because f is required to accept *any* Show instance as an
argument type, and so is happy to work for whatever particular type b
happens to inhabit.
Stuart Cook
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On 9/8/07, Ryan Ingram [EMAIL PROTECTED] wrote:
This does what you want, I think:
{-# LANGUAGE ExistentialQuantification #-}
module Exist where
data Showable = forall a. (Show a) = Showable a
instance Show Showable where
showsPrec p (Showable a) = showsPrec p a
show (Showable a) =
suspect
that even in a strict language, lambda-abstractions would cause
similar trouble.
[1] http://programming.reddit.com/info/2n6eh/comments
Stuart Cook
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On 9/9/07, Henning Thielemann [EMAIL PROTECTED] wrote:
If the signature would be
(Monad m) = ((a - b) - c) - m (a - b) - m c
it would be possible, and the implementation would be 'liftM'/'fmap'.
Thanks, that's the kind of insight I was looking for.
Hmm. A key distinction between (a - m
On 9/10/07, PR Stanley [EMAIL PROTECTED] wrote:
Can anyone come up with a better alternative?
*puts on his pointfree hat*
import Control.Arrow (())
import Data.List (group, sort)
countCS :: [Char] - [(Char, Int)] -- Char can be generalised to any Ord
countCS = map (head length) .
On 9/10/07, PR Stanley [EMAIL PROTECTED] wrote:
Hi
Any comments and/or criticisms would be most appreciated:
--count the occurrences of char in string
countC :: Char - [Char] - Int
countC x xs = sum [1 | c - xs, c == x]
That's a clever implementation, but I think there are clearer ways of
On 9/14/07, Jules Bean [EMAIL PROTECTED] wrote:
{-# OPTIONS -fglasgow-exts #-}
(extensions are only for deriving (Monad), it's not important)
If that's the case, you should be able to write (assuming GHC 6.6+)
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
though I don't know how well other
On 9/16/07, Ryan Ingram [EMAIL PROTECTED] wrote:
Is there a way to make GHCi not print the result
of an action but still make my variables get bound?
This seems to be a common question (I myself asked it recently), so
I've added an entry to the GHCi page on the wiki.
On 9/17/07, Martin Lütke [EMAIL PROTECTED] wrote:
What is the url for the wiki entry?
There was already a page at
http://haskell.org/haskellwiki/GHC/GHCi
so I put it there, but I also took the liberty of creating some #REDIRECTs, so
http://haskell.org/haskellwiki/ghci
should work just
On 9/23/07, Thomas Hartman [EMAIL PROTECTED] wrote:
-- this is the usual implementation right?
myreverse xs = foldl f [] xs
where f accum el = el : accum
This is often written
reverse = foldl (flip (:)) []
which I quite like, because you can contrast it with
foldr (:) []
which of
On 10/3/07, PR Stanley [EMAIL PROTECTED] wrote:
Without looking at the standard prelude, define the
higher-order library function curry that converts a function
on pairs into a curried
function, and conversely, the function uncurry
that converts a
On 10/4/07, Dominic Steinitz [EMAIL PROTECTED] wrote:
Look at the type of (.).(.).(.)
Indeed, this generalizes to functions of any arity on the RHS:
Prelude :t (.)
(.) :: (b - c) - (a - b) - a - c
Prelude :t (.).(.)
(.).(.) :: (b - c) - (a - a1 - b) - a - a1 - c
Prelude :t
On 10/6/07, Bertram Felgenhauer [EMAIL PROTECTED] wrote:
This is a language extension, you need -fbang-patterns
to allow it, or with a recent ghc (6.7, 6.9 or a 6.8 rc)
a {-# LANGUAGE BangPatterns #-} pragma, or -XBangPatterns.
LANGUAGE pragmas (including BangPatterns) work just fine in 6.6,
On 10/16/07, Bayley, Alistair [EMAIL PROTECTED] wrote:
Just a minor point, but would mind explaining exactly what lexicographic
ordering implies? It appears to me that e.g. version 9.3 of a package
would be preferred over version 10.0. That strikes me as
counter-intuitive.
