Monad Transformer 'distribute' Function

Question

Given the following Typeclassopedia exercise:

--Implement join :: M (N (M (N a))) -> M (N a), 
--given distrib :: N (M a) -> M (N a) and 
--assuming M and N are instances of Monad.

distrib :: (Monad m, Monad n) => n (m a) => m (n a)
distrib = undefined

I get the following compile-time error.

ghci> :l MonadTransformers.hs
[1 of 1] Compiling Main             ( MonadTransformers.hs, interpreted )

MonadTransformers.hs:7:34:
    Expected a constraint, but `n (m a)' has kind `*'
    In the type signature for `distrib':
      distrib :: (Monad m, Monad n) => n (m a) => m (n a)
Failed, modules loaded: none.

How can I resolve it?

score 3 · Accepted Answer · answered Apr 15 '15 at 02:49

3

That should be

distrib :: (Monad m, Monad n) => n (m a) -> m (n a)

=> comes between the constraint and the rest of the type.

-> is the infix type constructor for functions.

For example

(+) :: Num a => a -> a -> a
       -----     \  /      \
        \         arguments result
         constraint

answered Apr 15 '15 at 02:49

rampion

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I blame ~~Scala~~ (myself) for the `=>`. Thanks – Kevin Meredith Apr 15 '15 at 02:52
Kevin Meredith: Yeah, I imagine it doesn't help that in Scala, constraints are (implicit) arguments. – rampion Apr 15 '15 at 02:53
Are constraints resolved as implicits in Haskell too? – Kevin Meredith Apr 15 '15 at 14:41
sort of. type classes constraints are transformed by the compiler into dictionary arguments. which dictionary is used at call time is determined by type inference. – rampion Apr 16 '15 at 03:40

Monad Transformer 'distribute' Function

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