How do I make MonadRandom a Functor?

Question

It seems that MonadRandom from the random-fu package is not a Functor, as I am getting errors like:

Could not deduce (Functor m) arising from a use of ‘_1’
from the context (MonadRandom m)

I've tried adding the following code:

instance Functor MonadRandom where
 fmap = liftM
instance Applicative MonadRandom where
 pure = return
 (<*>) = ap

but I get the error:

The first argument of ‘Functor’ should have kind ‘* -> *’,
 but ‘MonadRandom’ has kind ‘(* -> *) -> Constraint’
In the instance declaration for ‘Functor MonadRandom’
The first argument of ‘Applicative’ should have kind ‘* -> *’,
 but ‘MonadRandom’ has kind ‘(* -> *) -> Constraint’
In the instance declaration for ‘Applicative MonadRandom’

• Xeo: MonadRandom is a type-class, you can't make those instances of other type-classes.
• Zeta: You should add least add the function's signature, so that we know the context where you want to use _1.
• leftaroundabout: @Xeo: actually you can, with {-# LANGUAGE KindSignatures #-} import GHC.Exts try e.g. class Foo (c :: * -> Constraint) instance Foo Num. Whether that makes any sense at all is a different question...

Answer 1

MonadRandom is a class, not a type with kind * -> *, like Maybe for example. Usually, you would use something like

instance MonadRandom m => Functor m where
 fmap = liftM
instance MonadRandom m => Applicative m where
 pure = return
 (<*>) = ap

However, in this case the instances of MonadRandom are already functors, so now the instances are ambiguous! Instead, you should to add the Functor constraint at your function:

yourFunction :: (MonadRandom m, Functor m) => ...
-- instead of yourFunction :: (MonadRandom m) => ...