I believe the
On 10/17/07, John Meacham [EMAIL PROTECTED] wrote:
if anyone is interested, Although I bet this has been implemented a
hundred times over, I have attached my lazy naturals module below just
for larks. It is quite efficient as such things go and very lazy. for
instance (genericLength xs 5)
On 10/17/07, Peter Verswyvelen [EMAIL PROTECTED] wrote:
So in that case, the result should be a list of ordered pairs like:
[(egg, 2), (cheese, 1)]. Or a pair of two lists, like ([egg,
cheese), (2,1)]. Otherwise you would not know which frequency belongs
to which element?
However, I suspect
On 10/18/07, Simon Peyton-Jones [EMAIL PROTECTED] wrote:
I don't believe GHC relies on any class laws. It'd be pretty dangerous to do
so, I think.
Incidentally, I consider it a slight infelicity that the H98 spec
doesn't seem to mention the implied laws of classes like Eq and Ord,
not even to
On 11/2/07, Petr Hoffmann [EMAIL PROTECTED] wrote:
import System.Cmd
main = do
System.Cmd.system echo hello output.txt -- use the external
application to create an output file
o1 - readFile output.txt
System.Cmd.system echo bye output.txt -- the second call to
On 11/2/07, Andrew Butterfield [EMAIL PROTECTED] wrote:
I'm puzzled - when I run this on GHCi (v6.4, Windows XP) I get the
following outcome
*Mainmain
The process cannot access the file because it is being used by another
process.
hello
*Main
Under GHCi 6.6 I get this:
*Main main
bye
On 11/8/07, Tim Newsham [EMAIL PROTECTED] wrote:
Data.Maybe has functions for processing Maybe's but nothing useful
for creating maybe. I think the following would be a very useful
addition, a guarded function:
guarded :: (a - Bool) - (a - b) - a - Maybe b
guarded p f x | p x
On 11/8/07, [EMAIL PROTECTED]
[EMAIL PROTECTED] wrote:
Would somebody try to solve it, before I unveil the solution? It isn't
difficult.
*** SPOILER WARNING ***
Here's my attempt, which I wrote without peeking:
let fibs' = 1 : 2 : zipWith (+) fibs' (tail fibs')
rabbits = 1 : 0 :
On 11/8/07, [EMAIL PROTECTED]
[EMAIL PROTECTED] wrote:
I have a vague impression that the solution of Alfonso Acosta:
rs_aa = let acum a1 a2 = a2 ++ acum (a1++a2) a1 in 1 : acum [1] [0]
is also somehow related to this limit stuff, although it is simpler,
and formulated differently.
I
On Dec 17, 2007 10:47 PM, Nicholls, Mark [EMAIL PROTECTED] wrote:
The constructor of a newtype must have exactly one field but `R' has
two In the newtype declaration for `Rectangle'
It doesn't like
newtype Rectangle = R Int Int
A newtype can only have one constructor, with one argument,
On Dec 19, 2007 11:28 AM, Andre Nathan [EMAIL PROTECTED] wrote:
I guess I could do away with StateT and just pass the PsMap around as a
parameter, but I guess that wouldn't be the haskell way...
I wouldn't say that. Manual state-passing is a perfectly legitimate
technique, and can be clearer in
On Dec 30, 2007 2:43 PM, Jake McArthur [EMAIL PROTECTED] wrote:
Really? This code doesn't even really make any sense to me. In order
to be an instance of Bar, t has to already be an instance of Foo,
implying that the function hi is already defined for t. What would the
function in this example
On Jan 11, 2008 2:01 AM, Michael Roth [EMAIL PROTECTED] wrote:
Hello list,
Exists there a way to write this cleaner without writing countless
set_xyz helper functions?
The set_xyz methods have to be written, but that doesn't mean *you*
have to write them. You can use Template Haskell to
On Jan 18, 2008 1:46 AM, Isaac Dupree [EMAIL PROTECTED] wrote:
Neil Mitchell wrote:
Hi
passall, passany :: [a - Bool] - a - Bool
passall ps v = and $ map ($v) ps
passany ps v = or $ map ($v) ps
or something similar defined anywhere? Such that one can write
nearly; using Prelude:
://code.haskell.org/~scook0/haddock/bimap/
Darcs:
darcs get http://code.haskell.org/bimap
Stuart Cook
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On Mon, Feb 4, 2008 at 7:56 PM, Neil Mitchell [EMAIL PROTECTED] wrote:
A few design differences from your one:
* I called my module BiMap rather than Bimap - I debated this with
a collegue, and we settled on the capital M, but it was a very close
call.
