#1050: Using an inferred type as a type signature fails
----------------------------------------+-----------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: low | Milestone: _|_
Component: Compiler (Type checker) | Version: 6.6
Severity: minor | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
----------------------------------------+-----------------------------------
See the thread starting here
http://www.haskell.org/pipermail/glasgow-haskell-users/2006-
December/011714.html.
Here's a short example:
{{{
class C a b where
op :: a -> a
-- f :: C a b => a -> a
f x = op x
}}}
It doesn't get much simpler than that! With the type sig, GHC can't see
that the `(C a b)` provided can satisfy the `(C a b1)` which arises from
the call to op. However, without the constraint, GHC simply abstracts
over the constrains arising in the RHS, namely `(C a b1)`, and hence
infers the type
{{{
f :: C a b1 => a -> a
}}}
It is extremely undesirable that the inferred type does not work as a type
signature, but I don't see how to fix it easily. It doesn't affect many
programs, I think; hence low priority
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1050>
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
