Re: [Haskell-cafe] Re: 0/0 1 == False
On Sat, 12 Jan 2008 13:23:41 +0200, Kalman Noel
[EMAIL PROTECTED] wrote:
Achim Schneider wrote:
Actually, lim( 0 ) * lim( inf ) isn't anything but equals one, and
the anything is defined to one (or, rather, is _one_ anything) to be
able to use the abstraction. It's a bit like the difference between
eight pens and a box of pens. If someone knows how to properly
formalise n = 1, please speak up.
Sorry if I still don't follow at all. Here is how I understand (i. e.
have learnt) lim notation, with n ∈ N, a_n ∈ R. (Excuse my poor
terminology, I have to translate this in my mind from German maths
language ;-). My point of posting this is that I don't see how to
accommodate the lim notation as I know it with your term. The limit of
infinity? What is the limit of infinity, and why should I multiplicate
it with 0? Why should I get 1?
(1) lim a_n = a(where a ∈ R)
(2) lim a_n = ∞
(3) lim a_n = − ∞
(4) lim { x → x0 } f(x) = y (where f is a function into R)
(1) means that the sequence of reals a_n converges towards a.
(2) means that the sequence does not converge, because you can
always find a value that is /larger/ than what you hoped might
be the limit.
(3) means that the sequence does not converge, because you can
always find a value that is /smaller/ than what you hoped might
be the limit.
(4) means that for any sequence of reals (x_n ∈ dom f) converging
towards x0, we have lim f(x_n) = y. For this equation again, we
have the three cases above.
Suppose lim a_n = a , lim b_n = b, c_2n = a_n, c_2n+1 = b_n.
What is lim c_n ?
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Re: [Haskell-cafe] Re: 0/0 1 == False
On Sat, 12 Jan 2008 13:55:19 +0200, Kalman Noel [EMAIL PROTECTED] wrote: Cristian Baboi: Suppose lim a_n = a , lim b_n = b, c_2n = a_n, c_2n+1 = b_n. What is lim c_n ? If my intuition was of any importance here, it would claim that c_n diverges, because if I roughly approximate c_n by the sequence c' = ⟨a,b,a,b,...⟩, then I note that c' oscillates, so c_n »roughly oscillates«. Kalman You mean something like this x=a:b:x ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Thu, 10 Jan 2008 19:38:07 +0200, Tillmann Rendel [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: Although it could be argued that laziness is the cause of some very obscure bugs... g Niko Example, PLEASE. Prelude sum [1..100] *** Exception: stack overflow Not true in Hugs. Prelude Data.List.foldl' (+) 0 [1..100] 5050 Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Fri, 11 Jan 2008 09:16:12 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Thank you Duncan, you took the words out of my mouth. :) On Jan 10, 2008 5:42 PM, Duncan Coutts [EMAIL PROTECTED] wrote: So let's imagine: ones = 1 : ones ones' = repeat 1 where repeat n = n : repeat n So you're suggesting that: ones == ones = True but ones' == ones' = _|_ Well if that were the case then it is distinguishing two equal values and hence breaking referential transparency. We can fairly trivially prove that ones and ones' are equal so == is not allowed to distinguish them. Fortunately it is impossible to write == above, at least using primitives within the language. If one can prove ones == ones = True with some method, why that method cannot be made to work on ones' ? Are you thinking about repeat (f x) by any chance ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Fri, 11 Jan 2008 09:11:52 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Some people seem to think that == is an equality predicate. This is a big source of confusion for them; until they realize that == is just another function returning Bool they will make claims like [1..]==[1..] having an unnatural result. The == function is only vaguely related to the equality predicate in that it is meant to be a computable approximation of semantic equality (but since it's overloaded it can be anything, of course). I think that confusion came from the fact that the type of == is called (Eq a)= a - a - Bool and not (Bla a) = a - a - Binary and not realizing it's just an overloaded polimorphic function. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Fri, 11 Jan 2008 09:11:52 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Some people seem to think that == is an equality predicate. This is a big source of confusion for them; until they realize that == is just another function returning Bool they will make claims like [1..]==[1..] having an unnatural result. The == function is only vaguely related to the equality predicate in that it is meant to be a computable approximation of semantic equality (but since it's overloaded it can be anything, of course). So let's imagine: ones = 1 : ones ones' = repeat 1 where repeat n = n : repeat n (==) :: Eq a = a - a - Bool -- what is (y (y) ) by the way ? -- how about ( y id ) ? y f = f (y f). ones :: Num a = [a] ones = y (1 :) repeat :: a - [a] repeat = \n - y (n:) ones' :: Num a = [a] ones' = repeat 1 = (\n-y(n:)) 1 = y (1 : ) To be able to test them for equality, we must have Eq a. So, the reason we cannot test them for equality is that we cannot test y (a : ) == y (a : ) where a == a is testable. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Fri, 11 Jan 2008 13:29:35 +0200, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Freitag, 11. Januar 2008 10:54 schrieb Wilhelm B. Kloke: Wolfgang Jeltsch [EMAIL PROTECTED] schrieb: However, the fact that (0 / 0) == (0 / 0) yields False is quite shocking. It doesn?t adhere to any meaningful axiom set for Eq. So I think that this behavior should be changed. Think of a set implementation which uses (==) to compare set elements for equality. The NaN behavior would break this implementation since it would allow for sets which contain NaN multiple times. You forget, that the intention of NaN is denial of membership of any set of numbers. This doesn’t matter. The Set data type I’m talking about would not know about NaN and would therefore allow multiple NaNs in a set. This is a good thing because one can define natural numbers with such sets :-) Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
On Fri, 11 Jan 2008 14:21:45 +0200, [EMAIL PROTECTED] wrote: Ketil Malde: Wolfgang Jeltsch: However, the fact that (0 / 0) == (0 / 0) yields False is quite shocking. It doesn’t adhere to any meaningful axiom set for Eq. Tough luck, but that's how floating point works, and what the numericalists know, and possibly even love (although I have my doubts). Sanitizing this behavior would make Haskell less usable for real-world numerical problems. As a compromise, what about an option to make NaN (and presumably the infinities) cause an immediate exception? (And, cetero censeo, exceptions for Int overflow as well.) People, you are monsters. First, despite the *common, well known* truth that Haskell is not Mathematics, this illusion seems to be extremely persistent! Haskell is a victim - no, some users are victims of its success as a formal language, not just as a coding tool... They *want* to have Eq as they imagine the equality, including the comparison between incomparable. This is BTW a long standing philosophical problem. For centuries some speculative guys tried to analyse such assertions as God == God, or death==death. Or myself==myself. Of course, even if they produced some cute conclusions, they had no whatsoever sense for the others. Now we have the modern variants of it: NaN == NaN, bottom == bottom ... Well, Haskell has this referential transparency thing which say that a function is a function and you will never be able to build anything else :-) Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why purely in haskell?
