#8390: regression in handling of type variables in constraints on instances
which
do not appear in the instance head
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Reporter: aavogt | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: GHC rejects valid program | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
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Comment (by aavogt):
If it helps, the error with ghc-7.7 is:
{{{
inf.hs:26:10: Warning:
Could not deduce (Fun g a b0)
arising from the ambiguity check for an instance declaration
from the context (Fun f b c, Fun g a b)
bound by an instance declaration:
(Fun f b c, Fun g a b) => Fun (Compose f g) a c
at inf.hs:26:10-56
The type variable ‛b0’ is ambiguous
In the ambiguity check for:
forall f g a c b. (Fun f b c, Fun g a b) => Fun (Compose f g) a c
In the instance declaration for ‛Fun (Compose f g) a c’
Ok, modules loaded: Main.
}}}
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8390#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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