref
- linkend="options-language" />.</para>
<para>In its default mode, GHC does not accept datatype contexts,
as it has been decided to remove them from the next version of the
language standard. This behaviour can be controlled with the

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