Hello!
I am working with TypeReps, and while writing some functions I have
noticed that I could use lenses to simplify them; however, I have stumbled
upon some difficulties.
First I’ll try to clarify which functions I want to write:
* a function for converting TypeRep of, say, `Maybe x` to
On Thu, 12 Sep 2013 18:24:24 +0400, Tom Ellis
tom-lists-haskell-cafe-2...@jaguarpaw.co.uk wrote:
On Thu, Sep 12, 2013 at 09:21:20AM -0400, Scott Lawrence wrote:
Something's always bothered me about map and zipWith for ByteString.
Why is it
map :: (Word8 - Word8) - ByteString -
On Wed, 11 Sep 2013 00:20:26 +0400, Thiago Negri evoh...@gmail.com wrote:
I hope these jokes do not cause people to be afraid to post new ideas.
Agreed. I would also like to clarify that my message was much more a joke
on
the incomprehensibility of legal acts than on the original proposal.
This might do for businesses, but clearly not adequate if we want
Haskell/Cucumber (ever) to be suitable for use in government.
Here I’d like to suggest a more rigorous approach, which hopefully will be
considered for implementation instead of the original proposal.
Here’s a small example, which, when compiled, gives an error. Why?
{-# LANGUAGE FlexibleInstances, ImpredicativeTypes,
TemplateHaskell #-}
import Control.Lens
class Item a where
name :: a - String
instance Item (String, Int) where
name = fst
Oops.
Ben Gamari bgamari.f...@gmail.com писал(а) в своём письме Tue, 04 Jun
2013 04:41:53 +0300:
To be perfectly clear, ByteString and Text target much different
use-cases and are hardly interchangeable. While ByteString is, as the
name suggests, a string of bytes, Text is a string of
silvio silvio.fris...@gmail.com писал(а) в своём письме Mon, 03 Jun 2013
22:16:08 +0300:
Hi everyone,
Every time I want to use an array in Haskell, I find myself having to
look up in the doc how they are used, which exactly are the modules I
have to import ... and I am a bit tired of
I'll just leave it here: http://hackage.haskell.org/trac/ghc/wiki/Records
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Andrew Cowie and...@operationaldynamics.com писал(а) в своём письме Fri,
15 Feb 2013 17:05:13 +0300:
So my question is: what's an appropriate Haskell mechanism for building
code that is OS / arch / distro specific? It's not like I have autoconf
running generating me a config.h I could
Ouch, forgot the Cafe.
Would you object to this particular optimisation (replacing an algorithm
with an entirely different one) if you were guaranteed that the space
behaviour would not change?
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Junior White efi...@gmail.com писал(а) в своём письме Tue, 29 Jan 2013
12:25:49 +0300:
The only different in the two program is in the first is q - [1..n], qs
- queens' (k-1), and the second is qs - queens' (k-1), q - [1..n].
In the first case `queens' (k-1)` is being recomputed for every q
Junior White efi...@gmail.com писал(а) в своём письме Tue, 29 Jan 2013
12:40:08 +0300:
Hi Artyom,
Thanks! But I don't understand why in the first case queens' (k-1)
is
being recomputed n times?
Because your list comprehension is just a syntactic sugar for
concatMap (\q -
Junior White efi...@gmail.com писал(а) в своём письме Tue, 29 Jan 2013
12:59:31 +0300:
So this is a problem in lazy evaluation language, it will not appear in
python or erlang, am i right?
Not quite. Compilers of imperative languages don’t perform CSE (common
subexpression elimination)
Shachaf Ben-Kiki shac...@gmail.com писал(а) в своём письме Tue, 29 Jan
2013 09:09:37 +0300:
That code is from base 4.5. Here's base 4.6:
quotRem x@(I32# x#) y@(I32# y#)
| y == 0 = divZeroError
-- Note [Order of tests]
| y == (-1) x ==
Hi!
I’ve always thought that `quotRem` is faster than `quot` + `rem`, since
both `quot` and `rem` are just wrappers that compute both the quotient
and the remainder and then just throw one out. However, today I looked
into the implementation of `quotRem` for `Int32` and found out that it’s
Right, it was fixed in GHC 7.4.2.
hackage.haskell.org/trac/ghc/ticket/5688
The trouble with old behaviour was that it had been creating a security
breach (anybody could pass 1e1000 where an Integer was expected
and cause a segmentation fault).
