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@lelf.lu> wrote:

Christopher Done <chrisdone@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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