The thing is that y already is a *builtin* function in Haskell. On Fri, 11 Jan 2008 15:59:50 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: So let's imagine: ones = 1 : ones ones' = repeat 1 where repeat n = n : repeat n (==) :: Eq a = a - a - Bool -- what is (y (y) ) by the way ? -- how about ( y id ) ? y f = f (y f). ones :: Num a = [a] ones = y (1 :) repeat :: a - [a] repeat = \n - y (n:) ones' :: Num a = [a] ones' = repeat 1 = (\n-y(n:)) 1 = y (1 : ) To be able to test them for equality, we must have Eq a. So, the reason we cannot test them for equality is that we cannot test y (a : ) == y (a : ) where a == a is testable. Yes, thanks. I actually do think that many things would be easier if every recursion would be translated to its fixpoint, making the term tree completely finite and defining y internal, as it's arcane, black magic. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Thu, 10 Jan 2008 10:22:03 +0200, Mitar [EMAIL PROTECTED] wrote: Hi! Why is 0/0 (which is NaN) 1 == False and at the same time 0/0 1 == False. This means that 0/0 == 1? No, because also 0/0 == 1 == False. I understand that proper mathematical behavior would be that as 0/0 is mathematically undefined that 0/0 cannot be even compared to 1. There is probably an implementation reason behind it, but do we really want such hidden behavior? Would not it be better to throw some kind of an error? Mitar I think it's a bug. Here is why: let f = (\x - x/0) in f 0 == f 0 Referential transparency say that f 0 must equal to f 0, but in this case it is not. :-) Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Thu, 10 Jan 2008 10:48:51 +0200, Benja Fallenstein [EMAIL PROTECTED] wrote: Hi Mitar, On Jan 10, 2008 9:22 AM, Mitar [EMAIL PROTECTED] wrote: I understand that proper mathematical behavior would be that as 0/0 is mathematically undefined that 0/0 cannot be even compared to 1. My understanding is that common mathematical practice is that comparing an undefined value to anything (including itself) always yields false; x /= x is sometimes used to formalize x is undefined. Why let a = a in a /= a is not False ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Thu, 10 Jan 2008 10:48:51 +0200, Benja Fallenstein [EMAIL PROTECTED] wrote: Hi Mitar, On Jan 10, 2008 9:22 AM, Mitar [EMAIL PROTECTED] wrote: I understand that proper mathematical behavior would be that as 0/0 is mathematically undefined that 0/0 cannot be even compared to 1. My understanding is that common mathematical practice is that comparing an undefined value to anything (including itself) always yields false; x /= x is sometimes used to formalize x is undefined. How about this: f 1 = 1 f 2 = 2 f 3 /= f 3 Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
and there is no such thing as the same bottom right ? On Thu, 10 Jan 2008 11:13:05 +0200, Ketil Malde [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] writes: I think it's a bug. Here is why: let f = (\x - x/0) in f 0 == f 0 Referential transparency say that f 0 must equal to f 0, but in this case it is not. :-) I think you are wrong. Referential transparency says that you can replace any occurence of 'f 0' with another expression of the same value, it does not say anything about the behaviour of (==). -k Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Thu, 10 Jan 2008 11:23:51 +0200, Yitzchak Gale [EMAIL PROTECTED] wrote: Cristian Baboi wrote: and there is no such thing as the same bottom right ? Yes and no. Semantically, Yes and No is bottom ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Thu, 10 Jan 2008 10:22:03 +0200, Mitar [EMAIL PROTECTED] wrote: Hi! Why is 0/0 (which is NaN) 1 == False and at the same time 0/0 1 == False. This means that 0/0 == 1? No, because also 0/0 == 1 == False. I understand that proper mathematical behavior would be that as 0/0 is mathematically undefined that 0/0 cannot be even compared to 1. There is probably an implementation reason behind it, but do we really want such hidden behavior? Would not it be better to throw some kind of an error? NaN is not 'undefined' (0/0) /= (0/0) is True (0/0) == (0/0) is False You can use these to test for NaN. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 0/0 1 == False
On Fri, 11 Jan 2008 08:34:10 +0200, Don Stewart [EMAIL PROTECTED] wrote: NaN is not 'undefined' (0/0) /= (0/0) is True (0/0) == (0/0) is False You can use these to test for NaN. You can also use isNaN :) Prelude isNaN (1/0) False Prelude isNaN (0/0) True Not true in Hugs. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: US Homeland Security program language security risks
On Sun, 06 Jan 2008 17:19:31 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 6. Januar 2008 15:54 schrieb Achim Schneider: Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 6. Januar 2008 15:18 schrieb Andrew Coppin: Daniel Fischer wrote: Just because I don't know: what bugs would be possible in a language having only the instruction return () Bug #1: You cannot write any nontrivial programs. ;-) That's not a bug, that's a feature. That's an interesting task: Design a non-touring complete, restricted language in which every expression is decidable, without making the language unusable for usual programming problems. I'm not a logician, but didn't Gödel prove that you couldn't express the (full) arithmetic of natural numbers in such a language? Of course it might be possible to express a sufficiently interesting part of it, but I should be surprised. Neither do I, but if you have non turing algorithms, maybe you could do it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Consensus about databases / serialization
I recommend you read Extending the database relational model to capture more meaning by E.F. Codd. On Wed, 02 Jan 2008 13:50:46 +0200, Peter Verswyvelen [EMAIL PROTECTED] wrote: As I'm a selfmade man, I never really studied relational databases in detail. My intuition told me that the relational part was not really suitable for the 3D data, 2D images, animation curves, state machines, and other data I encountered in the videogame and animation business. I could always get away with files, and for the applications I needed to deploy, plugging in a couple of extra gigabytes of RAM and serializing the object state to disk was more practical, cheaper and faster. However, a couple of years ago I started studying computer science (I seem to do the theory after the practice, weird behavior ;-) at the Open University, and one of the exams I did was about databases. Initially this course convinced me that databases are actually very nice, but the course ended with a topic on object oriented databases, which were designed to make storing data like 3D models, graphs, networks, and complex datastructures more practical. Duh. Since then, I did deploy a few commercial applications for customers using databases, which worked fine for the typical simple/flat database data. I hated embedding a dynamic untyped language like SQL, as much as I hated embedding code in HTML or XML. IMHO it feels UGLY and unsafe. Regarding the other popular data storage format - XML - I did use that a lot, but it seems like going back to the stone ages, when hierarchical stores/databases got invented (and ditched?) Now, initially after an introduction to Microsoft's LINQ, and recently having read a very brief overview of HAppS, it seems I'm not the only one with those feelings. Ouch, this introduction got way to long, sorry about that ;-) Finally some practical questions: .regarding Haskell and databases, the page http://haskell.org/haskellwiki/Libraries_and_tools/Database_interfaces describes a few, but which are the ones that are stable and practical? Any user experiences? .HApps is not listed in the page above, because it does not use databases? Is HApps reliable or experimental, and does it scale well? Any success stories? .regarding Haskell and serialization, I don't think that implementing Read/Show is a good way for real serialization, so what other options exist? I could find some libraries at http://hackage.haskell.org/packages/archive/pkg-list.html#cat:Data, but again which are the most practical and stable? When programming in C++/MFC and C#/.NET, I tended to develop my own serialization frameworks because I used that for many things, like logging commands to disk, performing undo/redo, intra and inter process cut/copy/paste, save/load, etc. .Regarding serialization, I'm kinda curious how ADTs and even GADTs are stored and retrieved in a relational database? I guess it could be done using BLOBs and serialization to ByteStrings, so bypassing a lot of the database table structures? .If I would want to experiment with say HAppS, the way I understand it, I first would first have to study Scratch your boilerplate and Template Haskell, and maybe some other language features? I'm still new to Haskell, and the road to understanding all language elements and extensions is very long, so sequentially learning it would be insane I guess. I have no practical experience with TH, but I spent a long time trying to do aspect oriented programming in C# without success, so TH looks uber to me. Thanks a lot and best wishes for 2008? Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
It give Something. On Mon, 31 Dec 2007 10:27:38 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: How about a :: Something a = let x = x in x :t a would give (a - [a]), not Something, or am I mistaken? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Mon, 31 Dec 2007 10:40:13 +0200, Achim Schneider [EMAIL PROTECTED] wrote: It seems like you're trying to solve the halting problem. I didn't know that. Slurping any infinite data structure makes your program perform rather badly, I'm afraid. What is infinite in let x = x in x ? What I see is that I cannot prevent building of infinite data structures without investing some time to find a way. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Mon, 31 Dec 2007 10:59:28 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: What is infinite in let x = x in x ? ^ || |___/| \/ a = let x = x in x is actually only quite verbose for a = undefined, the only difference being that undefined is a nicer _|_ than let x = x in x. I could have written this instead: a :: Something a = a Which is nicer than undefined. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Mon, 31 Dec 2007 11:14:39 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: I could have written this instead: a :: Something a = a Which is nicer than undefined. [EMAIL PROTECTED] ~ % ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude let a = undefined Prelude a *** Exception: Prelude.undefined Prelude let a = a Prelude a Interrupted. nope, it isn't. Well, it depends on what you think is nicer. For me it looks shorter than undefined and it don't rely on the library. And then let a = a is more defined than the other two... only the equation is _|_, not the value that is assigned. Value that is equated? Term? Jonathan, please beat me. This is over my head. If let a = a is more defined, then why (print a) won't stop ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Mon, 31 Dec 2007 11:14:39 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: On Mon, 31 Dec 2007 10:59:28 +0200, Achim Schneider [EMAIL PROTECTED] wrote: I could have written this instead: a :: Something a = a Which is nicer than undefined. [EMAIL PROTECTED] ~ % ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude let a = undefined Prelude a *** Exception: Prelude.undefined Prelude let a = a Prelude a Interrupted. nope, it isn't. If you try it again Prelude a *** Exception: Stack overflow Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
Nice! Thank you.
