[Haskell-cafe] GHCi infers a type but refuses it as type
lrpalmer at gmail.com
Tue Jun 23 20:19:31 EDT 2009
On Tue, Jun 23, 2009 at 6:05 PM, Eric Dedieu <papa.eric at free.fr> wrote:
> 1) How can I use transformers instead of the mtl? This is in no
> tutorial, and searchinf for "mtl" on the haskell wiki yields no
> result at all.
> cabal install transformers (you need cabal-install to do this... consult
#haskell if confused)
perhaps: > ghc-pkg hide mtl (I think mtl and transformers fight over the
Then import Control.Monad.Trans.State instead of Control.Monad.State. All
else should be well, if I am not forgetting something.
> 2) What should be the Haskell98-compatible type signature for my play2
> function, instead of (MonadState [a] m, Eq a) =>
> a -> m Bool ?
That depends on the monad library. In transformers it would be:
(Monad m, Eq a) => a -> StateT [a] m Bool
State s is just a type synonym for StateT s Identity, so this works for
State as well.
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