[Haskell-cafe] type checking that I can't figure out ....

[Vasili I. Galchin]

Hi Michael,

      Let me look tomorrow morning. In any case, many thanks!

Kind regards,

Vasili

On Tue, Jun 2, 2009 at 11:12 PM, Michael Snoyman <michael at snoyman.com>wrote:

> > remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t)
> > remLookupFwd re
> >   = do fwd <- gets resFwdMap
> > --       let { Just reinfo = M.lookup fwd re }                    --
> PROBLEM
> >        reinfo <- liftMaybe $ M.lookup re fwd                      --
> PROBLEM
> >        return reinfo
> >
> > liftMaybe :: Monad m => Maybe a -> m a
> > liftMaybe Nothing = fail "Nothing"
> > liftMaybe (Just x) = return x
>
> I made two changes:
>
> 1. You had the arguments to M.lookup backwards.
> 2. lookup does not return any generalized Monad, just Maybe (I think that
> should be changed). I added the simple liftMaybe function to convert the
> Maybe result into something that will work with your state monad.
>
> Michael
>
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