15.01.2013 2:41 пользователь Arnaud Bailly
You could create a new thread which would be 'forever' executing your
extensive computation and updating some IOVar accordingly. The drawing
function would do nothing except reading that IOVar and displaying its
contents. Since drawing is cheap, this can be done at any reasonable rate.
28.12.2012
Compiled just fine on my machine.
Ubuntu 12.10, Haskell Platform 2012.2.0.0, GHC 7.4.2, cabal-install
1.16.0.1.
Christopher Howard christopher.how...@frigidcode.com писал(а) в своём
письме Sat, 15 Dec 2012 06:52:22 +0300:
Hey guys, to teach myself Haskell I wrote a little arcade game
Hello!
I saw a question on StackOverflow about the difference between isAlpha and
isLetter today. One of the answers stated that the two functions are
interchangeable, even though they are implemented differently.
I decided to find out whether the difference in implementation influences
Jonas Almström Duregård jonas.dureg...@chalmers.se писал(а) в своём
письме Mon, 19 Nov 2012 01:31:01 +0300:
Hi,
You can make a datatype that captures exactly the expressions you want
(see
code below). Essentially you want to make sure that a subtraction or
addition never has another
Instead of attacking the problem textually, try to create a datatype which
would describe your expressions, then generate all values of this
datatype, filter those you don’t need, and convert the rest into Strings.
Currently your expressions are represented by “String” — conversion is
very
Sorry! I replied without reading your message properly.
I could then work directly with parsing trees, and generate all binary
trees of fixed lengths.
But most of them would be unnecessary, so it seems like I'm attacking
it from the wrong angle.
They won’t be unnecessary if you generate them
The following algorithm generates all possible expressions and throws away
most
of unnecessary duplicates.
import qualified Data.Map as M
data Expr = Num Int
| Add Expr Expr
| Sub Expr Expr
| Mul Expr Expr
| Div Expr Expr
Rendering function is
Indentation messed up… I have pasted the code here: http://hpaste.org/77864
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Hello,
I have accidentally written my version of polyvariadic composition
combinator, `mcomp`. It differs from Oleg’s version (
http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three
aspects: a) it is simpler, b) it works without enumerating basic cases
(all existing
Евгений Пермяков permea...@gmail.com писал в своём письме Mon, 30 Jul
2012 09:47:48 +0300:
Can someone tell me if there are any primitives, that used to detect
machine type overflows, in ghc haskell ? I perfectly understand, that I
can build something based on preconditioning of variables,
Hi Café, I would greatly appreciate your help. Entering the following in
GHCi
produces an error message…
-- import Data.Time
-- getCurrentTime = print
interactive:1:20:
No instance for (Show UTCTime)
arising from a use of `print'
Possible fix: add an
Oh, I’m sorry. I have Haskell Platform 2011.2.0.1, not 2012.2.0.1. So I
guess it was a bug in GHCi…
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I’m having the same problem on my Windows 7 laptop. The solution I’ve
found is to use Internet Explorer — it isn’t perfect, but for some reason
it is the only browser capable of handling these links.
On Fri, May 25, 2012 at 01:19:11AM +0200, Nicu Ionita wrote:
Hi cafe,
I have a problem
http://www.haskell.org/haskellwiki/Monomorphism_restriction
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Challenge accepted! I have written a solution in Haskell; please merge :)
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Andreas Pauley apau...@gmail.com писал(а) в своём письме Sun, 20 May
2012 20:33:13 +0300:
I want to see how elegant a solution I can get in a functional
language, given that the problem description is not really elegant at
all. It has a few annoying exceptions to the normal rules, typical of
Yves Parès limestr...@gmail.com писал(а) в своём письме Thu, 12 Jan 2012
13:14:16 +0200:
uvector is deprecated, its functionnalities has been ported into vector.
Yes, but a Ratio a instance hasn't been ported.
I admit that I have overlooked the “deprecated” warning, and agree that it
is
You can use Data.Vector.Unboxed. There isn't an instance for Ratio a, but
it is easy to write one, since it would be very similar to Complex a.
http://hackage.haskell.org/packages/archive/vector/0.9.1/doc/html/Data-Vector-Unboxed.html#t:Unbox
___
Also, uvector already supports unboxed Ratios:
http://hackage.haskell.org/package/uvector
In fact, I am surprised that Data.Vector doesn't have a Ratio
instance, but has a Complex instance. Any ideas, why?
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Gregg Reynolds d...@mobileink.com писал(а) в своём письме Fri, 30 Dec
2011 17:23:20 +0200:
Regarding side-effects, they can be (informally) defined pretty simply:
any non-computational effect caused by a computation is a side-effect.
