Best tip I have is to pay more attention to the 'forall's. Compose / . / <<< "fixes" the 'forall', which can cause type checking to fail elsewhere or, if "a" is higher-rank, there (it forces "b" to be monomorphic; higher-rank "a" might require a polymorphic "b'). Application / '$' (in GHC) doesn't "fix" the forall.
I think the difference might be "deeper", but for me it's able where the "choice" for the forall is made.
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