comp.lang.haskell Google Group

Transparent results cache

2009-11-18T18:29:07Z

I wrote something that is possibly a bit horrifying. I seem to have got
away with it for now. I don't normally use things like unsafePerformIO
but it occurred to me that I could write a kind of transparent results
cache for functions. I'd certainly be interested in comments about how
I screwed it up or shouldn't even be thinking such awful things. At

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2009-11-04T12:00:29Z

happs coming alone?

2009-10-30T04:51:35Z

see this? [link] lisp has a nice webserver

monads for lambda calculus

2009-10-20T05:04:28Z

Greetings Comp.Lang.Haskell Community,
I have a program that serves a lambda calculus reducer, but I want to
play with it and see what various class representations give, such as
functors and monads.
So consider
Then, of course fold,
So that we can give an instance of functor,
Next we can try to write an instance of monads. We have an obvious

* extended deadline: FLOPS 2010: Last Call for Papers *

2009-10-20T02:42:40Z

------------------------------ ------------------------------ ----------------
LAST CALL FOR PAPERS
Tenth International Symposium on Functional and Logic Programming
(FLOPS 2010)
April 19-21, 2010
Sendai, Japan
[link]
** EXTENDED SUBMISSION DEADLINES **

Just beginning problem

2009-10-17T21:52:52Z

I am just beginning to learn Haskell. Just for starters I'm watching a
video lecture from Channel 9. The lecturer defines a function like this
"myadd (x,y) = x + y". When I enter this into GHCi I get the error message
":1:12: parse error on input `='". Could I get a simple explanation of
this?

Rational -> Double conversion?

2009-10-08T09:57:43Z

Hello Haskellers,
I'm a novice Haskell user. I'm wondering how to get an approximate
value in Double of a Rational. I'm doing a little numerical
calculation using Rational to avoid truncation error, but want the
final results printed as Doubles that approximate original Rationals.
(I need this to read the results in Fortran.)

* FLOPS 2010: Last Call for Papers *

2009-10-03T02:42:10Z

Logger monad

2009-10-01T15:16:56Z

It has been a long time since I really had to think about how monads are
implemented so I thought I should take a refresher course, in probably
reinventing the wheel by implementing from scratch a sequential logging
monad I need for a project. The point of it is that client code can
append to it, but not do anything else with it if I hide the Log and

Kenlighten - A social network for knowledge seekers and providers

2009-09-29T07:51:48Z

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Laziness of sequence

2009-09-18T16:03:25Z

During an interesting discussion in de.alt.sysadmin.recovery we touched
laziness issues of mapM.
Just 4
Just [1,2,3]
_|_
This can be stripped down to the laziness behavior of sequence and from its
definition the strictness issues of (>>=).
Now the question arises, how to define sequence, to allow indefinite long

whither operational semantics?

2009-09-18T00:07:52Z

hi,
sounds like folks criticize haskell (and probably rightly so) for not
having a good definition, so implementations can ethically vary
widely. so which fp actually does have well nailed-down semantics?
plus a useful-in-the-real-world implementation (or two or three)?
thanks.

overflow behavior

2009-09-13T22:39:32Z

Hi
I would like to know if you could point me to a document specifying the
behavior of haskel with integer overflow. I mean when you have two
integers, what happens when you multiply them? Is overflow checked?
Thanks

* FLOPS 2010: 2nd Call for Papers *

2009-09-03T15:26:05Z

------------------------------ ------------------------------ ----------------
SECOND CALL FOR PAPERS
Tenth International Symposium on Functional and Logic Programming
(FLOPS 2010)
April 19-21, 2010
Sendai, Japan
[link]
Submission deadline: October 16, 2009

HTML tables in haddock

2009-08-31T13:26:33Z

Hi there!
I suppose, the answer to my question is a plain "no", but I'll try it
anyway, because it would be all too nice: Is it possible to use a HTML
table in the output of a haddock run, performed on a *.hs file? I
would like to display an overview in the description part of the HTML
output, formatted with such a table. But the haddock manual seems to

foundational question

2009-08-24T01:46:53Z

hello all
it is easy to convince oneself that the function id
(id x) = x
is such that ((id id) x)==(id x) evaluates as True for any x,
confirming the expectation that (id id) = id. this observation
suggests the following argument:
let f be any function whose principal type, like that of id, is (s ->

FLIBUGmeeting reminder august 19

2009-08-17T09:10:48Z

This is a reminder of the next meeting of FLIBUG:
August 19 2009 at 7.30 PM
Calle del Pi 3 Principal Interior (first floor)
Barcelona
Spain
[link]
Best wishes, Jos

handling large data structure

2009-08-10T10:41:03Z

I am trying to deal in a convenient way with large data structure
loaded from CSV-like files which typically contain +100000 records,
with several files at once. The process-flow looks like:
- read a line (lazily)
- split the line into list of ByteString using a separator (say \t)
- convert this list into the (large) data structure

F={<1, 0>, }

2009-08-09T14:25:53Z

F is a function. What is the lambda expression that is semantically
equivalent to F?

