7

I'm trying to implement a Monad instance. As a simpler example, assume the following: 

data Maybee a = Notheeng | Juust a 

instance Monad Maybee where
   return x = Juust x
   Notheeng >>= f = Notheeng
   Juust x >>= f = f x
   fail _ = Notheeng

This should be the standard implementation of Maybe as far as I know. However, this doesn't compile, because the compiler complains:

No instance for (Applicative Maybee)

and similarly he wants a Functor instance once the Applicative is given.

So: Simple question: Must I always implement Functor and Applicative before I can implement a Monad?

jub0bs
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trevore
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    I anticipate a lot of questions about this, in the wake of GHC 7.10's release. Should we create a *Functor-Applicative-Monad proposal* tag? – jub0bs Apr 24 '15 at 10:43

3 Answers3

11

Yes, it wasn't the case before, it's a change that was introduced in ghc7.10 under the name of Functor-Applicative-Monad Proposal.

Nicolas
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8

It is compulsory to define instances for Functor and Applicative (the second one is a new requirement in newer versions of Haskell), but it's actually no big deal because if you don't want to hand-write your own instances you can just use these ones:

import Control.Applicative (Applicative(..))
import Control.Monad       (liftM, ap)

-- Monad m

instance Functor m where
    fmap = liftM

instance Applicative m where
    pure  = return
    (<*>) = ap
Jeremy List
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6

With GHC 7.10 and above, you must implement Functor and Applicative. The class definitions for Monad mandate the superclass instances:

class Functor f => Applicative f where ...
class Applicative m => Monad m where ...

Note that once you have a Monad instance, the Functor and Applicative instances can be defined generically with no additional effort:

import Control.Monad

-- suppose we defined a Monad instance:
instance Monad m where ...

instance Functor m where
    fmap = liftM

instance Applicative m where
    pure = return
    (<*>) = ap
András Kovács
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