CODEWITHSUNDEEP
haskell
OderWat
OderWat

Reputation: 5739

Apply a constant subtraction using map in Haskell

I simply want to substract 1 from every list element in [1,2,3,4] like

map (-1) [1,2,3,4] -- does not work

map ((-)1) [1,2,3,4] -- gives [0,-1,-2,-3] which is "-listvalue" + 1

map (\x = x - 1) [1,2,3,4] -- what I want but so verbose

map (+ (-1)) [1,2,3,4] -- well :)

map (flip (-) 1) [1,2,3,4] -- I heard of flip before

(map . flip (-)) 1 [1,2,3,4] -- Kinda nice mapped substration composing thingy

map pred [1,2,3,4] -- OK cheated and won't help with -2

Do I miss a form which would be "correct" and / or what Mr. Haskell use?

Upvotes: 4

Views: 1440

Answers (2)

Luis Casillas
Luis Casillas

Reputation: 30237

You've run into a minor wart of Haskell's design. The expression (-1) could in principle be interpreted in two ways:

  1. A section that partially applies the subtraction operator to its second argument.
  2. The application of the unary sign operator to the number 1.

Haskell goes with #2. To make up for this the subtract function is provided to cover case #1.

Upvotes: 4

Jim Jeffries
Jim Jeffries

Reputation: 10081

You can use the subtract function instead

map (subtract 1) [1,2,3,4]

Upvotes: 6

Related Questions