T1897a.hs:9:1: error:
• Could not deduce (Wob a0 b)
from the context: Wob a b
bound by the inferred type for ‘foo’:
forall a b. Wob a b => b -> [b]
at T1897a.hs:9:1-24
The type variable ‘a0’ is ambiguous
• In the ambiguity check for the inferred type for ‘foo’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
When checking the inferred type
foo :: forall a b. Wob a b => b -> [b]