Things I'm unhappy about are for instance
f(x) \in L(\R)
where f \in L(\R) is meant
F(x) = \int f(x) \dif x
where x shouldn't be visible outside the integral
O(n)
which should be O(\n - n) (a remark by Simon Thompson in
The Craft of Functional
How can I convert an Int into a Double?
You don't convert to, you convert from :-)
The function 'fromIntegral' is probably what you want.
And what function can I use to convert from Double to Int (the inverse of
fromIntegral) ?
Use the functions in the RealFrac class.
On Tuesday 25 January 2005 02:25, Jan-Willem Maessen wrote:
On Jan 24, 2005, at 8:53 PM, Jorge Adriano Aires wrote:
And it would say nothing about things like:
return 4 return 5 ==?== return 5
I can live with it.
I feel obliged to point out (because the repeated references
Just did a search after my last post and learned
that FiniteMap is bad. Discoverd that Data.Map is
the intended replacement. Downloaded it and
modified it to work with 6.2. Blazingly fast!
Yay.
Hi, just curious,
How much trouble was getting it to work with ghc 6.2 and adapting your
Right, but we are dealing with the type system here. Remember Haskell
monoids are functors on types, not on values ... (ie the base objects the
'category theory' is applied to are the types not the values)...
Therefore we only consider the types when considering Monads.
How so? Functors map
We face a severe problem here, not only that IO a is not an instance of Eq,
which takes this whole discussion outside the realm of Haskell, on top of
that we find the horrible fact that x /= x may be true in the IO Monad,
consider
x = getLine = putStrLn
or anything similar -- actually
A constant c :: a is just morphism(function) c : 0 - a, where 0 is the
initial object (empty set).
--- Rant2 correction
Opss I messed up here. Should be terminal should 1- a (terminal object/unit
set). At least that's how I usually think of constants in haskell 1 is
()... so I think I
This isn't obvious to me. So x is an action, and it does not always
produces the same side effects when executed. But why should that make
x/=x? It is the same action, it gets one line from the input, and then
prints it...
OK, but then the different side-effects could not be used to
(Sorry about the recurrent self answers)
Maybe (not sure) it is sensible to
sapecify return::(a - IO a), as an action with no side effects such that
return x === return x iff x === x.
return x === return y iff x === y-- this is what I meant to write.
But even that is not enough,
What would happen if this was the definition?
instance MonadPlus [] where
mzero = []
mplus a b
| a == [] = b
| otherwise = a
Isn't the above a monoid as well?
Yes.
Is there only on correct definition of a monad/monoid on lists - or does
anything that satisfies
One common example is using MonadPlus for some backtracking algorithm,
then instantiatiating it to Maybe or List instance depending on wether
you just want one solution or all of them.
Backtracking only works with the first kind, even if you're only
interested in the first solution. This
I think it would be helpful if all these classes came with their laws
prominently attached in their Haddock documentation or wherever.
Agree.
The trouble with MonadPlus is that the precise set of associated laws is
either unspecified or not the most useful (I assume there's a paper on
the
Am Sonntag, 23. Januar 2005 15:58 schrieb Jorge Adriano Aires:
I'm not arguing that definition would be wrong. It is a monoid. This is
the instance for ():
instance MonadPlus() where
mzero = ()
mplus a b = ()
Maybe I'm stupid, but:
class Monad m = MonadPlus m where
mzero
But only some instances (such as []) satisfy this:
(mplus a b) = c = mplus (a = c) (b = c)
Other instances (IO, Maybe) satisfy this:
mplus (return a) b = return a
I think mplus should be separated into two functions.
How would we implement the first kind in the Maybe instance of
snip
Only the monoid Maybe a is not very nice (nor is the monoid IO a),since the
second argument of the composition is in general ignored.
snip
So I think, rather than separating mplus, one should think about whether it
is sensible to make Maybe and IO instances of MonadPlus in the first
Perhaps one could have top-level implicit parameters (or top-level
contexts in general):
module (?myvar :: IORef Int) = Random where
Hi!
I suggested something very similar to this some months ago, syntax and all.
Nice to see I'm not the only one thinking along this lines.
On Friday 07 January 2005 12:03, Ketil Malde wrote:
Naive use of foldl. I tend to think the default foldl should be
strict (ie. replaced by foldl') -- are there important cases where it
needs to be lazy?