Mine was also originally BiMap, but
I've updated the bimap package to version 0.2.
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bimap-0.2
The main difference is a pretty comprehensive interface shakeup: the
Either variants have been dropped, all L variants have had the L
removed from their name, and a few functions
On Tue, Feb 5, 2008 at 9:11 PM, Neil Mitchell [EMAIL PROTECTED] wrote:
Hi
The main difference is a pretty comprehensive interface shakeup: the
Either variants have been dropped, all L variants have had the L
removed from their name, and a few functions have been curried. The
end
On Tue, Feb 5, 2008 at 11:33 PM, Christian Maeder
[EMAIL PROTECTED] wrote:
Neil Mitchell wrote:
Yes, an MTL dependency is nothing to worry about at all, and isn't
worth even thinking about removing given its actually used.
I would appreciate haskell98 portability!
My development version
On Wed, Feb 6, 2008 at 11:43 AM, Stuart Cook [EMAIL PROTECTED] wrote:
My development version has removed the need for
Control.Monad.Exception and Control.Arrow. The only remaining H98
incompatibility I can think of is the use of foldl' in fromList.
Version 0.2.1 features:
* almost-H98
On Sun, Feb 10, 2008 at 12:19 AM, ChrisK [EMAIL PROTECTED] wrote:
For Bimap is there anything like Data.Map.insertWithKey ?
No. I wanted to implement the insertWith family, but it wasn't clear
to me what should happen if the value produced by the user's function
already exists, bound to
-0.1
Darcs:
http://code.haskell.org/~scook0/unix-pty-light/
Haddock:
http://code.haskell.org/~scook0/haddock/unix-pty-light/
Stuart Cook
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A while ago I wrote a little data structure that allows weighted
random selection-without-replacement from a collection of values in
O(log n) time.[1] I'm now in the process of packaging it up for
Hackage, but I'm looking for good names for both the type and its
operations.
The name I have at the
On Tue, Feb 19, 2008 at 1:27 PM, Isaac Dupree [EMAIL PROTECTED] wrote:
the without replacement thing is more specific. Although maybe the
design could accomodate selection-with-replacement in the same package too
Once you have without-replacement, with-replacement is easy: just
re-use the old
On Fri, Feb 29, 2008 at 7:09 PM, Lloyd Smith [EMAIL PROTECTED] wrote:
I mixed up my types when finding the allocated and unallocated,
but I am not sure why it produces an error when unallocated and
allocated are never used? Shouldn't the two functions be compiled
down to the same thing?
On Tue, Apr 8, 2008 at 1:51 PM, Jackm139 [EMAIL PROTECTED] wrote:
I have an assignment to make a program to test whether two lists use the
same characters for each string.
e.g.
sameCharacter [rock, cab] [cork, abc]
True
I would start with something smaller: try defining a function that
On Tue, Apr 22, 2008 at 4:12 AM, Evan Laforge [EMAIL PROTECTED] wrote:
Has there been any work on improving update syntax in haskell?
Possibly some improvement could be made with a typeclass or two and a
few custom operators, to unify some of the disparate syntax. Maybe
more improvement
On Sat, Apr 26, 2008 at 4:07 AM, Bulat Ziganshin
[EMAIL PROTECTED] wrote:
2. ghc supports this part of standard in a rather awkward way - you
need to generate .hs-boot files using some switch (look into docs).
which is like .h files generated automatic from .cpp. once these files
On Sat, May 24, 2008 at 1:11 AM, Thomas Hartman [EMAIL PROTECTED] wrote:
World.hs:42:0:
No instances for (Eq (a (M.Map String Player)),
Eq (a (M.Map ItemId Item)),
Eq (a (M.Map PlayerId Player)),
Eq (a (M.Map RoomId Room)),
On Thu, Jul 3, 2008 at 8:00 PM, Dougal Stanton [EMAIL PROTECTED] wrote:
Here's a snippet from the parser for one option (others omitted for clarity):
options :: [OptDescr (Opts - Opts)]
options =
[ Option b [bus] (ReqArg busNum NUM) Bus number
, ...
