[Haskell-cafe] When using functional dependencies to combine...
Martijn van Steenbergen
martijn at van.steenbergen.nl
Thu Dec 18 18:17:19 EST 2008
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:
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?
Thanks much. :-)
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