Hi,
One simple example would be,
reverse = foldl (flip (:)) []
J.A.
On Friday 07 January 2005 12:03, Ketil Malde wrote:
Naive use of foldl. I tend to think the default foldl should be
strict (ie. replaced by foldl') -- are there important cases where it
needs to be lazy?
Hi,
One simple example would be,
reverse = foldl (flip (:)) []
A better example
No, it would work with strict foldl too. In fact in the absence
of optimization it would work better (uses less time and space).
The optimization required is inlining and strictness analysis.
Is this also true if your just going to use the first few elements after
reversing it?
A function
On Sunday 09 January 2005 21:30, Marcin 'Qrczak' Kowalczyk wrote:
Jorge Adriano Aires [EMAIL PROTECTED] writes:
No, it would work with strict foldl too. In fact in the absence
of optimization it would work better (uses less time and space).
The optimization required is inlining
(+) is
usually strict on both arguments (although in principle it does not
have to be true because of overloading, which implies that a compiler
can only optimize particular specializations of sum, not generic sum).
Since you mention it, there was some talk about this in the #haskell channel,
Jorge,
Besides the case where 'a' is the same as 'b', there is also another
interesting case. That is when you have both, Foo A B and Foo B A.
This is a
known property (named DoubleFoo) [...]
Again, with -fallow-undecidable-instances:
\begin{code}
class (Foo a b, Foo b a) =
Hello!
I got a multi-parameter type class:
class Foo a b | a - b where
foo_method1 :: ...
foo_method2 :: ...
...
And some particular cases are important on their own, like the one where 'a'
and 'b' are the same, I call elements with this property, Bar. So I defined:
class Foo a
Maybe I should have included a more interesting example in the previous mail.
So I had this class:
class Foo a b | a - b where
foo_method1 :: ...
foo_method2 :: ...
...
Besides the case where 'a' is the same as 'b', there is also another
interesting case. That is when you have
(opss just noticed I did a reply-to)
The following is closer to the original, but doesn't work when the whole
list is folded (i.e., p always satisfied):
foldlWhile f p a = head . dropWhile p . scanl f a
Serge's version returns the last 'a' that satisfies 'p', while yours
Not really.
Hello,
A hash-table becomes rather useless without mutable state AFAICS.
Without it, one might almost just as well use a list of pairs...
Could you please elaborate? Is there motive why an Hash Table, implemented as
FiniteMap of Lists, for instance, wouldn't be worth to using over a simple
The generate function is exported:
generate :: Int - StdGen - Gen a - a
Thanks, that's the one I was missing.
J.A.
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Hello all,
When using Quickcheck, is there some way to extract generated data values
to the IO Monad?
I know I can collect and print information about test cases, but that's
not enough. Data may be pretty complex, and there may be no parsers for
it. If a test suddenly goes wrong,
Hello all,
When using Quickcheck, is there some way to extract generated data values to
the IO Monad?
I know I can collect and print information about test cases, but that's not
enough. Data may be pretty complex, and there may be no parsers for it. If a
test suddenly goes wrong, just having
So, yes, it is useful, but should it be included in a standard Monad
module? After all, this module contains mostly trivial functions ;)
BTW. You can write this function using foldM:
compM l a = foldM (#) a l
where # is an often used reverse application operator:
x # f = f x
Hello,
I needed to compose monadic functions, since I couldn't find anything in
Control.Monad so I defined my own. I first came up with a left associative
version:
compLM [f1,..., fn] a = (f1 a = f2) = ... = fn
compLM:: Monad m = [a-m a] - (a-m a)
compLM mfs a = foldl (=) (return a)
Hello, When trying
type CTPar = ([Double],Int,Int)
ctPar ::(?ctPar::CTPar)=CTPar
ctPar@(us,n,j) = ?ctPar
I got this error message in ghci is:
Illegal overloaded type signature(s)
in a binding group for ctPar, us, n, j
that falls under the monomorphism restriction
On style, yes. A concrete example of what I asked before.
The following doesn't type check for obvious reasons.
In your opinion what is the most elegant way to fix it?
Like I said, I'm starting to feel like defining my own operators is the way to
go. On the other hand, this kind of situation
Hello,
Lately I've been coding functions where I have to use Int elements, both as
Ints (e.g. as an array or list index) and Doubles/Floats (e.g. when
performing arithmetic operations with Doubles/Floats).
