1HaskellADay 1Liners June 2016

• June 13th, 2016:
  You want this list: [1, -1, 1, -1, ...]
  How would you produce this value in #Haskell ?
  • Wai Lee Chin Feman @wchinfeman
    https://gist.github.com/skatenerd/08d70c45499e1610206a
    (set plop to be identity, and set transformstate to be (*) -1)
  • Philipp Maier @AkiiZedd `iterate negate 1’
  • Patrick Mylund @pmylund concat $ repeat [1, (-1)]
    • Gary Fixler @gfixler No need for the parens in a list.
  • Jeff Foster @fffej and Kevin Meredith @Gentmen
    iterate (* (- 1)) 1
  • Spencer Janssen @spencerjanssen and Андреев Кирилл @nonaem00
    cycle [1, -1]
    • Philipp Maier @AkiiZedd:
      I’m curious: Since concat is O(n) wouldn’t it take more and more time depending on how many items you take?
    • Patrick Mylund @pmylund Looks like they compile to the same thing https://gist.github.com/patrickmn/9a92ab2a088018b2c0631f3bcfd60ebe
    • Philipp Maier @AkiiZedd I’m actually surprised the compiler can optimise this away :o Thanks for showing me ddump-simpl!
    • Eyal Lotem @EyalL concat is foldr (++), not foldl. O(1) work is done to produce the next item. [1,-1]++([1,-1]++(...
• David Turner @DaveCTurner I'd actually write 'cycle [1,-1]' but I like the elegant, alliterative obscurity of   'iterate negate 1'
• Fatih Karakurt @karakfa alt=1:[-x|x<-alt]