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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