Proposal: Add isLeft/isRight to Data.Either

[Andreas Abel]

On 01.12.12 5:42 PM, Ivan Lazar Miljenovic wrote:
> On 2 December 2012 08:42, Andreas Abel <andreas.abel at ifi.lmu.de> wrote:
>> On 30.11.12 7:14 PM, Henning Thielemann wrote:
>>>
>>> Agda uses isLeft and isRight in some QuickCheck properties. And then
>>> there is this application:
>>>
>>>      do
>>>         -- ps0 :: [NamedArg ParseLHS]
>>>         ps0 <- mapM classPat ps
>>>         let (ps1, rest) = span (isLeft . namedArg) ps0
>>>         (p2, ps3) <- uncons rest -- when (null rest): no field pattern or
>>> def pattern found
>>>         guard $ all (isLeft . namedArg) ps3
>>>         let (f, lhs)      = fromR p2
>>>             (ps', _:ps'') = splitAt (length ps1) ps
>>>         return $ Right (f, LHSProj x ps' lhs ps'')
>>>
>>> Looks at least interesting ... :-)
>>
>>
>> Well, maybe this code could be improved, avoiding isLeft and fromR(ight).
>>
>> Basically, what I want to to here is:
>> - I have a list of things: ps
>> - I have a classification of these things into Left and Right: classPat
>> - I want to succeed if exactly one of these things is classified as a Right
>> - I want to extract that Right thing, modify it a bit: lhs
>> - I want to obtain all the Left things unchanged: ps' and ps''
>>
>> These standard functions do not do the job:
>> - partitionEithers: loses the order of my things
>> - lefts/rights: ditto
>
> How do they lose the order?  Or do you mean lose the order of "This
> Left is before that Right"?

Yes.

>> The crux here is that the standard list functions like filter, span, break
>> etc. use "Bool" as decision type.  They would be more general with Either,
>> using Left as falsehood and Right as truth.
>>
>>    span  :: (a -> Either b c) -> [a] -> ([c], [a])
>>    break :: (a -> Either b c) -> [a] -> ([b], [a])
>>    partition :: (a -> Either b c) -> [a] -> ([c], [b])
>
> Isn't your either-based partition function just partitionEithers?

Yes.

>> How would you do it?
>>
>> Cheers,
>> Andreas
>>
>> --
>> Andreas Abel  <><      Du bist der geliebte Mensch.
>>
>> Theoretical Computer Science, University of Munich
>> Oettingenstr. 67, D-80538 Munich, GERMANY
>>
>> andreas.abel at ifi.lmu.de
>> http://www2.tcs.ifi.lmu.de/~abel/
>>
>>
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>
>

-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/