haskell - Every monad is monoid? -


since every monad monoid on sequencing operation. why doesn't monad inherit monoid in haskell?

it doesn't have monad even, works every applicative. yes, define:

class (functor f, monoid (f ())) => applicative f

but means have provide monoid instance every time write applicative instance. can quite annoying, since monoid instance not used often.

a better solution create newtype wrapper around f (), , can provide monoid instance applicative functors once , all. there's 1 readily available in the reducers package.


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