On Mon, 31 Dec 2007 12:56:42 +0200, Ryan Ingram [EMAIL PROTECTED]
wrote:
On 12/30/07, Cristian Baboi [EMAIL PROTECTED] wrote:
Thank you.
data Something = This | S Something
ppp :: Something - String
ppp This =
ppp (S x) = 'S':(ppp x)
How can I prevent one to pass 'let x = S x in x' to ppp ?
{-# LANGUAGE GADTs, EmptyDataDecls #-}
data Z
data S a
data Something a where
This :: Something Z
S :: Something a - Something (S a)
ppp :: Something a - String
ppp This =
ppp (S x) = 'S' : ppp x
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Re: [Haskell-cafe] Re: Web server continued
On Mon, 31 Dec 2007 14:36:02 +0200, Joost Behrends [EMAIL PROTECTED] wrote: I forgot 2 things: The distinction between '=' and '==' is much like in C, although mixing them up is not so dangerous like in C. ':=' and '=' like in Wirth languages would be nicer. Strangely nobody reacted on this. That a=a+1 is an infinite recursion here What is more strange is that a = a + 1 and a = 1 + a are somehow distinct. The second give a stack overflow almost instanly, but the first don't. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sat, 29 Dec 2007 21:12:21 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Try writing a program that makes me say hello world as many times as you press my nose. Be careful what you wish for. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sat, 29 Dec 2007 21:49:16 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 29 Dec 2007, at 5:01 AM, Cristian Baboi wrote: By portable I mean: works on the same machine, with the same OS, but with different Haskell implementation. Ah, you can't. But, again, what are you trying to do? Re-compiling your software for each implementation seems like a perfectly reasonable thing to do, given the differences between them. Recompiling my software will not save a function created by the software at runtime. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 18:39:51 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 30 Dec 2007, at 10:14 AM, Cristian Baboi wrote: On Sat, 29 Dec 2007 21:49:16 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 29 Dec 2007, at 5:01 AM, Cristian Baboi wrote: By portable I mean: works on the same machine, with the same OS, but with different Haskell implementation. Ah, you can't. But, again, what are you trying to do? Re-compiling your software for each implementation seems like a perfectly reasonable thing to do, given the differences between them. Recompiling my software will not save a function created by the software at runtime. Which is a different problem than the one solved by dynamic linking. Again, why do you want to do this? I think they are not as different as you think they are. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 18:34:08 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: True, but again, what are you trying to do? I've already did what I was trying to do. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] The semantics of constructor patterns
In section 4.3.3., chapter 4: Structured types and the semantics of pattern-matching, by S.Peyton Jones and Philip Wadler, there is this equation: Eval[[\(s p1 p2 ... pt).E]] (s a1 a2 ...at) = Eval[[\p1 ... \pt.E]] a1 ... at The text say: To apply \(s p1 ... pt).E to an argument A we first evaluate A to find out what sort of object it is. Finally, if A was built with the same constructor as the patern, then the first rule applies. What I don't get it : (s a1 a2 ... at) must be the value of A in the semantic domain. Let call that value a. Then how can one know if a was built with (s a1 a2 ... at) and not with (egg b1 b2) ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 19:10:44 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 30. Dezember 2007 17:55 schrieb Cristian Baboi: On Sun, 30 Dec 2007 18:34:08 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: True, but again, what are you trying to do? I've already did what I was trying to do. Congrats. How? By talking with you. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 19:02:11 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 30 Dec 2007, at 10:54 AM, Cristian Baboi wrote: On Sun, 30 Dec 2007 18:39:51 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 30 Dec 2007, at 10:14 AM, Cristian Baboi wrote: On Sat, 29 Dec 2007 21:49:16 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 29 Dec 2007, at 5:01 AM, Cristian Baboi wrote: By portable I mean: works on the same machine, with the same OS, but with different Haskell implementation. Ah, you can't. But, again, what are you trying to do? Re-compiling your software for each implementation seems like a perfectly reasonable thing to do, given the differences between them. Recompiling my software will not save a function created by the software at runtime. Which is a different problem than the one solved by dynamic linking. Again, why do you want to do this? I think they are not as different as you think they are. I think they're very different --- dynamic libraries can be built by running the compiler, whatever you're asking for can't. More generally, dynamic libraries are supported by every production- quality compiled language in existence; I know of no language that can do what you're asking for. I think, again, that what you really want is a reason to discredit Haskell. A simple question: Can you write the value of x to a file where x = (1:x) ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The semantics of constructor patterns
On Sun, 30 Dec 2007 19:13:47 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 30 Dec 2007, at 11:10 AM, Cristian Baboi wrote: In section 4.3.3., chapter 4: Structured types and the semantics of pattern-matching, by S.Peyton Jones and Philip Wadler, there is this equation: Eval[[\(s p1 p2 ... pt).E]] (s a1 a2 ...at) = Eval[[\p1 ... \pt.E]] a1 ... at The text say: To apply \(s p1 ... pt).E to an argument A we first evaluate A to find out what sort of object it is. Finally, if A was built with the same constructor as the patern, then the first rule applies. What I don't get it : (s a1 a2 ... at) must be the value of A in the semantic domain. Let call that value a. Then how can one know if a was built with (s a1 a2 ... at) and not with (egg b1 b2) ? I suspect that s and egg are constructors, in which case their images are (by definition) mutually exclusive. I have not seen that definition in the book. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 19:16:04 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: If you want help, you would have an easier time getting it if you came here with /programming/ problems. Most of us enjoy programming quite a bit, and are eager to help with such, but this sort of thing is more of a distraction, really. Thank you, but I don't need help with programming. I can do that myself. The help I need is for understanding Haskell. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 19:19:11 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: When you can convince me that continuing this discussion will be of use /to the Haskell community/, I will until then, goodbye. I won't even try to do that. Goodbye. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The semantics of constructor patterns
Thank you. The thing is that when talking about the semantic of Prolog,
one can choose any set as the semantic domain to start, and then a reason
is given for choosing the Herbrand universe.
On Sun, 30 Dec 2007 19:23:00 +0200, Benja Fallenstein
[EMAIL PROTECTED] wrote:
Hi Cristian,
On Dec 30, 2007 6:10 PM, Cristian Baboi [EMAIL PROTECTED] wrote:
What I don't get it :
(s a1 a2 ... at) must be the value of A in the semantic domain. Let call
that value a.
Then how can one know if a was built with (s a1 a2 ... at) and not with
(egg b1 b2) ?
Because the semantic domain is chosen so that (s a1 a2 ... at) and
(egg b1 b2) are distinct objects.
More precisely, the domain corresponding to (for example) the type
data T = C1 T11 T12 | C2 T21 T22
should be isomorphic to the domain
[[T]] = lift (([[T11]] * [[T12]]) + ([[T21]] * [[T22]]))
where * is cartesian product, + is disjoint sum (e.g., X+Y = {(1,x) |
x in X} union {(2,y) | y in Y}, and 'lift' adds the bottom element to
the domain.
So here, C1 and C2 correspond to the two sides of the disjoint sum,
meaning you can tell them apart.
Hope that helps?
