T2714.hs:8:5: error:
• Couldn't match type ‘a’ with ‘f0 b’
‘a’ is a rigid type variable bound by
the type signature for:
f :: forall a b. ((a -> b) -> b) -> forall c. c -> a
at T2714.hs:7:1-42
Expected type: ((a -> b) -> b) -> c -> a
Actual type: ((a -> b) -> b) -> f0 (a -> b) -> f0 b
• In the expression: ffmap
In an equation for ‘f’: f = ffmap
• Relevant bindings include
f :: ((a -> b) -> b) -> forall c. c -> a (bound at T2714.hs:8:1)