[Haskell-cafe] GHCi infers a type but refuses it as type signature

[Luke Palmer]

On Tue, Jun 23, 2009 at 6:05 PM, Eric Dedieu <papa.eric at free.fr> wrote:

> So
>
> 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
Control.Monad module)

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.

Luke
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