- Benja
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Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 20:00:05 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 30. Dezember 2007 18:16 schrieb Cristian Baboi: A simple question: Can you write the value of x to a file where x = (1:x) ? Not in finite time and space :) I used the word 'value' by mistake. A notation of the value of x. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 20:24:23 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 30. Dezember 2007 19:04 schrieb Cristian Baboi: On Sun, 30 Dec 2007 20:00:05 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 30. Dezember 2007 18:16 schrieb Cristian Baboi: A simple question: Can you write the value of x to a file where x = (1:x) ? Not in finite time and space :) I used the word 'value' by mistake. A notation of the value of x. I suppose let x = 1:x in x is not what you're after? Yes! Can Haskell do the same ? I mean this: module Module where a= let x=1:x in x main = do something to write a (a notation for a) to file The function must work if one change a to let x=2:x in x, let x=1:2:3:x and variations on the same theme. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sun, 30 Dec 2007 21:07:44 +0200, Daniel Fischer [EMAIL PROTECTED] wrote: Am Sonntag, 30. Dezember 2007 19:31 schrieb Cristian Baboi: I mean this: module Module where a= let x=1:x in x main = do something to write a (a notation for a) to file The function must work if one change a to let x=2:x in x, let x=1:2:3:x and variations on the same theme. Can Java, C? I think most of us can write a C program that can print circular lists. What do you mean by 'notation'? I might have used the wrong word. Can you write the NUMBER 1 on a piece of paper ? I think you cannot. You must use some encoding for it. The same thing with let x = x in x. How can one print numbers in Haskell ? I don't understand. When I say print (2+4), why 6 is printed and not 10 - 4 ? Would main = do txt - readFile Module.hs let definitions = parseModule txt case lookup a definitions of Nothing - putStrLn No definition for a Just rhs - writeFile Notation.hs (prettyprint rhs) satisfy you? Yes, but there is a little problem with 'txt- readFile Module.hs and case lookup a ... ' You see, I don't know if I must print a or b. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
Thank you. data Something = This | S Something ppp :: Something - String ppp This = ppp (S x) = 'S':(ppp x) How can I prevent one to pass 'let x = S x in x' to ppp ? On Sun, 30 Dec 2007 23:25:19 +0200, Chaddaï Fouché [EMAIL PROTECTED] wrote: 2007/12/30, Chaddaï Fouché [EMAIL PROTECTED]: 2007/12/30, Cristian Baboi [EMAIL PROTECTED]: A simple question: Can you write the value of x to a file where x = (1:x) ? Yes, but you'll have to write it yourself, because Haskell can't decide by itself that this value is infinite and try to print it with a recursive definition, if it could do that, it could decide the halting problem and that's not possible. Sorry, I'll try to be more precise since Cristian is pretty sticky with details when he can use them in its sense... In the particular case of x = (1:x), it can be detected that the structure is circular in memory and so infinite, but in the general case (for example if you replace (1:x) by (1:f x)) it can't be decided if the value is finite or infinite. You can't do a printer that do what you want in Haskell itself but you could probably do it with a VM or with compiler support, but it would be extremely tricky, also it would _always_ use the running form, even if this form isn't the optimal for storage (for example a filter of a huge list that results in an empty list would take enormously more place than with a normal printer). Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
How about a :: Something a = let x = x in x ppp a On Mon, 31 Dec 2007 09:11:12 +0200, Cristian Baboi [EMAIL PROTECTED] wrote: Thank you. data Something = This | S Something ppp :: Something - String ppp This = ppp (S x) = 'S':(ppp x) How can I prevent one to pass 'let x = S x in x' to ppp ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sat, 29 Dec 2007 01:42:54 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: Here is how I want print to be in Haskell print :: (a-b) - (a-b) with print = id, but the following side effect: - I want to call the print function today, and get the value tomorrow. Sorry, simply couldn't resist: Put a very long timing loop in the middle. Well, that kind of loop won't work. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sat, 29 Dec 2007 02:08:14 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: How can one make portable dynamic libraries then ? If by portable you mean, works on /both/ OSs, then GHC-specific should be portable enough for you... By portable I mean: works on the same machine, with the same OS, but with different Haskell implementation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Sat, 29 Dec 2007 13:01:44 +0200, Cristian Baboi [EMAIL PROTECTED] wrote: On Sat, 29 Dec 2007 02:08:14 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: How can one make portable dynamic libraries then ? If by portable you mean, works on /both/ OSs, then GHC-specific should be portable enough for you... By portable I mean: works on the same machine, with the same OS, but with different Haskell implementation. I should have used compatible. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Sending bottom to his room
In The Implementation of Functional Programming Languages by S.P. Jones, section 2.5.3, page 32 it is written: Eval [[*]] a b = a x b Eval [[*]] _|_ b = _|_ Eval [[*]] a _|_ = _|_ but in section 2.5.2 it is said that _|_ is an element of the value domain. What business does it have on the left side of the '=' ? Can you help me send him to his room ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Sending bottom to his room
Sorry, I think a and b are from the value domain. On Sat, 29 Dec 2007 13:14:09 +0200, Cristian Baboi [EMAIL PROTECTED] wrote: In The Implementation of Functional Programming Languages by S.P. Jones, section 2.5.3, page 32 it is written: Eval [[*]] a b = a x b Eval [[*]] _|_ b = _|_ Eval [[*]] a _|_ = _|_ but in section 2.5.2 it is said that _|_ is an element of the value domain. What business does it have on the left side of the '=' ? Can you help me send him to his room ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Sending bottom to his room
On Sat, 29 Dec 2007 13:30:03 +0200, Luke Palmer [EMAIL PROTECTED] wrote: On Dec 29, 2007 11:14 AM, Cristian Baboi [EMAIL PROTECTED] wrote: In The Implementation of Functional Programming Languages by S.P. Jones, section 2.5.3, page 32 it is written: Eval [[*]] a b = a x b Eval [[*]] _|_ b = _|_ Eval [[*]] a _|_ = _|_ but in section 2.5.2 it is said that _|_ is an element of the value domain. What business does it have on the left side of the '=' ? I don't know the book you're talking about, but I suspect that this is not a definition of a function in a language, but rather the denotational semantics for a function. Yes. Eval is the thing that do that. Just as mathematics is allowed to categorize all turing machines into two categories (those that halt and those that do not), even though to actually do this is impossible, so too can mathematics talk about what a function returns when given _|_, even though it is impossible in general to know when you actually do have _|_ or you're just waiting for a value. What confused me is the Eval seems to be defined by recursion, but maybe it is not. It would have been clear if it was written Eval [[*]] env = x where x is extended to handle _|_ The recursivity I was talking about is: Eval([[\x.E]], env) a = Eval([[E]], env[x=a]) Eval([[E1 E2]],env) = Eval([[E1]],env) (Eval([[E2]],env)) It appears as if lambda calculus is defined by lambda calculus. These are equations that Eval must satisfy, but the text call '=' 'define' ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Sending bottom to his room
On Sat, 29 Dec 2007 16:01:51 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Cristian Baboi [EMAIL PROTECTED] wrote: It appears as if lambda calculus is defined by lambda calculus. Yes. id (lambda calculus) = lambda calculus. You might try to point back to yourself when being asked who you are to see the advantage of this technique. The next question is if id is well defined. There is such a function ? How many of them ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
Thank you. It sounds like a limit. xn -- x for n -- :-) How can I get that maximal value when I start from a non maximal one ? [1 .. ] and x=1:x are maximal ? On Fri, 28 Dec 2007 11:21:36 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: How can I test that partial order in Haskell ? You can't. It's kinda internal. More specifically, if you try to compute a value, which is not maximal in this order, you'll get an error or loop forever. But you still can use such values in your programs if you don't try to compute them. The point is, maximal values are the only interesting ones, and all others are just approximations. In fact, you only have to worry about non-maximal values when you use recursion. The fundamental property of Haskell is that it computes the least fixed point. For example, if f maps (_|_) to (_|_), then let x = f x in x will evaluate to (_|_) - and you loop forever, see above. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 12:01:51 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: It sounds like a limit. xn -- x for n -- :-) Yeah, that's right. If xn is monotone, then I only have to worry when it is not bounded, right ? How can I get that maximal value when I start from a non maximal one ? Not a problem. For example, const 1 maps each value (even (_|_)) to the maximal value 1. [1 .. ] and x=1:x are maximal ? Can you give me some examples of non maximal ones ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
