May 2016 1Liners

One-liners

• May 24th, 2016:
  Given f :: a -> [a] -> b, g :: a -> c
  Write h :: c -> [c] -> b, point-free, in terms of f and g
  where h x y = f (g x) (map g y)
• May 16th, 2016: The next 3 #1Liner are of a piece, using
  data CmpV a =
   Vec { len :: Int, top :: a,  elts :: [a],
         cmp :: CmpV a -> CmpV a -> Ordering }
• Give the point-free definition of:
  twoVs :: CmpV a -> CmpV b -> ([a], [b])
• instance Ord (CmpV a) where
   compare v1 = uncurry (cmp v1) .  (v1,)
  Make compare point-free by removing v1 from above definition
• An Ord-instance needs an Eq-instance:
  instance Eq a => Eq (CmpV a) where
   v1 == v2 = elts v1 == elts v2
  point-free-itize (==)
• May 16th, 2016: You have the following lambda:
  \x y -> x == y || fn x y
  where fn :: a -> a -> Bool
  Point-free-itize
• obadz @obadzz without any fancy uses of the (a ->) Applicative :)
  curry $ foldr (||) False . flip map [(==), fn] . flip uncurry
• obadz @obadzz with fancy use of (a ->) Applicative :)
  curry $ liftA2 (||) (uncurry (==)) (uncurry fn)
• Noah Luck Easterly @walkstherain
  curry $ uncurry (||) . (uncurry (==) &&& uncurry fn)
• May 5th, 2016:
  sames :: Eq a => [a] -> [a] -> Int
  Counts equal values at the same indices in two lists.
  What is the point-free definition?
  
  • joomy @cattheory
    sames = curry (length . filter (uncurry (==)) . uncurry zip)
  • bazzargh @bazzargh and then Fatih Karakurt @karakfa
    ((length . filter id) .) . zipWith (==)
  • me: sum <<- fromenum="" li="" nbsp="" zipwith="">
  • Noah Luck Easterly @walkstherain
    sames = ((sum . map fromEnum) .) . zipWith (==)
  • `getSum . foldMap (Sum . fromEnum)` seems better than `foldr (bool id succ) 0` but both satisfy `[Bool] -> Int`
  • Андреев Кирилл @nonaem00
    let it = (length .) . (filter id .)  . zipWith (==)