[Haskell-cafe] When using functional dependencies to combine...

[Daniel Fischer]

Am Freitag, 19. Dezember 2008 00:17 schrieb Martijn van Steenbergen:
> Good evening everyone,
>
> My program reads:
> > module Boom where
> >
> > import Control.Monad.State
> >
> > type SucParser s = StateT [s] []
> >
> > newtype WithUnit s a = WithUnit (SucParser s (a, ()))
> >
> > foo :: SucParser s [s]
> > foo = get
> >
> > bar :: WithUnit s [s]
> > bar = WithUnit get
>
> The compiler complains:
>
> Boom.hs:13:0:
>      Couldn't match expected type `([s], ())'
>             against inferred type `[s]'
>      When using functional dependencies to combine
>        MonadState s (StateT s m),
>          arising from the instance declaration at <no location info>
>        MonadState ([s], ()) (StateT [s] []),
>          arising from a use of `get' at Boom.hs:13:15-17
>      When generalising the type(s) for `bar'
>
> I'm wondering if I'm making a silly mistake or if there's something less
> trivial going on here. Could someone please explain the error and give a
> hint on how to fix it?

class MonadState s m | m -> s where
    get :: m s ...

For SucParser,
get :: StateT [s] [] [s].
To wrap it in WithUnit, it would need type
StateT [s] [] (([s],()),[s])

Easy fix:
bar :: WithUnit s [s]
bar = WithUnit $ do
    s <- get
    return (s,())

Better define

liftUnit :: SucParser s a -> WithUnit s a
liftUnit m = WithUnit $ do
    a <- m
    return (a,())

and
bar = liftUnit get

Or make WithUnit more general (working with arbitrary monads) and give it a 
MonadTrans instance.
>
> Thanks much. :-)
>
> Martijn.