[Haskell-cafe] Declaring each instance of a typeclass to be also an instance of another typeclass

[Ryan Ingram]

No, but you can do this:

> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
> import Control.Monad (liftM)

> newtype Functorize m a = F { unF :: m a } deriving (Eq, Show, Monad)

Insert any other type classes you care about potentially inheriting
from the parent Monad into the deriving clause, like MonadPlus.
Unfortunately it's not possible to derive "everything my internal type
has", which makes it difficult to include things like MonadState or
other multiparameter classes.

> inF f = F . f . unF
> instance Monad m => Functor (Functorize m) where
>    fmap f = inF (liftM f)

  -- ryan


On Sun, Jan 11, 2009 at 5:12 AM, Mikhail Glushenkov
<the.dead.shall.rise at gmail.com> wrote:
> Hi,
>
> Is it possible to write something like this:
>
>> {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
>>
>> import Control.Monad (liftM)
>>
>> instance (Monad a) => Functor a where
>>     fmap = liftM
>
> without having to use UndecidableInstances (and preferably, other type system
> extensions too)?
>
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