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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>In Haskell, the context of all functions in a mutually recursive
group must be identical.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><a
href="http://haskell.org/onlinereport/decls.html#generalization">http://haskell.org/onlinereport/decls.html#generalization</a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Simon<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] <b>On Behalf
Of </b>Luis Cabellos<br>
<b>Sent:</b> 14 January 2010 11:26<br>
<b>To:</b> haskell@haskell.org<br>
<b>Subject:</b> [Haskell] Signature error because the order<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>Hi,<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal> I found than this piece of code causes an error
in GHC ( <b><a href="http://haskell.pastebin.com/m5e3f4a4c">http://haskell.pastebin.com/m5e3f4a4c</a></b>
):<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal><span style='font-family:"Courier New"'>data Tree a = Leaf a
| Node (Tree a) (Tree a)</span><o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>--prettyShow ::
(Show a, Num a) => Tree a -> String -- WORKS</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>prettyShow :: (Num
a, Show a) => Tree a -> String -- FAILS</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>prettyShow (Leaf a)
= show a</span><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>prettyShow (Node a
b) = (simple a) ++ (simple b)</span><o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>simple :: (Show a,
Num a) => Tree a -> String</span><o:p></o:p></p>
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<p class=MsoNormal><span style='font-family:"Courier New"'>simple (Leaf x) =
prettyShow (Leaf x)</span><o:p></o:p></p>
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<p class=MsoNormal><span style='font-family:"Courier New"'>simple x =
"(" ++ prettyShow x ++ ")"</span><o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal>With the error:<o:p></o:p></p>
</div>
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<p class=MsoNormal><span style='font-family:"Courier New"'>test.lhs:14:0:</span><o:p></o:p></p>
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<p class=MsoNormal><span style='font-family:"Courier New"'>
Couldn't match expected type `Show a'</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
against inferred type `Num a1'</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
When matching the contexts of the signatures for</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
prettyShow :: forall a. (Num a, Show a) => Tree a -> String</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
simple :: forall a. (Show a, Num a) => Tree a -> String</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
The signature contexts in a mutually recursive group should all be
identical</span><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>
When generalising the type(s) for prettyShow, simple</span><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><span style='font-family:"Courier New"'>Failed, modules
loaded: none.</span><o:p></o:p></p>
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<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal>But if I change the order of <b>Show</b> and <b>Num</b> in
the signature of <i>prettyShow</i> it works.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>It's a GHC problem? it's normal than signature is
order-dependent?<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>I use <b>GHC 6.10.3</b><o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal>Thanks, Luis Cabellos<o:p></o:p></p>
</div>
<p class=MsoNormal><br>
-- <br>
<br>
LC, ("There is no Dana, only Zuul." - Cazafantasmas [1984]) <o:p></o:p></p>
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