Thank you. This is what I understand so far: - the maximal values is what is usually called data - the non maximal are what is usually called code - in functional languages one can be given non maximal values (code) as input Is that right ? On Fri, 28 Dec 2007 12:01:51 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: It sounds like a limit. xn -- x for n -- :-) Yeah, that's right. How can I get that maximal value when I start from a non maximal one ? Not a problem. For example, const 1 maps each value (even (_|_)) to the maximal value 1. [1 .. ] and x=1:x are maximal ? Yes. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 12:01:51 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: [1 .. ] and x=1:x are maximal ? Yes. How can a Haskell implementation recognize a maximal value ? x=1:x for example. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 12:03:04 +0200, apfelmus [EMAIL PROTECTED] wrote: Cristian Baboi wrote: http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. Exactly, together with 10 and 11 (when the distributed processes are on different machines). But there is good reason that those things can't be done in Haskell. With extensional equality (two functions are considered equal if they yield the same result on every possible argument) number 5 is undecidable. I didn't know 5 as a number is undecidable :-) Similarly, there cannot be functions print :: (Int - Int) - String compile :: String - (Int - Int) with compile . print = id A print function based on an intensional representation (assembly, byte code, etc.) would have to distinguish extensionally equal functions print f ≠ print g although f = g which is not allowed. Ok. I understand that there cannot be pairs like (print,compile) above. But I was told that compile (quality number 10), can be defined. About print above, I do not intend to make a Haskell String from a Haskell function. This is why I said something about mixing levels of abstractions. More importantly, I don't quite understand your question. If you definitively need 9-12 for a practical problem at hand, then you may want to take a look at the functional language Clean http://clean.cs.ru.nl/ which is similar to Haskell but offers 9-12 in some form. How can be Clean similar to Haskell and at the same time satisfy 9-12 ? I tryed Clean. I like it very much. I have some silly little problems with it: - its only 32 bit - it doesn't support Unicode - I don't like those type adnotations. Look as ugly as Lisp () to me. - it's not free - the linux version seems abandoned. - there are some issues with # expressions (let before) that I don't fully understand - the IO seems easyer in Haskell than in Clean Otherwise I like it better than Haskell. In all other cases, an email thread is not a good (often not even successful) way to get a coherent world view on Haskell (or on something else) since this necessarily involves nitpicking philosophical questions. In my experience, interrogating one person in real-time in audio and interrogating books are the best ways to do that. In my experience, different people have different world views, and one cannot get that from books only. Concerning books, maybe The Haskell Road to Logic, Maths and Programming http://www.cwi.nl/~jve/HR is for you. Thank you. More books on http://haskell.org/haskellwiki/Books You don't have to buy them, borrow them from a library. I'l try borrow them when I'll be visiting Munich next year, but I don't think I'll have enough time to read them. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
Here is how I want print to be in Haskell print :: (a-b) - (a-b) with print = id, but the following side effect: - I want to call the print function today, and get the value tomorrow. On Fri, 28 Dec 2007 12:03:04 +0200, apfelmus [EMAIL PROTECTED] wrote: Cristian Baboi wrote: http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. Exactly, together with 10 and 11 (when the distributed processes are on different machines). But there is good reason that those things can't be done in Haskell. With extensional equality (two functions are considered equal if they yield the same result on every possible argument) number 5 is undecidable. Similarly, there cannot be functions print :: (Int - Int) - String compile :: String - (Int - Int) with compile . print = id A print function based on an intensional representation (assembly, byte code, etc.) would have to distinguish extensionally equal functions print f ≠ print g although f = g which is not allowed. Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
But I guess it won't work because the compiler will optimize it and the call will disappear. On Fri, 28 Dec 2007 14:58:53 +0200, Cristian Baboi [EMAIL PROTECTED] wrote: Here is how I want print to be in Haskell print :: (a-b) - (a-b) with print = id, but the following side effect: - I want to call the print function today, and get the value tomorrow. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 15:29:00 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: take 5 [1,2,3,4,5,undefined] (What would GHCi print? [1,2,3,4,5] Then please square that with your comment that we don't care how much of the list gets evaluated.) I don't care if it evaluates it, reads it's answer from some internal table, or consult it's lawyer. I just want it to agree with my reasoning about complete partial orders; that's all I need. I don't know how to explain this clearer. This purity of Haskell will allways interfere with practical matters that cannot be described in Haskell. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 16:10:15 +0200, Jules Bean [EMAIL PROTECTED] wrote: It won't work because haskell functions can't have side-effects. I'm not quite sure what you're trying to say here. Jules What I am trying to say is: - storing something to disk does not mean one must have a function to convert that something to a Haskell String - I want to be able to use a function defined in one process in other process even if the two does not run at the same time Cristian Baboi wrote: But I guess it won't work because the compiler will optimize it and the call will disappear. On Fri, 28 Dec 2007 14:58:53 +0200, Cristian Baboi [EMAIL PROTECTED] wrote: Here is how I want print to be in Haskell print :: (a-b) - (a-b) with print = id, but the following side effect: - I want to call the print function today, and get the value tomorrow. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 16:34:23 +0200, apfelmus [EMAIL PROTECTED] wrote: Jules Bean wrote: Cristian Baboi wrote: But I guess it won't work because the compiler will optimize it and the call will disappear. I'm not quite sure what you're trying to say here. (That's the world view on Haskell issue I mean. I know this feeling with physics: what the heck are they talking about, this is just mathematically wrong, ill-defined or otherwise void of contents?) Let me ask you 3 simple questions. Can one use Haskell to make dynamically linked libraries (DLL on Windows, so on Linux) ? If yes, what is in them ? If not, why not ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 18:32:05 +0200, Jules Bean [EMAIL PROTECTED] wrote: Cristian Baboi wrote: Let me ask you 3 simple questions. Can one use Haskell to make dynamically linked libraries (DLL on Windows, so on Linux) ? The short answer is yes. The long answer is that this is not a feature of haskell, but rather a feature of certain programs, which happen to be mostly but not entirely written in haskell, in particular, the haskell compiler. GHC can produce dynamically linked libraries. What you choose not to notice is the context in which I asked these questions. The context is: Haskell functions as first-class objects. What I am interested in the first place is dynamically linked libraries written in Haskell and used in Haskell. The interface with other languages like C come second. If yes, what is in them ? Object code, mostly. Sometimes a little data. What is the definition of an entry point in Haskell ? What is the semantics of those entry points ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 18:27:29 +0200, Achim Schneider [EMAIL PROTECTED] wrote: Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Freitag, 28. Dezember 2007 07:49 schrieben Sie: On Thu, 27 Dec 2007 18:19:47 +0200, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Donnerstag, 27. Dezember 2007 16:34 schrieb Cristian Baboi: I'll have to trust you, because I cannot test it. let x=(1:x); y=(1:y) in x==y . I also cannot test this: let x=(1:x); y=1:1:y in x==y In these examples, x and y denote the same value but the result of x == y is _|_ (undefined) in both cases. So (==) is not really equality in Haskell but a kind of weak equality: If x doesn’t equal y, x == y is False, but if x equals y, x == y might be True or undefined. Thank you. I can only notice that y always has an even number of 1, which is not the case for x :-) Both have an infinite number of 1. Why do you say “always”? It seems that you think of x and y as “variables” whose values change over time. This is not the case. They both have a single value for all time: the infinite list consisting only of 1. Does that then mean that [1] = [1,1] No. If you try to learn Haskell don't listen to me. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Wikipedia on first-class object
On Fri, 28 Dec 2007 19:18:33 +0200, ChrisK [EMAIL PROTECTED] wrote: This thread is obviously a source of much fun. I will play too. Well, it starts with Wikipedia ... :-) What is the definition of an entry point in Haskell ? Haskell does not have such a concept. At all. An implementation may have such a concept. Then a Haskell module know nothing about them. Most people on this list define Haskell as any attempt at an implementation of one of the standards which define Haskell, most recently the Hakell 98 standard. This can be nhc / yhc / ghc / hugs / winhugs / helium / jhc. Some of these compile to native code, some compile to byte code for a virtual machine. If an implementation can compile separately, then it might support dynamic libraries. If so then a specific version of that compiler will define its own implementation specific concept of an entry point. How can one make portable dynamic libraries then ? What is the semantics of those entry points ? It depends. For recent ghc versions, see its user manual: http://haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#ffi-library http://haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html The conclusion: Portable Haskell Dynamic libraries does not exists. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Function Stores
