Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so:
(+) 3 4 = 7
and
(+) 4 3 = 7
but
(-) 3 4 = -1
and
(-) 4 3 = 1
--- On Thu, 9/17/09, Tom Doris <tomdoris@gmail.com> wrote:
From: Tom Doris <tomdoris@gmail.com>
Subject: Re: [Haskell-beginners] map question
To: "Joost Kremers" <joostkremers@fastmail.fm>
Cc: beginners@haskell.org
Date: Thursday, September 17, 2009, 6:06 AM
This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers <joostkremers@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
--
Joost Kremers, PhD
University of Frankfurt
Institute for Cognitive Linguistics
Grüneburgplatz 1
60629 Frankfurt am Main, Germany
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