> 2009/9/17 Joost Kremers <joostkrem...@fastmail.fm> >> >> 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. >> >> TIA >> >> Joost
The reason that "map (-1) [1,2,3,4]" doesn't work as you'd expect it to is that "-" is ambiguous in Haskell (some may disagree). "-1" means "-1" in Haskell, i.e. negative 1, not "the function that subtracts 1 from its argument". "(-) 1" is the function that subtracts its argument from 1, which is not what you were looking for either! You're looking for the function that subtracts 1 from its argument, which is `subtract 1'. Prelude> map (subtract 1) [1..4] [0,1,2,3] Note that `subtract' is just another name for `flip (-)', i.e. subtraction with its argument in reverse order. -- Deniz Dogan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