Here is what I was able to come up with: module Store where data FStore1 a b = Empty1 | FS1 ( a-b , FStore1 a b ) store1 :: (a-b) - (FStore1 a b) store1 f = let x = FS1 (f, x) in x data FStore2 a b = Empty2 | FS2 ( a-b , FStore2 a b, FStore2 a b ) store2 :: (a-b) - (FStore2 a b) store2 f = let x = FS2 (f,x,x) in x First I tryed store1, but after a while it went out of business. Second I tryed other business model with store2. Same result. Then I tryed to make a street of stores. After a while, they all went out of business. Then I tryed to make several streets of stores. After a while they all went out of business. Then I tryed to make an infinite street of stores. After a while they all went out of business. Then, following some rumors that there is a store on that street with some function in stock that might be willing to give it away, I noticed that the street disappeared, and I cannot go to that store. Can you help me find it ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: Re: [Haskell-cafe] Wikipedia on first-class object
--- Forwarded message --- From: Cristian Baboi [EMAIL PROTECTED] To: Yitzchak Gale [EMAIL PROTECTED] Cc: Subject: Re: [Haskell-cafe] Wikipedia on first-class object Date: Thu, 27 Dec 2007 12:21:44 +0200 I think I found the answer to why functions cannot be written to files. This is by design. Haskell must be free. Enabling writing functions to files, might make it ilegal in some countries. :-) On Thu, 27 Dec 2007 11:10:21 +0200, Yitzchak Gale [EMAIL PROTECTED] wrote: Like any type, only certain operations make sense on functions. Strings can be compared to each other for equality and written to a disk, and you can take the logarithm of a float, but none of those operations make sense for functions. In particular, two functions are equal only if they produce the same value for every input, and in general it is impossible for a computer to check that. -Yitz Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
Thinking about files and types, I recalled that in Pascal files must have types. On Thu, 27 Dec 2007 12:40:22 +0200, Yitzchak Gale [EMAIL PROTECTED] wrote: I'm not sure that in Haskell one can say that storing a value of some type to disk is an operation defined on that type. It is. For example: hPutStr :: Handle - String - IO () The result type of this function is IO (), which means an IO action. In this case, the semantics of the action are that, when performed, it writes the bytes into a file. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 14:37:51 +0200, Yitzchak Gale [EMAIL PROTECTED] wrote: I wrote: Like any type, only certain operations make sense on functions... Yes, but one can store the result of an operation to disk except in the particular case the result happen to be a function. No, you can only store the result of an operation to disk in the particular case that the result type represents a list of bytes. Otherwise, you have to serialize it first... But it is not clear at all how you could define a general serialization method for functions. Isn't that confusing levels of abstractions ? Of course functions are bytes, 'cause they are already stored as bytes in RAM. That is just the point. A function in Haskell is an abstraction, not bytes in RAM. The compiler might implement the same function in several places, with different bytes in each place. Or it might decide to combine it into other functions, and not store any bytes in RAM at all for this function. The function itself represents a way of doing a calculation. It is not an object that can do the calculation. I think you try to say that the time cannot be stored. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 14:37:51 +0200, Yitzchak Gale [EMAIL PROTECTED] wrote: I wrote: Like any type, only certain operations make sense on functions... Yes, but one can store the result of an operation to disk except in the particular case the result happen to be a function. No, you can only store the result of an operation to disk in the particular case that the result type represents a list of bytes. Otherwise, you have to serialize it first... But it is not clear at all how you could define a general serialization method for functions. Isn't that confusing levels of abstractions ? Of course functions are bytes, 'cause they are already stored as bytes in RAM. That is just the point. A function in Haskell is an abstraction, not bytes in RAM. The compiler might IMPLEMENT the same function in several places, with different bytes in each place. Or it might decide to combine it into other functions, and not store any bytes in RAM at all for this function. See ? The function itself represents a way of doing a calculation. It is not an object that can do the calculation. Then trees of functions are ... I'm not sure that in Haskell one can say that storing a value of some type to disk is an operation defined on that type. It is. For example: hPutStr :: Handle - String - IO () And this is a property of the type String ? The function hPutStr appears in the definition of the type String ? Ah, you are thinking of operation on a type in the OOP sense. Sorry, I wasn't clear. When I said only certain operations make sense on each type, I just meant that there are only certain things you can do with the type. In Haskell, the things you can do with a type are the functions you can define that mention that type in their signature. How can one define in the language STORAGE of things ? Storage of numbers for example ? You said that the TYPE of a function forbids me somehow to store it in a file. Now I understand that not the type forbids me, but the lack of a function with apropriate types. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 14:02:36 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Comparing functions is certainly possible in Haskell, but there's no standard function that does it. If course, it might not terminate, but the same is true for many other comparable objects in Haskell, e.g., infinite lists (which are isomorphic to Nat-T). The list [1 .. ] is a single value in Haskell ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: Re: [Haskell-cafe] Wikipedia on first-class object
How about x below: let x=(1:x) in x ? Is x a single value in Haskell ? --- Forwarded message --- From: Cristian Baboi [EMAIL PROTECTED] To: Lennart Augustsson [EMAIL PROTECTED] Cc: haskell-cafe@haskell.org haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Wikipedia on first-class object Date: Thu, 27 Dec 2007 16:08:58 +0200 On Thu, 27 Dec 2007 14:02:36 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Comparing functions is certainly possible in Haskell, but there's no standard function that does it. If course, it might not terminate, but the same is true for many other comparable objects in Haskell, e.g., infinite lists (which are isomorphic to Nat-T). The list [1 .. ] is a single value in Haskell ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 14:42:37 +0200, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello Cristian, Thursday, December 27, 2007, 12:19:08 PM, you wrote: Yes, but one can store the result of an operation to disk except in the particular case the result happen to be a function. how can values of type T be saved to disk? I don't know. I'm a beginner in Haskell, and I down't know about T. You mean they cannot ? I was under the impression that the purpose of computers cannot be fulfiled if we cannot get the result of computations out of the computers. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
Good to know. I intended to use Haskell for algorithms, but it seems it is not so good at them. On Thu, 27 Dec 2007 17:52:19 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 27 Dec 2007, at 9:47 AM, Cristian Baboi wrote: I don't know. I'm a beginner in Haskell, and I down't know about T. You mean they cannot ? I was under the impression that the purpose of computers cannot be fulfiled if we cannot get the result of computations out of the computers. Haskell is not a computer programming language; Haskell implementations are not required to run on computers. Haskell is a formal notation for computation (completely unrelated to the Von Neuman machine sitting on your desk). It can be implemented on Von Neuman machines, because they are still universal Turing machines, but it is /not/ a radical attack on the problem of programming peripherals! I suppose it can run on pebbles. Any language can be emulated on pebbles; unlike most languages, Haskell can be compiled directly to them. jcc I know, and in this case one doesn't need IO. The result is a nice collection of asorted pebbles. Which is why Haskell treats IO as a domain specific language. jcc Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 16:50:10 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Absolutly. Every expression in Haskell denotes a value. Now, we've not agreed what value means, but to me it is a value. :) It is one value, or several ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[4]: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 16:12:04 +0200, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello Cristian, Thursday, December 27, 2007, 3:51:17 PM, you wrote: Yes, but one can store the result of an operation to disk except in the particular case the result happen to be a function. how can values of type T be saved to disk? I don't know. I'm a beginner in Haskell, and I down't know about T. here T is any type. you said that values of ANY TYPE can be saved to disk, so show us the way The way is toward west. I said except functions. I'll add instances of IO. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re: [Haskell-cafe] Wikipedia on first-class object
