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

Mateusz Kowalczyk fuuzetsu at fuuzetsu.co.uk
Sat Aug 17 11:40:12 CEST 2013

On 17/08/13 10:11, Christopher Done wrote:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.
First thing I thought was ‘inverse’…
> 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.
I do this a lot as well. Why not skip the ‘g’ all together and have ‘f .
reverse . f’ if that's all we're doing? You could even call it fromEnd
at that point and we end up with a rather intuitive ‘fromEnd (drop 10)’.
Maybe even just have an operator.
> 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.

Is this a proposal for addition to something or is it just general

Mateusz K.
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