Ovidiu Deac
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/