How do you usually deal with this? Some options I can think of are,
1. using
Hello,
Is it even possible to make a global variable in Haskell?
If yes, how?
Thanks.
(short answer, no time now...)
Look here:
http://www.haskell.org/pipermail/haskell-cafe/2002-January/002589.html
Hope it helps ;)
J.A.
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Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
I appreciate your comment.
I agree that the type of findM should be the one you suggested,
and it still fits my original purpose. It's no more than a step arout.
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
Now fixed in the HEAD, and will be in 5.04.2
Thanks for pointing it out.
Simon
Thanks :)
J.A.
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On Thursday 14 November 2002 18:47, Iavor S. Diatchki wrote:
hello,
Well, actually you must be right since the pure field defines a pure
(projection) function... Hmmm, ok, can someone explain this to me,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m
If I use an STRef to a record, will a new record be created
each time I want
to update a single field? Or can I expect GHC to optimize it
and have the field of the record updated in place?
You'll get a new record for each update. This might not be so bad
though, depending on the
But like I just showed, sometimes paring them may not be a natural
approach though...
Yeah, I understand what you mean. In the examples you give, you could
always try to make the appended stuff (the zs, etc) the same length by
appending Nothings, but probalby not a general solution.
Yes
Evening,
I'm trying to write a utility that reads in some user preferences from
a pre-determined file, does some work, and exits. Sounds simple enough.
The problem I'm having is with the preferences: How do I make it
available throughout the entire program? (FWIW, most of the work is
Hi,
thx for this reply.
Is there any overhead using this mutable?
I just thought I should point out that Mutable is not an haskell type.
You can see in the Utils module that it is just a type synonim for IORef:
http://icfpcontest.cse.ogi.edu/simulator/pfe.cgi?Utils#Mutable
Hi, all, I installed the latest version of Haskell mode for emacs.
Whenever I load haskell major mode by opening a haskell file, Xemacs gives
me the following error message:
(1) (error/warning) Error in `post-command-hook' (setting hook to nil):
(void-variable imenu--index-alist)
My
-
[moving to haskell-cafe]
I want to read some data from a text file. I accomplished it but I want to
read data from file as a Integer list for example text file has [1,2,3,5]
-
Wait you actually have [1,2,3,5]
-
and when I read this data
However the user of the program give some argument then program is
starting. And I want to use this arguments for example at the end of the
program. What will I do.
Good question. I've had a bad time with this, some in this list proably got
tired of me saying the same thing over and
Tried using Andrew Cookes haskell.sty and liked it a lot, but seems like the
(X)emacs haskell mode doesn't support the \begin{code} \end{code} literate
programming style, just the one where code is preceded by ' '.
Anyone has got an hack available for this?
J.A.
On Friday 22 March 2002 16:31, you wrote:
Jorge Adriano [EMAIL PROTECTED] writes:
Tried using Andrew Cookes haskell.sty and liked it a lot, but seems like
the (X)emacs haskell mode doesn't support the \begin{code} \end{code}
literate programming style, just the one where code is preceded
of 'psgml-mode' is highly recommended. It is, of course, a
;; part of Debian GNU/Linux.
;; Modified by Marco Pantaleoni [EMAIL PROTECTED]
;; to allow execution of an hook on mode switching.
;; Also added a standard mode hook and some documentation strings.
;; Modified by Jorge Adriano [EMAIL
On Friday 22 March 2002 23:27, Jorge Adriano wrote:
I got some tips from some people at #xemacs in openprojects.org and cam up
with the file in attachment. I know nothing about e-lisp so beware!!
Opss I forgot:
The original file is Copyrighted by David Welton and it is located here:
http
I got some tips from some people at #xemacs in openprojects.org and cam up
with the file in attachment. I know nothing about e-lisp so beware!!
Last e-mail today. Forgot to actually tell what was this for (need ti get
some sleep :)... It's easy to guess by looking at the file anyway the idea
Thanks for all your answers :-)
J.A.
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Why is it necessary to leave a blank line between comments and code?
I'm using LaTeX in my lhs files, with the code inside a verbatim
environment, and I'd rather start writing my code right after the
\begin{verbatim}.
I'd also like to know if anyone as changed is xemacs configuration file in
On Saturday 09 February 2002 11:38, Jorge Adriano wrote:
If you make it strict on the (,), like:
test3 l =
let s = foldr (\x (a,b) - ((,)$!x+a)$!x-b) (1,1) l
in s
Things will get worst.