I'll have to trust you, because I cannot test it. let x=(1:x); y=(1:y) in x==y . I also cannot test this: let x=(1:x); y=1:1:y in x==y On Thu, 27 Dec 2007 17:29:12 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: One value. One infinite value. On Dec 27, 2007 3:53 PM, Cristian Baboi [EMAIL PROTECTED] wrote: On Thu, 27 Dec 2007 16:50:10 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Absolutly. Every expression in Haskell denotes a value. Now, we've not agreed what value means, but to me it is a value. :) It is one value, or several ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK part001.htm - is OK http://www.eset.com Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
I don't know. I'm a beginner in Haskell, and I down't know about T. You mean they cannot ? I was under the impression that the purpose of computers cannot be fulfiled if we cannot get the result of computations out of the computers. Haskell is not a computer programming language; Haskell implementations are not required to run on computers. Haskell is a formal notation for computation (completely unrelated to the Von Neuman machine sitting on your desk). It can be implemented on Von Neuman machines, because they are still universal Turing machines, but it is /not/ a radical attack on the problem of programming peripherals! I suppose it can run on pebbles. Any language can be emulated on pebbles; unlike most languages, Haskell can be compiled directly to them. jcc I know, and in this case one doesn't need IO. The result is a nice collection of asorted pebbles. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 17:39:25 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 27 Dec 2007, at 6:51 AM, Cristian Baboi wrote: On Thu, 27 Dec 2007 14:42:37 +0200, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello Cristian, Thursday, December 27, 2007, 12:19:08 PM, you wrote: Yes, but one can store the result of an operation to disk except in the particular case the result happen to be a function. how can values of type T be saved to disk? I don't know. I'm a beginner in Haskell, and I down't know about T. You mean they cannot ? I was under the impression that the purpose of computers cannot be fulfiled if we cannot get the result of computations out of the computers. Haskell is not a computer programming language; Haskell implementations are not required to run on computers. Haskell is a formal notation for computation (completely unrelated to the Von Neuman machine sitting on your desk). It can be implemented on Von Neuman machines, because they are still universal Turing machines, but it is /not/ a radical attack on the problem of programming peripherals! I suppose it can run on pebbles. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 18:45:23 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: On 27 Dec 2007, at 9:57 AM, Cristian Baboi wrote: Good to know. I intended to use Haskell for algorithms, but it seems it is not so good at them. Very sad. The entire point of Haskell is that it allows the user to transcend the algorithm as a way of expressing computations. I hope someday you may understand Haskell, rather than just criticizing it. I'm begining to understand it. Criticizing it's just a tehnique to allow me to understand it better. This is what I understood: - there is no distinction of data from functions. This seem more like a matter of definiton: what I call X, the X + Y or just X. - functions can be manipulated the same way as data. This does not sound right. - functions can be manipulated as easy as data. This seems better. - functional programming is declarative. One may take a picture of all those pebbles, but their arrangemant does not make sense to him because no part of it resemble the original description. - one cannot print things that cannot be traversed in a sequential way The last two seems to be in contradiction. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 18:13:57 +0200, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Donnerstag, 27. Dezember 2007 16:57 schrieb Cristian Baboi: On Thu, 27 Dec 2007 17:52:19 +0200, Jonathan Cast Which is why Haskell treats IO as a domain specific language. Good to know. I intended to use Haskell for algorithms, but it seems it is not so good at them. Why is I/O needed for algorithms? And the fact that I/O is embedded into Haskell as a kind of a domain specific language doesn’t mean that Haskell is bad at I/O. As Simon Peyton Jones put it: Haskell is the world’s finest imperative programming language. An algorithm is a finite recipe that allow one to solve a class of problems in a mechanical way. To be able to use the algorithm one must be able to read it. To be able to communicate the algorithm, one must be able to write it. When I mentioned that IO is not needed for pebbles, I was joking. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 18:14:53 +0200, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Donnerstag, 27. Dezember 2007 15:53 schrieb Cristian Baboi: On Thu, 27 Dec 2007 16:50:10 +0200, Lennart Augustsson [EMAIL PROTECTED] wrote: Absolutly. Every expression in Haskell denotes a value. Now, we've not agreed what value means, but to me it is a value. :) It is one value, or several ? It is one value with parts that are values themselves. It is one value or a SET of values ? What are the parts ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 18:19:47 +0200, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Donnerstag, 27. Dezember 2007 16:34 schrieb Cristian Baboi: I'll have to trust you, because I cannot test it. let x=(1:x); y=(1:y) in x==y . I also cannot test this: let x=(1:x); y=1:1:y in x==y In these examples, x and y denote the same value but the result of x == y is _|_ (undefined) in both cases. So (==) is not really equality in Haskell but a kind of weak equality: If x doesn’t equal y, x == y is False, but if x equals y, x == y might be True or undefined. Thank you. I can only notice that y always has an even number of 1, which is not the case for x :-) Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 17:35:54 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: Only on Von Neuman machines. Haskell implementations are not required to run on Von Neuman machines. That's why the language is called functional. (Imperative languages, by contrast, are just abstractions of the underlying Von Neuman architecture, which is probably the source of your confusion). Can you tell me what is it that make a language imperative ? When I learned about formal grammars and languages, there was no discussion about this. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Wikipedia on first-class object
On Thu, 27 Dec 2007 17:39:25 +0200, Jonathan Cast [EMAIL PROTECTED] wrote: Haskell is not a computer programming language; Haskell implementations are not required to run on computers. Haskell is a formal notation for computation (completely unrelated to the Von Neuman machine sitting on your desk). It can be implemented on Von Neuman machines, because they are still universal Turing machines, but it is /not/ a radical attack on the problem of programming peripherals! How do you call that thing that implement Haskell ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] CAF's in Haskell
On Wed, 26 Dec 2007 17:16:22 +0200, Neil Mitchell [EMAIL PROTECTED] wrote: Hi, Are CAF's specified in the Haskell report? I couldn't find them mentioned. If not, why do all Haskell compilers support them? Is there some paper which persuaded everyone they were a good idea, or some reference I could look up? I found this in the wiki http://www.haskell.org/haskellwiki/Constant_applicative_form ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Wikipedia on first-class object
http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing and Referential transparency excuse
On Tue, 25 Dec 2007 11:05:59 +0200, Derek Elkins [EMAIL PROTECTED] wrote: On Mon, 2007-12-24 at 11:15 +0200, Cristian Baboi wrote: How can I define a function to do the inverse operation ? g :: String - ( a - b ) This time I cannot see how referential transparency will deny it. What's the excuse now ? Referential transparency has nothing to with this. The problem here is parametricity. However, if you give a non-parametric type (or add appropriate class constraints on a and b, or add (phantom) type information to the input), e.g. String - (Int - Int) then there is no trouble at all. Thank you. I finished reading the Haskell Language Report and I noticed that the class Show and Read are toys. I'll try to define some String - (a - b) functions some other time. I wonder if parametricity is the only problem. I guess that I'll have some problem with the arity of the result, but currying may help. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Printing and Referential transparency excuse
While reading the Haskell language report I noticed that function type is not an instance of class Read. I was told that one cannot define them as an instance of class Show without breaking referential transparency or printing a constant. f :: (a-b)-String f x = bla bla bla How can I define a function to do the inverse operation ? g :: String - ( a - b ) This time I cannot see how referential transparency will deny it. What's the excuse now ? I'm at the begining of chapter 7, but I have the feeling I'll not find the answer in there. Thank you. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Printing and Referential transparency excuse
On Mon, 24 Dec 2007 11:27:11 +0200, apfelmus [EMAIL PROTECTED] wrote: Cristian Baboi wrote: How can I define a function to do the inverse operation ? g :: String - ( a - b ) This time I cannot see how referential transparency will deny it. What's the excuse now ? The new excuse is that a better name for g is full-fledged-compiler :: String - (Int - Int) (the function returned by g better not have a polymorphic type). Which programming language should the argument String be written in? Cobol, what else ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Functions are first class values in C
Let me show you an example to prove it.
