[Haskell-beginners] map question

[Daniel Fischer]

Am Donnerstag 17 September 2009 13:31:38 schrieb Joost Kremers:
> Hi all,
>
> I've just started learning Haskell and while experimenting with map a bit,
> I ran into something I don't understand. The following commands do what I'd
> expect:
>
> Prelude> map (+ 1) [1,2,3,4]
> [2,3,4,5]
> Prelude> map (* 2) [1,2,3,4]
> [2,4,6,8]
> Prelude> map (/ 2) [1,2,3,4]
> [0.5,1.0,1.5,2.0]
> Prelude> map (2 /) [1,2,3,4]
> [2.0,1.0,0.6666666666666666,0.5]
>
> But I can't seem to find a way to get map to substract 1 from all members
> of the list. The following form is the only one that works, but it doesn't
> give the result I'd expect:
>
> Prelude> map ((-) 1) [1,2,3,4]
> [0,-1,-2,-3]
>
> I know I can use an anonymous function, but I'm just trying to understand
> the result here... I'd appreciate any hints to help me graps this.

(-) a b = a - b, so  (((-) 1) x) = 1 - x and you've mapped (\x -> 1-x) over the list.
You want to map (\x -> x-1), which is

map (subtract 1) list

or

map (flip (-) 1) list

(or map (+ (-1)) list)

>
> TIA
>
> Joost