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

The lens docs even have an example of another helper function, "involuted" for functions which are their own inverse.

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"live" & involuted reverse %~ ('d':) "lived"

inv f g = involuted f %~ g http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Le... -- Dan Burton On Sat, Aug 17, 2013 at 1:43 PM, Dan Burton wrote:

This is indeed a job for lens, particularly, the Iso type, and the "under" function. Lens conveniently comes with a typeclassed isomorphism called "reversed", which of course has a list instance.

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under reversed (take 10) ['a'.. 'z'] "qrstuvwxyz"

-- Dan Burton On Aug 17, 2013 10:23 AM, "Anton Nikishaev" wrote:

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Christopher Done writes:

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Anyone ever needed this? Me and John Wiegley were discussing a decent name for it, John suggested inv as in involution. E.g.

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.

And it's here only because reverse^-1 ≡ reverse, is not it? I only can see how f ∘ g ∘ f^-1 can be a pattern.

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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.

-- lelf

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