Tuesday, January 5, 2016

December 2015 1HaskellADay 1-Liners

One-liners
  • December 30th, 2015: You have a string of 'digits' in base whatever to convert to an Int
    debase :: [Int] -> Int -> Int
    debase [12,21,3] 26 ~> 8661
    Define debase
    • Gautier DI FOLCO @gautier_difolco
      import Data.Bifunctor
      debase = curry (sum . uncurry (zipWith (*)) . bimap reverse (flip iterate 1 . (*)))
    • bazzargh @bazzargh
      debase a b = sum $ zipWith (*) (reverse a) (map (b^) [0..])
    • obadz @obadzz
      or debase l b = foldl (\ p n -> p * b + n) 0 l
    • bazzargh @bazzargh
      that's better than mine. how about:
      flip (foldl1 . ((+) .) . (*))
  • December 12th, 2015: #math You have this sequence: [1,1,1,1,1,1,2,1,1,1,3,3] What is this pattern? Is there one? Write #haskell to generate this list.
  • December 3rd, 2015: Lens-y again Points-free-itize the following correctName :: Row -> Row correctName r = set lastName (init (view lastName r)) r
  • December 3rd, 2015: Let's get a little lens-y with this one: accumer :: Getter a t a -> t -> [a] -> [a] accumer f s acc = ans where ans = view f s:acc
    • Define the curried-accumer function that curries away the acc-argument.
    • What would the curried definition be if the function type were: accumer :: Getter a t a -> t -> Set a -> Set a
  • December 3rd, 2015: define minimax :: Ord eh => eh -> (eh, eh) -> (eh, eh) such that, e.g.: minimax 1 (2,3) ~> (1,3) minimax 10 (2,3) ~> (2,10)
    • Thomas Dietert @thomasdietert In that case, minimax n (x,y) = (minimum [n,x,y], maximum [n,x,y])
    • joomy @cattheory minimax = liftM2 (***) min max
  • December 1st, 2015: define (->>) :: (a -> m ()) -> (a -> m ()) -> a -> m () All monadic effects must be evaluated.
    • Jérôme @phollow (->>) f g a = f a >> g a 
      • then liftM2 (>>)
      • Nicoλas @BeRewt but the full applicative is: liftA2 (*>)

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