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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The overlap warning checker simply doesn’t take account of
GADTs. There’s a long-standing project suggestion to fix this:<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://hackage.haskell.org/trac/ghc/wiki/ProjectSuggestions">http://hackage.haskell.org/trac/ghc/wiki/ProjectSuggestions</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'>Perhaps a good GSoc project.<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 lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> glasgow-haskell-users-bounces@haskell.org
[mailto:glasgow-haskell-users-bounces@haskell.org] <b>On Behalf Of </b>Conal
Elliott<br>
<b>Sent:</b> 31 March 2009 03:45<br>
<b>To:</b> glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> spurious "non-exhaustive" pattern warnings?<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-family:"Courier New"'>--
Why do I get warnings about non-exhaustive pattern matches in the<br>
-- following code? Is the compiler just not clever enough to notice that<br>
-- the uncovered cases are all type-incorrect?</span> (ghc 6.11.20090115)<span
style='font-family:"Courier New"'><br>
<br>
{-# LANGUAGE TypeFamilies, EmptyDataDecls, TypeOperators<br>
, GADTs,
KindSignatures<br>
,
FlexibleInstances, FlexibleContexts<br>
#-}<br>
{-# OPTIONS_GHC -Wall #-}<br>
<br>
import Control.Applicative (Applicative(..))<br>
<br>
data Z<br>
data S n<br>
<br>
infixr 1 :<<br>
<br>
data Vec :: * -> * -> * where<br>
ZVec :: Vec Z a<br>
(:<) :: a -> Vec n a -> Vec (S n) a<br>
<br>
-- todo: infix op for SVec<br>
<br>
instance Functor (Vec n) where<br>
fmap _ ZVec = ZVec<br>
fmap f (a :< u) = f a :< fmap f u<br>
<br>
instance Applicative (Vec Z) where<br>
pure _ = ZVec<br>
ZVec <*> ZVec = ZVec<br>
<br>
-- Warning: Pattern match(es) are non-exhaustive<br>
--
In the definition of `<*>':<br>
--
Patterns not matched:<br>
--
(_ :< _) _<br>
--
ZVec (_ :< _)<br>
<br>
instance Applicative (Vec n) => Applicative (Vec (S n)) where<br>
pure
a
= a :< pure a<br>
(f :< fs) <*> (x :< xs) = f x :< (fs <*> xs)<br>
<br>
-- Warning: Pattern match(es) are non-exhaustive<br>
--
In the definition of `<*>':<br>
--
Patterns not matched:<br>
--
ZVec _<br>
--
(_ :< _) ZVec<br>
<br>
</span><o:p></o:p></p>
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