This is one of the functor laws, the other one being `fmap id = id'. If it doesn't hold your type *should* not have a Functor instance, but the compiler doesn't verify this for you.

HTH,

Adam

On Thu, Dec 31, 2015 at 2:15 AM, David Banas <capn.freako@gmail.com> wrote:

Is this true, in general?:

fmap h (fmap g f) == fmap (h . g) f

Is there a simple proof?

Thanks,
-db

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