#8392: Suggest AllowAmbiguousTypes
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Reporter: rwbarton | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Keywords: | Operating System:
Architecture: Unknown/Multiple | Unknown/Multiple
Difficulty: Unknown | Type of failure:
Blocked By: | None/Unknown
Related Tickets: | Test Case:
| Blocking:
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The error message I get with GHC HEAD on the example program in #8390:
{{{
tf.hs:38:10:
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 tf.hs:38:10-56
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’
}}}
could perhaps benefit from an appended line:
{{{
(To defer this ambiguity check to use sites, enable
AllowAmbiguousTypes)
}}}
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8392>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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