#1834: No instance for <type family result type> in ghci command line
---------------------------------------------------------+------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: GHCi
Version: 6.9 | Severity: normal
Keywords: type family function class instance ghci | Testcase:
Architecture: Unknown | Os: Unknown
---------------------------------------------------------+------------------
In the following code, {{{Pred ()}}} no longer matches the instance for
{{{()}}} from the GHCi command line, even though {{{Pred () ~ ()}}}.
In 6.9.20071015 it still works, in 6.9.20071103 it no longer does.
{{{
{-# LANGUAGE TypeFamilies #-}
module Test where
x = reify (() :: Pred ())
type family Pred a
type instance Pred a = a
class Reify a where
reify :: a -> ()
instance Reify () where
reify _ = ()
}}}
The exact results:
{{{
GHCi, version 6.9.20071015:
*Test> x
()
*Test> reify (() :: Pred ())
()
GHCi, version 6.9.20071103:
*Test> x
()
*Test> reify (() :: Pred ())
<interactive>:1:0:
No instance for (Reify (Pred ()))
arising from a use of `reify' at <interactive>:1:0-20
Possible fix: add an instance declaration for (Reify (Pred ()))
In the expression: reify (() :: Pred ())
In the definition of `it': it = reify (() :: Pred ())
}}}
And, just for completeness, 6.8 shows the same regression:
{{{
GHCi, version 6.8.0.20071002:
*Test> x
()
*Test> reify (() :: Pred ())
()
GHCi, version 6.8.1.20071031:
*Test> x
()
*Test> reify (() :: Pred ())
<interactive>:1:0:
No instance for (Reify (Pred ()))
arising from a use of `reify' at <interactive>:1:0-20
Possible fix: add an instance declaration for (Reify (Pred ()))
In the expression: reify (() :: Pred ())
In the definition of `it': it = reify (() :: Pred ())
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1834>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
