[Haskell-beginners] tuple update

[Ertugrul Söylemez]

Ovidiu Deac <ovidiudeac at gmail.com> wrote:

> Does anybody know if there are any functions like these in the Haskell
> libray?
>
> updateSnd f (x,y) -> (x, f y)
>
> updateFst f (x,y) -> (f x, y)

While the function arrow mentioned by Brent is enough for many purposes,
I just want to have mentioned Edward Kmett's amazing data-lens library,
which provides prefab lenses for tuples:

    (3, 5) ^. fstLens = 3
    (3, 5) ^. sndLens = 5

    (fstLens ^= 4) (3, 5) = (4, 5)
    (sndLens ^= 4) (3, 5) = (3, 4)

    (fstLens ^%= f) (3, 5) = (f 3, 5)

This is especially useful when your tuple is part of the implicit
argument of a state monad (data-lens-fd package):

    access fstLens :: State (a, b) a

                       -- starting state (3, 5)
    fstLens ~= 10      -- (10, 5)
    fstLens += 1       -- (11, 5)
    fstLens %= negate  -- (-11, 5)

    focus fstLens $ do
        -- Do something with only the fst value here.


Greets,
Ertugrul

-- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
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