ByteString.Lazy.Char8 question

2009-08-08T14:24:33Z

I would like to test the efficiency of ByteSring.Lazy.Char8. The small
program hereafter does not compile when I switch from the standard
Prelude (no import) to the ByteSring.Lazy.Char8 module.
Any idea why ghc (6.10.4) infers a [ByteString] for contents?
thanks.
ld.
import Prelude hiding (getContents, length, words)

What is <+>?

2009-08-07T22:35:00Z

I've been trying to understand Xmonad's configuration, and this
particular line is confusing me, as I haven't (yet) seen it in any
haskell tutorials or references:
manageHook = manageDocks <+> manageHook defaultConfig
Can anyone point me to some only documentation where the <+> symbol is
explained (or, if it's an xmonad-only thing, to let me know)?

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2009-08-07T09:09:09Z

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another newbie problem

2009-08-06T22:58:55Z

this should be a no-brainer for somebody that knows haskell, but i'm
stumped. what's wrong with this instance declaration? thanks if you
can help.
data Cell = Ex | Oh | Blank deriving Eq
type Board = [Cell]
instance Show Board where
show board = "ERROR {Hugs} - Cannot use type synonym in instance

newbie problem

2009-08-05T12:18:38Z

i'm using Hugs on windows XP. i have a file of kosher midi data at
filepath, and there's no file at newfilepath. then i say:
(readFile filepath) >>= (writeFile newfilepath)
and the new file is NOT kosher midi data: the sequencer won't open it
and the player won't play it. does this make sense to someone? thanks

how to enable multi-parameter type classes in Hugs

2009-08-04T19:42:54Z

Under ghci my module with a 2-parameter type class runs fine after
:set -fglasgow-exts
But when I start Hugs and try to load the module, it complains with
ERROR ... - Haskell 98 does not suppoer dependent parameters
I thought I somewhere read that Hugs would support this. But how can I
enable this feature?

ANN: Book Constraint Handling Rules

2009-08-04T09:08:48Z

Book now available:
CONSTRAINT HANDLING RULES
[link]
Thom Frühwirth, Cambridge University Press, ISBN 9780521877763, 2009.
Constraint Handling Rules (CHR) is both a versatile theoretical
formalism based on logic and an efficient practical high-level
programming language based on rules and constraints.

Re: Alternatives to C: ObjectPascal, Eiffel, Ada or Modula-3?

2009-07-21T13:25:47Z

And it's achievable in Haskell, especially with a bunch of the work
that's been done in GHC 6.6, 6.8, 6.10, and especially if you have
multiple cores. (I'm not just talking about things like short cut
fusion, but a bunch of library improvements like in ByteStrings.)
The caveat - and you may consider it a showstopper, but I don't - is

Parallelizing array-based functions

2009-07-18T01:10:36Z

If you have a function that mutates the elements of an array in-place and
leverages parallelism by recursively subdividing the problem and mutating
parts of the array separately, can this be expressed in Haskell?
In-place quicksort and many of the numerical methods from linear algebra
fall into this category.

haskell, unixodbc and freetds

2009-07-14T18:20:18Z

Hallo, alls -
I am trying to access a SQL Server 2003 database using Haskell on
an
Ubuntu 9.04 machine. I have installed and configured unixodbc and
freetds. I have tested the connection using isql, and everything
works dandy from there.
I have tried connecting to my database using both HDBC and Takusen,

Re: LISP vs HASKELL vs PROLOG

2009-07-13T20:58:07Z

Why in the world does it have to dirty the whole array? Is there any sane
reason for that or is this just an unfortunate deficiency of GHC? I just can't
believe that an array update is O(n). But but my tests updating a STArray
really is O(n) in GHC. I'm a bit shocked.
--larry

ICFP09 Final Call for Participation

2009-07-13T02:46:38Z

============================== ============================== =========
Final Call for Participation
The 14th ACM SIGPLAN International Conference
on Functional Programming (ICFP 2009)
[link]
Edinburgh, Scotland, 31 August - 2 September 2009

