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

[Ross Mellgren]

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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