#2418: desugaring type function application to constraint makes bug disappear
------------------------------+---------------------------------------------
Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.9 | Severity: normal
Keywords: type families | Testcase:
Architecture: Unknown | Os: Unknown
------------------------------+---------------------------------------------
When considering type functions, I find it helpful to desugar their
applications into additional constraints (`tf a` becomes `tf a~tfa=>tfa`).
But consider this example from a recent [http://www.haskell.org/pipermail
/haskell-cafe/2008-July/044911.html haskell-cafe thread]:
{{{
{-# OPTIONS_GHC -fglasgow-exts #-}
class Blah f a where blah :: a -> T f f a
class A f where type T f :: (* -> *) -> * -> *
-- wrapper :: forall a f tf . (Blah f a,T f~tf) => a -> tf f a
wrapper :: forall a f . (Blah f a) => a -> T f f a
wrapper x = blah x
}}}
for which `GHCi, version 6.9.20080514` yields:
{{{
C:\Documents and Settings\cr3\Desktop\TF.hs:8:12:
Could not deduce (Blah f a) from the context (Blah f1 a)
arising from a use of `blah'
at C:\Documents and Settings\cr3\Desktop\TF.hs:8:12-17
Possible fix:
add (Blah f a) to the context of the type signature for `wrapper'
In the expression: blah x
In the definition of `wrapper': wrapper x = blah x
C:\Documents and Settings\cr3\Desktop\TF.hs:8:12:
Couldn't match expected type `T f1 f1 a'
against inferred type `T f f a'
In the expression: blah x
In the definition of `wrapper': wrapper x = blah x
Failed, modules loaded: none.
}}}
Switching to the desugared version of the `wrapper` signature makes the
error go away, so the "desugared" and original version are not equivalent
in the current implementation of type families! See also
[http://www.haskell.org/pipermail/haskell-cafe/2008-July/044914.html].
For added fun, GHCi reports the sugared type when using the desugared one:
{{{
*Main> :t wrapper
wrapper :: (Blah f a) => a -> T f f a
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2418>
GHC <http://www.haskell.org/ghc/>
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