I wonder: can writing to memory be called a
Donn Cave d...@avvanta.com писал(а) в своём письме Fri, 30 Dec 2011
20:36:46 +0200:
That's why we use terms in a sense that apply meaningfully to computer
programming languages in general and Haskell in particular. To do
otherwise - for example to insist on a definition of pure that could
Chris Smith cdsm...@gmail.com писал(а) в своём письме Fri, 30 Dec 2011
22:04:21 +0200:
Computability is just a distraction here. The problem isn't whether
getAnIntFromUser is computable... it is whether it's a function at
all! Even uncomputable functions are first and foremost functions,
Chris Smith cdsm...@gmail.com писал(а) в своём письме Fri, 30 Dec 2011
22:28:36 +0200:
I really think that the notion of “purity” appeared to convince C
programmers. It would be silly to try to explain that “Int - IO Int”
isn't really a function from Int to Int, monads, blah blah blah. So,
I wonder, if there is any example of actual Haskell program cracked /
reverse engineered? GHC-generated code is already quite hard to understand…
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Hello!
The following program executes 1.5 seconds on my computer:
---CODE BEGIN-
module Main where
import Data.Array.IArray
main = print (answers ! 100)
nextAns :: (Int, Int, Float) - (Int, Int, Float)
nextAns (a, n, r) = if
2011/11/10 Daniel Fischer daniel.is.fisc...@googlemail.com:
There's a problem here, unsafeAt uses an Int index into the array, while
(!) uses the declared index type. Even skipping the bounds check, you'd
still have to calculate the Int index for the replacement of (!).
#ifdef
Hello!
The (!) operator is short and nice. Unfortunately, when doing heavy
computing, we have to use unsafeAt instead. It looks ugly and it is
ugly, also.
Some compilers for imperative languages like Free Pascal have an option to
turn on/off bounds checking for arrays. Wouldn't it be
GMP has a lot of functions, such as extracting roots, primality test,
Legendre symbol, factorial and so on. These can be written in Haskell, of
course, but isn't it better to use existing functions? They are also much
faster than similar functions from NumericPrelude, I believe.
I have
I'm writing a package (arithmoi) that will include reasonably fast
implementations of those, but I never find the time to finish it :(
Package is great, but sometimes it is useful to have such functions out of
box (for example, when solving SPOJ problems).
No, you can't, unfortunately (not
It may be related to the problem which I encountered too:
http://www.mail-archive.com/haskell-cafe@haskell.org/msg89460.html
Try using GLUT from http://github.com/haskell-opengl/GLUT
Hope that helps.
--Artyom
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Suppose iRecurse looks like this:
iRecurse = do
x - launchMissiles
r - iRecurse
return 1
As x is never needed, launchMissiles will never execute. It obviously is
not what is needed.
But in Haskell, standart file input|output is often lazy. It's a
combination of buffering and
Scott Lawrence byt...@gmail.com писал(а) в своём письме Tue, 31 May 2011
23:29:49 +0300:
On 05/31/2011 04:20 PM, Artyom Kazak wrote:
Suppose iRecurse looks like this:
iRecurse = do
x - launchMissiles
r - iRecurse
return 1
As x is never needed, launchMissiles will never execute
And I can declare an instance for (x, y) which does NOT implies (Show x):
instance Show (x, y) where
show _ = I'm tuple! Hooray!
Andrew Coppin andrewcop...@btinternet.com писал(а) в своём письме Fri,
20 May 2011 00:08:27 +0300:
Cannot deduce (Show x) from context (Show (x, y)).
Yes, of course there is! You can find the index of all functions, types,
typeclasses etc. here:
http://lambda.haskell.org/hp-tmp/docs/2011.2.0.0/doc-index.html
KC kc1...@gmail.com писал(а) в своём письме Tue, 17 May 2011 20:33:05
+0300:
Is there a way to tell when a function is included
I'm sorry, but I don't fully understand what do you want from putback.
If putback'ed action does io and then returns x, it's trivial:
putback x io = io return x
If you want putback'ed action to return x on its first call and do io on
second, third... calls, then you need to put
Barbara Shirtcliff ba...@gmx.com писал(а) в своём письме Wed, 04 May
2011 16:41:07 +0300:
Also, note that lexOrder s@[_] = [s] is nicer than lexOrder s |
length s == 1 = [s].
