ghc-7 assumes incoherent instances without requiring language `IncoherentInstances`
the attached examples works with ghc-7 and returns
*Main> a
[Spec1 Spec2]
*Main> b
[](One may wish that b also returned [Spec1 Spec2])
ghc-6 complains with
Splittable.hs:16:36:
Overlapping instances for Test a Spec2
arising from a use of `test' at Splittable.hs:16:36-43
Matching instances:
instance [overlap ok] Test a Spec2
-- Defined at Splittable.hs:20:13-24
instance [overlap ok] Test Bool Spec2
-- Defined at Splittable.hs:25:13-27
(The choice depends on the instantiation of `a'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the second argument of `($)', namely `test a b'
In the expression: map Spec1 $ test a b
In the definition of `test':
test a (Spec1 b) = map Spec1 $ test a b
Failed, modules loaded: none.After adding IncoherentInstances the file also goes through ghc-6 with the same results.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.6.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | sternkinder@gmail.com |
| Operating system | |
| Architecture |