[Haskell-cafe] inv f g = f . g . f

[Ivan Lazar Miljenovic]

On 17 August 2013 19:11, Christopher Done <chrisdone at gmail.com> wrote:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.

In terms of a decent name: as soon as I saw the subject, I thought you
were somehow inverting a function :/

In terms of how useful it is, I don't think I tend to use such an idiom.

>
> inv reverse (take 10)
> inv reverse (dropWhile isDigit)
> trim = inv reverse (dropWhile isSpace) . dropWhile isSpace
>
> That seems to be the only use-case I've ever come across.
>
> There's also this one:
>
> co f g = f g . g
>
> which means you can write
>
> trim = co (inv reverse) (dropWhile isSpace)
>
> but that's optimizing an ever rarer use-case.
>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com