Ross Mellgren schrieb:
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)
fromJust should be avoided, since it is partial and if it results in an error, the error message points to the implementation of fromJust, not its application. Pattern matching is better, but 'maybe' and 'fromMaybe' are best.