Friday, January 5, 2018

December 2017 1HaskellADay 1Liners problems and solutions

  • December 29th, 2017:
    given f :: Monad m => n -> a -> m (Maybe b)
    define g :: Monad m => n -> a -> m (a, Maybe b)
    using f and ... arrows? Kleisli category?
    • Bazzargh @bazzargh (\n a->liftM ((,) a) (f n a)) ... according to pointfree.io, that's `liftM2 fmap (,) . f` but I can't pretend to get the transformation
  • December 29th, 2017:
    given f :: a -> b
    define g :: [a] -> [Maybe c] -> [(b, c)]

    >>> g [1,2,3] [Just 7, Nothing, Just 10]
    [("1",7),("3",10)]

    when f = show
    • matt @themattchan
      g = catMaybes ... zipWith (fmap . (,) . f)
      where (...) = (.).(.)
    • garrison @GarrisonLJ g a b = map (f.id***fromJust) . filter (isJust . snd) $ zip a b
    • TJ Takei @karoyakani g = (catMaybes .) . zipWith ((<$>) . (,) . f)
  • December 29th, 2017: define f :: [(a,b)] -> ([a], [b])
    • Андреев Кирилл @nonaem00 and matt @themattchan unzip
    • Victoria C @ToriconPrime f = fmap fst &&& fmap snd
      • (in a vacuum, a more general type signature would be inferred, but the compiler limits itself as instruct)

Tuesday, January 2, 2018

December 2017 1HaskellADay problems and solutions