Question

The simple following code

import Control.Monad
import Control.Monad.Random

psum :: (MonadRandom r) => Int -> r Double -> r Double
psum n x = fmap sum $ replicateM n x

yields the error:

Could not deduce (Functor r) arising from a use of `fmap'
    from the context (MonadRandom r)

This is weird to me because of

class (Monad m) => MonadRandom m where ...

in Control.Monad.Random.Class source file, and since monads are functors, GHC should have deduced that r is a functor in my context. I also tried to import Control.Monad.Random.Class with no success.

Adding manually the Functor constraint on r works, but I find this quite ugly.

What am I missing here ?

Answer 1

Theoretically monads are functors, but sadly Functor is not a superclass of Monad for no good reason.

Instead of adding Functor r you can also use liftM instead of fmap.

Edit: There really seems to be no good reason. The classes were introduced together in Haskell 1.3, and superclasses already existed and were used for MonadPlus and MonadZero.