Well, that's what I expected, the elements of the list will b reduced to
head normal form
On Friday 08 February 2002 14:35, Ketil Z. Malde wrote:
Jorge Adriano [EMAIL PROTECTED] writes:
Haskell looks nice... Isabell looks beautiful :-)
I'm not familiar with Isabell, but aren't we comparing apples and
oranges here? E.g. you can prettify .lhs pretty nicely with one of
the LaTeX
I'd say that's because in the second case you also got to apply the (,),
besides the (+)/(-) constructor during the transversing...
Am I right?
opss... I meant to write: the (,) constructor besides the (+)/(-)...
J.A.
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On Sunday 27 January 2002 05:36, Hal Daume III wrote:
For your last question (about reduction to hnf), use the attached
code; search the haskell mailing list for deepseq for more.
Thanks... I found myself trying to define a function
deepSeq :: [a]-[a]
to evaluate all the elements of the
Am I the only one who'd like to have some the function specified by
scan_and_fold f e xs= (scanl f e xs, foldl f e xs)
In the Lists library. Or is it there somewhere and I missed it?
What about:
pair (f,g) x = (f x, g x)
cross (f, g) = pair(f.fst, g.snd)
I kind of like point free style.
I agree with others who mentioned that viewing monads as simply
providing a way to sequentialize things or to program imperatively is
the wrong way to look at them.
snip
Yes, Lists are the classical example.
That said, the EFFICIENCY of monads is often poorly understood. To
state the
[Obs: most answers I got end up in my pvt e-mail and not in the mailing
list... I replyed in pvt to those. I do feel it some cases that is probably
accidental as I do it all the time :), and the discussion ends leaving the
mailing list. So i'd just like to let you know that I for one am in
Hi,
I'm studying, among other things, Genetic Algorithms and Neural Networks and
I decided I'd use haskell to code some simple GAs and NNs along with my study.
Well, maybe it was not such a good idea after all, because I've been spending
way more time learning more Haskell then GAs and NNs :(
Hi, I've just started messing around with strictness. My goal now is
understanding when and how to use it.
I began with simple examples like making a strict foldl.
When trying to sum a list of 6 elements with lazy foldl I'd get a stack
space overflow, with a strict foldl I was able to sum
Then I tried:
sfibac :: IntPos - (IntPos,IntPos) -
(IntPos,IntPos) sfibac n (a,b)
| n == 0= (a,b)
| otherwise = sfibac (n-1) (b, (+b) $! a)
I'm sorry I meant:
sfibac :: IntPos - (IntPos,IntPos) -
The functions:
f [] w = (w,0)
f (x:xs) w = (nextw, steps+1)
where
(nextw, steps) = f xs (g x w)
f2 [] k w = (w,k)
f2 (x:xs) k w = f2 xs (k+1) (g x w)
f3 [] w = w
f3 (x:xs) w = f3 xs (g x w)
Oh oh ---
Hi all, got some questions on recursive functions.
I got a simple recursive function (f3)
In *some* situations I'll want to know how many recursive calls did it make.
I came up with two simple implementations to do that (f1) and (f2)
f [] w = (w,0)
f (x:xs) w = (nextw, steps+1)
How can I use the std functions (listed in the 'Standard Libraries for
Haskell 98' documentation) in ghc(i)?
I've read the ghc documentation, and seen the several categories (which can
be added with '-package category_name') available, but I still don't know how
to access functions like
Está para sair a 22 de Outubro... até eu estou a ficar surpreendido com a
evolução destes tipos. Qd penso do 1o Linux que instalei (à cerca de dois
anos? - andava eu no 4o ano) sinto-me como aquelas velhas que dizem eu ainda
sou do tempo..., err... em que tinha de andar a alterar XF86Config à
I just accidently sent a mail written in portuguese about SuSE 7.3.
I'm really sorry, it will not happen again.
My apologies
J.A.
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Just thought I'd report that the PolyGP link:
http://www.cs.ucl.ac.uk/staff/T.Yu/research.html
at the 'Haskell in Practice' section (http://www.haskell.org/practice.html)
seems to be broken.
J.A.
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Hi all,
I was thinking about making some GUIs in Qt/KDE for some haskell applications
(compiled with ghc).
My first idea was to simply run haskell applications with some args using the
GUI and get the results from some file or something like that.
Anyway someone told me that a better idea
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