Suggest AllowAmbiguousTypes
The error message I get with GHC HEAD on the example program in #8390 (closed):
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)
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |