From: Job Vranish <jvranish@gmail.com>
Subject: Re: [Haskell-cafe] Re: [Haskell-beginners] map question
To: "Gregory Propf" <gregorypropf@yahoo.com>
Cc: "Tom Doris" <tomdoris@gmail.com>, "Haskell-Cafe" <haskell-cafe@haskell.org>, joostkremers@fastmail.fm
Date: Thursday, September 17, 2009, 9:04 AM

(-) happens to be the only prefix operator in haskell, it also an infix operator.
so:
> 4 - 2
2
> -3
-3

> ((-) 5) 3  -- note that in this case (-) is treated like any regular function so 5 is the first parameter
2
> (5 - ) 3
2
> (-5 )
-5
> (flip (-) 5) 3 
-2


It's a little wart brought about by the ambiguity in common mathematical syntax.
If you play around in ghci you should get the hang of it pretty quick.

- Job

On Thu, Sep 17, 2009 at 11:08 AM, Gregory Propf <gregorypropf@yahoo.com> wrote:

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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners -----Inline Attachment Follows----- _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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