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

[Ross Mellgren]

You've applied two solutions to get the value out -- pattern matching  
(Just reinfo) and fromJust. You should use one or the other, but not  
both:

-- pattern matching
remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t)
remLookupFwd re
    = do fwd <- gets resFwdMap
         let { Just reinfo = M.lookup re fwd }                    --  
PROBLEM
         return reinfo

-- fromJust
remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t)
remLookupFwd re
    = do fwd <- gets resFwdMap
         let { reinfo = fromJust (M.lookup re  
fwd) }                    -- PROBLEM
         return reinfo

BTW, I would personally write this as one line (untested)

gets (fromJust . M.lookup re . resFwdMap)

-Ross

On Jun 3, 2009, at 1:18 PM, Vasili I. Galchin wrote:

> Hi Andrew (Bromage),
>
>        I reversed the  parameter order to Data.Map.lookup and  
> calling fromJust to pull out value from Maybe wrapper ... all as you  
> suggested:
>
> > remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t)
> > remLookupFwd re
> >   = do fwd <- gets resFwdMap
> >        let { Just reinfo = fromJust(M.lookup re  
> fwd) }                    -- PROBLEM
> >        return reinfo
>
>
> I am still getting a type mismatch:
>
>
> Swish\HaskellRDF\Dfa\Dfa.lhs:162:29:
>     Couldn't match expected type `Maybe t'
>            against inferred type `ReInfo t1'
>     In the expression: fromJust (M.lookup re fwd)
>     In a pattern binding: Just reinfo = fromJust (M.lookup re fwd)
>     In the expression:
>         do fwd <- gets resFwdMap
>            let Just reinfo = fromJust (M.lookup re fwd)
>            return reinfo
>
> Vasili
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