True, so perhaps better written as:
import Data.Maybe (fromMaybe)
gets (fromMaybe (error "could not find re in resFwdMap") . M.lookup
re . resFwdMap)
with more detail in error message as appropriate.
-Ross
On Jun 3, 2009, at 5:57 PM, Henning Thielemann wrote:
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.
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