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T8034.hs:6:3: error:
Couldn't match type ‘F a0’ with ‘F a’
NB: ‘F’ is a type function, and may not be injective
The type variable ‘a0’ is ambiguous
Expected type: F a -> F a
Actual type: F a0 -> F a0
In the ambiguity check for the type signature for ‘foo’:
foo :: forall a. C a => F a -> F a
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
When checking the class method: foo :: forall a. C a => F a -> F a
In the class declaration for ‘C’
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