[Haskell-cafe] Suggestions for improvement

[Dominique Devriese]

Gregory,

2010/10/3 Gregory Crosswhite <gcross at phys.washington.edu>:
>  On 10/3/10 1:45 PM, Dominique Devriese wrote:
>>
>> Additionally, you can't combine the functions (blowup . allButLast)
>> and lastToTheLength into a function that returns a pair like you seem
>> to attempt. You need a function like the following for that:
>>
>> comma :: (a ->  b) ->  (a ->  c) ->  a ->  (b,c)
>> comma f g x = (f x, g x)
>>
>> Then you could say:
>>
>> blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength
>
> It is worth noting that such a function already exists in the standard
> libraries;  it is the &&& operator in Control.Arrow:
>
>    blowup = uncurry (++) . (blowup . allButLast &&& lastToTheLength)

Or you can write it as (liftA2 (,)) as I noted a few lines further in my mail ;)

Dominique