]
where busNum n os = let
On Wed, Jul 9, 2008 at 10:01 AM, Neil Mitchell [EMAIL PROTECTED] wrote:
It seems that the qualified import syntax is a bit awkward. At the
moment, its common to see:
import qualified Data.Map as M
import Data.Map(Map)
i.e. import a module, give it an alias (M), and put some things in the
On Wed, Jul 16, 2008 at 12:54 AM, Henning Thielemann
[EMAIL PROTECTED] wrote:
Sooner or later you want generalize your datatypes. Then you can define
data A b = A b
and you do not need to import B any longer. I do not know if this is a
generally applicable approach, but it helped me in some
On Sun, Nov 29, 2009 at 8:42 AM, pbrowne patrick.bro...@comp.dit.ie wrote:
Question 3) Instances are not named so can they be imported?
Whenever you import a module, you automatically import all of its
instances as well. In fact, there is no way to *not* include instances
when importing a
2009/7/3 Luke Palmer lrpal...@gmail.com:
Keep in mind that with this solution *and* with the
ExistentialQuantification solution, there is no possibility of downcasting.
I.e. if you were planning on making a GraphicalWidget subclass, and them
somewhere seeing if a a Widget is actually a
On Thu, Aug 20, 2009 at 4:28 PM, Colin Paul
Adamsco...@colina.demon.co.uk wrote:
But how do you get Latin-1 bytes from a Unicode string? This would
need a transcoding process.
The first 256 code-points of Unicode coincide with Latin-1. Therefore,
if you truncate Unicode characters down to 8
On Thu, Aug 20, 2009 at 5:12 PM, Colin Paul
Adamsco...@colina.demon.co.uk wrote:
Yes, but surely this will work both ways. The same bytes on input
should come back on output, shouldn't they?
I would have thought so, but apparently this isn't actually what happens.
GHCi, version 6.8.2:
2008/8/1 Galchin, Vasili [EMAIL PROTECTED]:
Thanks Bulat! So since we are talking ;^) is there a function already
in Foreign that will allow me to ...
[a] - Ptr (Ptr ()) i.e. map a list of type a to an array of ptrs of type
a?
I think this is going to be a two-part operation: first
On Thu, Oct 16, 2008 at 12:53 AM, Bulat Ziganshin
[EMAIL PROTECTED] wrote:
data SomeStruct = SomeStruct
You can even go one step further and do
data SomeStruct
which will prevent you from accidentally trying to the dummy
constructor. However, you'll need
{-# LANGUAGE EmptyDataDecls #-}
On 7/18/07, Arie Groeneveld [EMAIL PROTECTED] wrote:
Ok, so when do I use nub instead of 'map head.group.sort' ?
Using nub gave me a lot of trouble in terms of time consumption
while handling long lists.
Well, nub is non-strict, so you can use it on infinite or partial
lists, provided you
On 7/27/07, Eric Y. Kow [EMAIL PROTECTED] wrote:
Solution #3 No lists, just records (lhs2TeX)
--
Advantages:
very convenient/compact; have to write
(i) Flag type
(ii) Settings record type/GetOpt in one go
(iii) default Settings
easy to
received my first monadic enlightenment from Bird's
Introduction to Functional Programming using Haskell, which also
uses the same approach. I think it's an excellent way to approach the
topic.
Stuart Cook
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On 8/2/07, Dave Tapley [EMAIL PROTECTED] wrote:
Using a BitmapFont I can get strings to appear but they demonstrate
the odd behaviour of translating themselves a distance equal to their
length every time my displayCallback function is evaluated.
I've never used OpenGL from Haskell, but it
From Fixing Haskell IO:
We can summarize the SDIOH (Standard Definition of IO in Haskell)
as a value of type IO a is a value, that performs, then delivers
a value of type a.
I think you've already made a critical mistake here. The quotes you
give all describe an IO value as something that when
On Mon, Mar 2, 2009 at 12:35 AM, Achim Schneider bars...@web.de wrote:
-Wall? The number of -W options enabled should scale (at least)
linearly with code size.
To make this a little more clear:
You should probably be using the -Wall compiler option, which will
produce a message for code
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