This law implies from fmap id = id, see https://www.fpcomplete.com/user/edwardk/snippets/fmap On 12/31/2015 03:27 AM, Adam Bergmark wrote:

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 mailto: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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