Out of scope errors with type families do not mention scope
{-# LANGUAGE TypeFamilies #-}
class C a where
type T a
c :: a -> T a
main = c noSuchThing
yields:
test_bad_error.hs:6:1: error:
• Couldn't match expected type ‘T a’ with actual type ‘T a0’
NB: ‘T’ is a type function, and may not be injective
The type variable ‘a0’ is ambiguous
• In the ambiguity check for the inferred type for ‘main’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
When checking the inferred type
main :: forall a. T a
This makes simple out-of-scope error seem very perplexing and is a huge regression in error quality.