I agree that that initial version was a little clumsy, but your
suggestion doesn't really seem to work:
Hi Cafe!
When I'm running examles from Gloss graphics library, all examples
except gloss-hello are crashing with the following error: GLUT:
Warning in gloss-easy: GL error: gloss-easy: stderr: hPutChar:
invalid argument (character is not in the code page). A window with
produced picture is showed
It's good to hear. Thanks!
2011/5/2 Jason Dagit dag...@gmail.com:
This is fixed in the git repository for GLUT, but I didn't push the fix to
hackage yet:
https://github.com/haskell-opengl/GLUT/pull/1
Sorry for the inconvenience!
Jason
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Oh. I have taken a wrong approach to the problem.
I have written Newton method with cutting precision if it's more than
N digits, and it finds an answer practically in no time. But still,
it's very good, thank you!
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Hi Café!
roots (http://hackage.haskell.org/package/roots) is a package to solve
equations like f(x)==0.
In RootFinder class there is an 'defaultNSteps' value, which is used as
maximal count of iterations functions like findRoot and traceRoot can
make. By default it is 250, but sometimes
Aaron Gray aaronngray.li...@gmail.com писал(а) в своём письме Sat, 12
Feb 2011 22:18:33 +0200:
I was wondering if anyone had an idea or estimate as to how large the
Haskell community is ?
Aaron
I've been subscribed to this list since the very start of the 2010 year,
and I counted 1168 ±
Some time ago I have read A State Monad Tutorial
(http://strabismicgobbledygook.wordpress.com/2010/03/06/a-state-monad-tutorial/).
While reading, I was fixing some minor mistakes (okay, a lot of
mistakes). After all, I had an idea to create PDF with fixed version.
So, here it is:
Hello!
I tried to implement the parallel Monte-Carlo method of computing Pi
number, using two cores:
--PROGRAM
module Main where
import Random
import Data.Ratio
import Data.List
import System.IO
import GHC.Conc
main = do
putStrLn pi 1
putStr n:
hFlush stdout
t - getLine
piMonte (read
Hi! I'm learning Haskell, and now I'm trying to make framework for
solving searching problems, such as Knight's Tour. For small boards it
answers instantly. For 7x8 board - 23 seconds. For 8x8 board - more
than 30 minutes (it hasn't finished yet). Where is the root of the
evil?
--program
module
2010/3/1 Daniel Fischer daniel.is.fisc...@web.de:
In the algorithm. You investigate far too many dead ends. Since for larger
boards, the number of dead ends increases fast, larger boards take much
much longer.
With one little change, I get
...
For a reason I don't understand, if the second
2010/3/1 Daniel Fischer daniel.is.fisc...@web.de:
Am Montag 01 März 2010 19:29:45 schrieb Artyom Kazak:
2010/3/1 Daniel Fischer daniel.is.fisc...@web.de:
In the algorithm. You investigate far too many dead ends. Since for
larger boards, the number of dead ends increases fast, larger boards
2010/2/24 Brandon S. Allbery KF8NH allb...@ece.cmu.edu:
On Feb 22, 2010, at 03:36 , Roman Cheplyaka wrote:
* Anthony Cowley acow...@seas.upenn.edu [2010-02-21 14:15:00-0500]
#! /usr/bin/env bash
./prog --RTS $*
./prog --RTS $@
Otherwise it will work wrong if arguments contain quoted
Hello everybody!
I want to write a little program, that will receive a string as command-line
argument and write it in the file. But if this string contains '+RTS', GHC
runtime won't pass the rest of the string to my program.
What can I do to avoid this?
Enclose it in double quotes (perhaps single quotes would also work)
No, I want my program to work the same way as UNIX echo does.
Without any double quotes.
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2010/2/21 Daniel Fischer daniel.is.fisc...@web.de:
Am Sonntag 21 Februar 2010 18:20:43 schrieb Artyom Kazak:
Enclose it in double quotes (perhaps single quotes would also work)
No, I want my program to work the same way as UNIX echo does.
Without any double quotes.
Okay, what about
Why fact2 is quicker than fact?!
fact2 :: Integer - Integer
fact2 x = f x y
where
f n e | n 2 = 1
| e == 0 = n * (n - 1)
| e 0 = (f n (e `div` 2)) * (f (n - (e * 2)) (e `div` 2))
y = 2 ^ (truncate (log (fromInteger x) / log 2))
fact :: Integer - Integer
fact 1 = 1
fact n = n * fact (n - 1)
I
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