No kind signatures

From HaskellWiki
Revision as of 16:46, 22 March 2013 by Lemming (talk | contribs) (How to live without KindSignatures)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Question

I have a datatype that needs other kinds than the compiler infers but my compiler does not support KindSignatures. E.g. I want to define

newtype MonadTransformer t m a = MonadTransformer t m a

with a :: *, m :: * -> *, t :: (* -> *) -> (* -> *), but the compiler infers a :: *, m :: *, t :: * -> * -> *!

Answer

You can achieve this using the phantom type of the Const functor.

import Control.Applicative (Const(Const))

newtype MonadTransformer t m a = MonadTransformer (Const (t m a) (m a))

monadTransformer :: t m a -> MonadTransformer t m a
monadTransformer = MonadTransformer . Const

runMonadTransformer :: MonadTransformer t m a -> t m a
runMonadTransformer (MonadTransformer (Const m)) = m

For more than one kind specification you still need only one Const, since you can use a tuple type like so Const (t m a) (t m a, m a, a). So to speak, the second type argument of Const allows you to specify examples of how to use the type parameters of MonadTransformer.