Send Beginners mailing list submissions to
[email protected]
To subscribe or unsubscribe via the World Wide Web, visit
http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
[email protected]
You can reach the person managing the list at
[email protected]
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. map question (Joost Kremers)
2. Re: map question (Daniel Fischer)
3. Re: map question (Brent Yorgey)
4. Re: map question (Joost Kremers)
5. How to import the Data.Char library in Hugs? (Benjamin L.Russell)
6. Re: How to import the Data.Char library in Hugs? (Adrian Neumann)
7. Re: How to import the Data.Char library in Hugs? (Daniel Fischer)
8. Re: map question (Tom Doris)
9. Re: map question (Magnus Therning)
10. Re: map question (Tommy M. McGuire)
----------------------------------------------------------------------
Message: 1
Date: Thu, 17 Sep 2009 13:31:38 +0200
From: Joost Kremers <[email protected]>
Subject: [Haskell-beginners] map question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
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
------------------------------
Message: 2
Date: Thu, 17 Sep 2009 13:50:27 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
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
------------------------------
Message: 3
Date: Thu, 17 Sep 2009 08:01:50 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Thu, Sep 17, 2009 at 01:31:38PM +0200, Joost Kremers wrote:
>
> 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]
By the way, the reason
map (+1) [1,2,3,4]
works but
map (-1) [1,2,3,4]
doesn't is because of an ugly corner of Haskell syntax: -1 here is
parsed as negative one, rather than an operator section with
subtraction. The 'subtract' function is provided exactly for this
purpose, so that you can write
map (subtract 1) [1,2,3,4]
instead.
-Brent
------------------------------
Message: 4
Date: Thu, 17 Sep 2009 14:01:47 +0200
From: Joost Kremers <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On Thu, Sep 17, 2009 at 01:50:27PM +0200, Daniel Fischer wrote:
> > 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.
Ah yes... Of course, I didn't realise that wrapping a binary operator in parens
turns it into a normal function.
And from <http://www.haskell.org/onlinereport/exps.html#sect3.5>, which someone
pointed me to off-list, I gather that in (- 1) the minus sign is interpreted as
the unary operator, not the binary one. Which means that (- 1) is not a
function, which means map will barf.
Ok, thanks, all makes sense now!
Joost
--
Joost Kremers
Life has its moments
------------------------------
Message: 5
Date: Thu, 17 Sep 2009 21:41:47 +0900
From: Benjamin L.Russell <[email protected]>
Subject: [Haskell-beginners] How to import the Data.Char library in
Hugs?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
My apologies if this is an extremely elementary question, but I am
having difficulties in importing the Data.Char library in Hugs.
In GHCi, the command "import Data.Char" works correctly, as follows:
--8<---------------cut here---------------start------------->8---
GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, for Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Prelude> import Data.Char
Prelude Data.Char>
--8<---------------cut here---------------end--------------->8---
However, in Hugs, the same command fails with an error, as follows:
--8<---------------cut here---------------start------------->8---
__ __ __ __ ____ ___ _________________________________________
|| || || || || || ||__ Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005
||---|| ___|| World Wide Web: http://haskell.org/hugs
|| || Bugs: http://hackage.haskell.org/trac/hugs
|| || Version: 20051031 _________________________________________
Haskell 98 mode: Restart with command line option -98 to enable
extensions
Type :? for help
Hugs> import Data.Char
ERROR - Syntax error in expression (unexpected keyword "import")
Hugs>
--8<---------------cut here---------------end--------------->8---
Does anybody know how to import the Data.Char library in Hugs?
-- Benjamin L. Russell
--
Benjamin L. Russell / DekuDekuplex at Yahoo dot com
http://dekudekuplex.wordpress.com/
Translator/Interpreter / Mobile: +011 81 80-3603-6725
"Furuike ya, kawazu tobikomu mizu no oto."
-- Matsuo Basho^
------------------------------
Message: 6
Date: Thu, 17 Sep 2009 15:02:21 +0200
From: Adrian Neumann <[email protected]>
Subject: Re: [Haskell-beginners] How to import the Data.Char library
in Hugs?
