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

[Dan Burton]

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

>>> "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-Lens-Iso.html#v:involuted

-- Dan Burton


On Sat, Aug 17, 2013 at 1:43 PM, Dan Burton <danburton.email at gmail.com>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.
>
> >>> under reversed (take 10) ['a'.. 'z']
> "qrstuvwxyz"
>
> -- Dan Burton
> On Aug 17, 2013 10:23 AM, "Anton Nikishaev" <me at lelf.lu> wrote:
>
>> Christopher Done <chrisdone at gmail.com> writes:
>>
>> > 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.
>>
>> > 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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>
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