Call for Participation: Commercial Users of Functional Programming Workshop 2009

2009-06-30T23:30:52Z

Commercial Users of Functional Programming Workshop (CUFP) 2009
Functional Programming As a Means, Not an End
Call for Participation
Sponsored by SIGPLAN
Co-located with ICFP 2009
______________________________ ___________________________

ICFP09 Call for Participation

2009-06-22T15:29:49Z

============================== ============================== =========
Call for Participation
The 14th ACM SIGPLAN International Conference
on Functional Programming (ICFP 2009)
[link]
Edinburgh, Scotland, 31 August - 2 September 2009

Haskell logo fail

2009-06-16T21:06:42Z

[link]

Newbie question...

2009-06-13T11:36:19Z

Hello,
I've been studying this fascinating language for a few days so I am
pretty new to all this (especially the error messages from the
interpreter!)
I wrote this simple function, which purpose is to generate a list of
proper divisors of a number:
proper_divisors :: Int -> [Int]
proper_divisors n = [d | d <- [1 .. (floor . sqrt) n], mod n d == 0]

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reads

2009-06-11T14:17:58Z

I would expect
reads "1.2" :: [(Int,String)]
to be evaluated to
[(1,".2")]
but using Hugs (Version Sep 2006) it is just
[]
Bug or feature?

INVITATION FLIBUG

2009-06-09T17:07:21Z

Hi
I am sorry to announce that the first FLIBUG (was FLIB) meeting has
been postponed to June 17.
At this moment the group has around 8 members. Adapated invitation
below.
Jos Koot
INVITATION
FLIBUG (Fringe/Functional Languages In Barcelona User Group) has
postponed its first meeting to June 17. Everyone who is interested is

Re: Fedora update error

2009-06-03T13:41:29Z

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Verilog parser..

2009-05-29T05:44:42Z

Has anyone written a verilog (structural netlist) parser in Haskell?
Thanks.

Overwrite implementation of show function

2009-05-28T17:35:47Z

import Data.Set as Set ( Set, fromList, toList )
test = fromList [fromList [1,2],fromList [3]]
Then 'show test' will result in the following string:
"fromList [fromList [1,2],fromList [3]]"
What I want is something like "[[1,2],[3]]"
Is there a way to overwrite the implementation of the show function?

Avoid unnecessary evaluations

2009-05-28T13:36:12Z

Assume that the evaluation of a function 'expensive' takes a lot of
time. Hence I want to avoid unnecessary evaluations of 'expensive'.
expensive :: Int -> (Int,Int)
expensive x = (x,x)
f :: Int -> Int
f x = fst (expensive x) + snd (expensive x)
Will 'expensive x' be evaluated twice to compute 'f x'?

DEFUN09: Final Call for Talks & Tutorials (co-located w/ ICFP09)

2009-05-28T04:48:08Z

Call for Talks and Tutorials
ACM SIGPLAN 2009 Developer Tracks on Functional Programming
[link]
Edinburgh, Scotland, September 3 and 5, 2009
The workshop will be held in conjunction with ICFP 2009
[link]

haddock and latex

2009-05-27T14:38:11Z

Is it possible to convert the default HTML output of Haddock into TeX/
LaTeX?
I mean, I frequently produce my program documentation now with a
haddock -h Example.hs
command, that generates Example.html.
But I would be able to integrate the generated text into a broader
Latex project, so it would be nice to have an Example.tex file as well

INVITATION: user group FLIB: Fringe/ Functional Languages In Barcelona.

2009-05-26T09:13:02Z

INVITATION
We, (Jose A. Ortega Ruiz j...@gnu.org, Andy Wingo wi...@pobox.com and
Jos Koot jos.k...@telefonica.net) have scheduled a meeting in
Barcelona (Spain) with the intention to form a user group FLIB:
Fringe/
Functional Languages In Barcelona. Everyone who is interested in
shaping this group is invited to share us.

ICFP09 Accepted Papers

2009-05-15T15:30:41Z

Accepted Papers
ICFP 2009: International Conference on Functional Programming
Edinburgh, Scotland, 31 August - 2 September 2009
[link]
The ICFP 2009 Program Chair and Committee are pleased to announce that
the following papers have been accepted for the conference.

FLOPS 2010: Preliminary Call for Papers

2009-05-13T21:08:11Z

PRELIMINARY CALL FOR PAPERS

Tenth International Symposium on Functional and Logic Programming
(FLOPS 2010)
April 19-21, 2010
Sendai, Japan
[link]