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
You do
> :l Data.Char
As far as I know you can't have multiple loaded modules unless you put
them in a file and load that.
Regards,
Adrian
Benjamin L.Russell schrieb:
> My apologies if this is an extremely elementary question, but I am
> having difficulties in importing the Data.Char library in Hugs.
>
> In GHCi, the command "import Data.Char" works correctly, as follows:
>
> --8<---------------cut here---------------start------------->8---
> GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> ___ ___ _
> / _ \ /\ /\/ __(_)
> / /_\// /_/ / / | | GHC Interactive, for Haskell 98.
> / /_\\/ __ / /___| | http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_| Type :? for help.
>
> Prelude> import Data.Char
> Prelude Data.Char>
> --8<---------------cut here---------------end--------------->8---
>
> However, in Hugs, the same command fails with an error, as follows:
>
> --8<---------------cut here---------------start------------->8---
> __ __ __ __ ____ ___ _________________________________________
> || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard
> ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005
> ||---|| ___|| World Wide Web: http://haskell.org/hugs
> || || Bugs: http://hackage.haskell.org/trac/hugs
> || || Version: 20051031 _________________________________________
>
> Haskell 98 mode: Restart with command line option -98 to enable
> extensions
>
> Type :? for help
> Hugs> import Data.Char
> ERROR - Syntax error in expression (unexpected keyword "import")
> Hugs>
> --8<---------------cut here---------------end--------------->8---
>
> Does anybody know how to import the Data.Char library in Hugs?
>
> -- Benjamin L. Russell
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 251 bytes
Desc: OpenPGP digital signature
Url :
http://www.haskell.org/pipermail/beginners/attachments/20090917/4aa56846/signature-0001.bin
------------------------------
Message: 7
Date: Thu, 17 Sep 2009 15:04:07 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] How to import the Data.Char library
in Hugs?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Am Donnerstag 17 September 2009 14:41:47 schrieb Benjamin L.Russell:
> My apologies if this is an extremely elementary question, but I am
> having difficulties in importing the Data.Char library in Hugs.
Hugs> :a Data.Char
Data.Char>
------------------------------
Message: 8
Date: Thu, 17 Sep 2009 14:06:23 +0100
From: Tom Doris <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: Joost Kremers <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
This works:
map (+ (-1)) [1,2,3,4]
2009/9/17 Joost Kremers <[email protected]>
> 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
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
http://www.haskell.org/pipermail/beginners/attachments/20090917/3d8634f3/attachment-0001.html
------------------------------
Message: 9
Date: Thu, 17 Sep 2009 14:54:19 +0100
From: Magnus Therning <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: Tom Doris <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Thu, Sep 17, 2009 at 2:06 PM, Tom Doris <[email protected]> wrote:
> This works:
>
> map (+ (-1)) [1,2,3,4]
map pred [1..4]
/M
--
Magnus Therning (OpenPGP: 0xAB4DFBA4)
magnusï¼ therningï¼org Jabber: magnusï¼ therningï¼org
http://therning.org/magnus identi.ca|twitter: magthe
------------------------------
Message: 10
Date: Thu, 17 Sep 2009 10:05:43 -0500
From: "Tommy M. McGuire" <[email protected]>
Subject: Re: [Haskell-beginners] map question
To: Magnus Therning <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
Cue The Evolution of a Haskell Programmer:
http://www.willamette.edu/~fruehr/haskell/evolution.html
:-)
Magnus Therning wrote:
> On Thu, Sep 17, 2009 at 2:06 PM, Tom Doris <[email protected]> wrote:
>> This works:
>>
>> map (+ (-1)) [1,2,3,4]
>
> map pred [1..4]
>
> /M
>
--
Tommy M. McGuire
[email protected]
------------------------------
_______________________________________________
Beginners mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/beginners
End of Beginners Digest, Vol 15, Issue 12
*****************************************
