This is a recurring problem[1] and I'm still looking for a really
satisfying solution. The only working and non-verbose solution I found
is the one Miguel suggests. Although I'm not too fond of splitting up
the monadic functions into separate type classes. A similar solution is
described elsewhere[2]. It also desribes how you can use Template
Haskell to regain the power of the do-notation with your own restricted
monad type class.
Kind regards,
Thomas
[1]
http://www.nabble.com/Monad-instance-for-Data.Set%2C-again-td16259448.html
[2]
http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros
Miguel Mitrofanov wrote:
{-# LANGUAGE MultiParamTypeClasses #-}
class Returnable m a where ret :: a -> m a
class Bindable m a b where bind :: m a -> (a -> m b) -> m b
newtype MOAMonad r m a = MOAMonad ((a -> m r) -> m r)
instance Monad (MOAMonad r m) where
return x = MOAMonad $ ($ x)
MOAMonad h >>= f = MOAMonad $ \p -> h $ \x -> let MOAMonad h' = f x
in h' p
fromMOAMonad :: Returnable m r => MOAMonad r m r -> m r
fromMOAMonad (MOAMonad h) = h ret
toMOAMonad :: Bindable m a r => m a -> MOAMonad r m a
toMOAMonad mx = MOAMonad $ \p -> bind mx p
class FMappable f a b where fmp :: (a -> b) -> f a -> f b
newtype MOAFunctor r f a = MOAFunctor ((a -> r) -> f r)
instance Functor (MOAFunctor r f) where
fmap f (MOAFunctor h) = MOAFunctor $ \p -> h $ p . f
fromMOAFunctor :: MOAFunctor r f r -> f r
fromMOAFunctor (MOAFunctor h) = h id
toMOAFunctor :: FMappable f a r => f a -> MOAFunctor r f a
toMOAFunctor fx = MOAFunctor $ \p -> fmp p fx
-- MOA stands for "Mother Of All"
On 26 Apr 2009, at 15:21, Neil Brown wrote:
Hi,
I have a Haskell problem that keeps cropping up and I wondered if
there was any solution/work-around/dirty-hack that could help. I keep
wanting to define class instances for things like Functor or Monad,
but with restrictions on the inner type. I'll explain with an
example, because I find explaining this in words a bit difficult.
Let's say I want to create a Monad instance for Set akin to that for
lists:
==
import Data.Set
import Prelude hiding (map)
instance Monad Set where
return = singleton
m >>= f = fold union empty (map f m)
-- Error: Could not deduce (Ord a, Ord b) from the context (Monad Set)
==
Everything fits (I think) -- except the type-class constraints.
Obviously my Monad instance won't work if you have things inside the
set that aren't Ord, but I can't work out how to define a restricted
instance that only exists for types that have Ord instances. I can't
express the constraint on the instance because the a and b types of
return and >>= aren't visible in the class header. Shifting the
constraint to be present in the type doesn't seem to help either (e.g.
newtype Ord a => MySet a = MySet (Set a)...).
Is there any way to get such instances as the one for Set working? I
cannot carry around a compare function myself in a data type that
wraps Set, because return cannot create such functions without the
original type-class instance. I don't actually need a Monad for Set,
but it neatly demonstrates my problem of wanting constraints on the
type inside a Monad (or a Functor, or an Applicative, etc).
I worked around a similar problem with Functor by opting for a new
Functor-like type-class with the constraints, but doing that with
Monad rules out using all the monad helper functions (liftM, mapM,
etc), and the do notation, which would be a step too far. All
suggestions are welcome, no matter how hacky, or how many GHC
extensions are required :-) (provided they don't break all the other
monads, e.g. redefining the signature of Monad).
Thanks,
Neil.
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