The example is limited to composition of unary functions defined on int
u::Int-Int
v::Int-Int
o ::(Int-Int)-(Int-Int)-(Int-Int)
o u v= \x-u(v(x))
#include stdio.h
#include conio.h
#include functional.h
int f1(int x){
return x+x;
}
int f2(int x){
return 2*x;
}
int g1(int x){
return x+1;
}
int g2(int x){
return x-1;
}
#define P1 P0 int main(){
#define P2 P1 printf(%d,%d,%d\n,2,
#define P3 O(f1,f2,P2)(2),
#define P4 O(g1,g2,P3)(3));
#define P5 P4 getch();
#define P6 P5 }
MAIN P6
Here is the file functional.h
#define FUNC2(x,y) x##y
#define FUNC1(x,y) FUNC2(x,y)
#define FUNC(x) FUNC1(x,__COUNTER__)
#define COMP(c,f,g,p) \
int c (int x) { return f(g(x)); }; \
p \
c
#define O(f,g,p) COMP( FUNC(a), f, g, p)
#define P0
#define MAIN
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Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 14:28:56 +0200, Paul Johnson [EMAIL PROTECTED] wrote: Cristian Baboi wrote: Let me show you an example to prove it. This reminds me of arguments in the late 80s and early 90s that C could do OO programming via function pointers, so there was no need for OO languages. What is Objective C ,if not just C with some syntactic sugar ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: Re: [Haskell-cafe] Functions are first class values in C
--- Forwarded message --- From: Cristian Baboi [EMAIL PROTECTED] To: Jules Bean [EMAIL PROTECTED] Cc: Subject: Re: [Haskell-cafe] Functions are first class values in C Date: Sat, 22 Dec 2007 15:58:50 +0200 On Sat, 22 Dec 2007 14:09:13 +0200, Jules Bean [EMAIL PROTECTED] wrote: Cristian Baboi wrote: Let me show you an example to prove it. That's not C. That's the C preprocessor, which is a textual substitution macro language. Well, the preprocessor is part of the language in a way. These two come together. Macros certainly aren't first class (you can't pass a macro to a function, only its expansion). In Haskell I cannot pass a function to a function, only its expansion. C does support function pointers, which are something like first class functions. The main things C lacks which people associate with true first-class function is: The ability to construct anonymous/local functions. If you look at the example you will see I've done that. The ability to capture local variables and return a function with some variables bound. If I can construct anonymous functions and constants, I can construct functions with some variables bound. The ability to write type-safe functions with polymorphic arguments. I didn't know this must be a property of first-class functions. C is staticaly typed, so type errors will be detected. Haskell is just C with some syntactic sugar :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 14:55:44 +0200, Peter Verswyvelen [EMAIL PROTECTED]
wrote:
Before I knew Haskell, the OO community started to embrace the concept
of interfaces more and more (aka purely abstract classes). Furthermore
in OO, many bugs are caused IMO by keeping track of mutable state and
caches (which is often premature optimization). So for complicated
tasks, I tended to use more and more immutable objects, e.g. objects
that could be constructed once, but not mutated. And then I noticed that
it was often not needed to precompute all the values that got passed to
the constructor, so I added C# properties that computed the inner
cached values once, lazily.
Lazy constant in C:
int C1 (){
return 7;
}
C1 is computed only when you apply the operator () to it.
IMO Haskell embraces the above ideas and much more, with the difference
it encapsulates these ideas nicely and concisely, so you need only a
fraction of the lines of code :)
This is why we have syntactic sugar :)
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Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 16:25:26 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: That's not C. That's the C preprocessor, which is a textual substitution macro language. Well, the preprocessor is part of the language in a way. These two come together. No. In fact, these are even two different programs, see man cpp. These two different programs come together. Macros certainly aren't first class (you can't pass a macro to a function, only its expansion). In Haskell I cannot pass a function to a function, only its expansion. What do you mean by expansion? Can you clarify this? f1=\x-x+1 f2=\x-2*x g=\x-x.f1 h=\x-x.(\x-x+1) h is g C does support function pointers, which are something like first class functions. The main things C lacks which people associate with true first-class function is: The ability to construct anonymous/local functions. If you look at the example you will see I've done that. No. Your compose macro is not a function; for example, you can't use it as an argument to itself (which is easy in Haskell: (.)(.)) Ok. The ability to capture local variables and return a function with some variables bound. If I can construct anonymous functions and constants, I can construct functions with some variables bound. See above. You can't. Ok. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 16:26:18 +0200, Miguel Mitrofanov
[EMAIL PROTECTED] wrote:
Lazy constant in C:
int C1 (){
return 7;
}
C1 is computed only when you apply the operator () to it.
But that's not the point of lazyness. Lazy value is computed only ONCE.
Ok. I guess I cannot be sure I'll call C1 only once.
How about this
int C1(){
static c1=-;
if(c1==-){
c1=7
}
return c1
}
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Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 16:55:08 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: In Haskell I cannot pass a function to a function, only its expansion. What do you mean by expansion? Can you clarify this? f1=\x-x+1 f2=\x-2*x g=\x-x.f1 h=\x-x.(\x-x+1) h is g Not clear. Try starting with function's expansion is... function's expansion is ... just like macro expansion. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 16:48:51 +0200, Peter Verswyvelen [EMAIL PROTECTED]
wrote:
Cristian Baboi wrote
Lazy constant in C:
int C1 (){ return 7; }
Not really, this is not lazy, since it always recomputes the value 7.
To have lazy values in C you would have to do something like:
struct ValueInt
{
int IsComputed;
union
{
int Value;
struct
{
int (*ComputeValue)(void *args);
void* Args;
};
};
};
int GetLazyInt (ValueInt* v)
{
if( !v-IsComputed )
{
v-Value = v-ComputeValue(v-Args);
v-IsComputed = true;
}
return v-Value;
}
But this of course, is totally useless in C and very bulky. It's also
impossible to know when to call freemem on the Args (hence garbage
collection in FP), when *not* to use lazy values but strict values
instead (hence strictness analysis in FP), etc...
I know FP have automatic garbage collection.
I know FP compilers use strictness analysis.
In C++ one can isolate memory management in constructors and destructors.
There are C compilers that are also able to do some optimizations.
I must say I had the same opinion as you had for many many years. I
always thought functions as first class values where just function
pointers, so what is it these Haskell/FP guys are so excited about? But
if you dig deeper, you'll see the magic... Notice you will have to give
yourself some time; it is very hard to get out of the imperative blob.
E.g. I'm still being sucked into the imperative blob after my first year
of Haskell hacking :)
PS: As I'm relatively new to Haskell, don't take the above C code too
seriously; it certainly will not reflect the way a real Haskell system
works.
I am new to Haskell, but not new to declarative programming. I programmed
in Prolog for several years, and I tryed LISP, but I don't liked the LISP
syntax.
I don't take my C example seriously either.
The thing is I think that for a language to have first-class functions,
it must be homoiconic if I understand the terms correctly.
Have you tryed to write a Haskell program that manipulate Haskell programs
?
Please don't tell me that Haskell compiler is written in Haskell, because
there are C compilers written in C.
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Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 17:13:55 +0200, Philippa Cowderoy [EMAIL PROTECTED] wrote: function's expansion is ... just like macro expansion. No, it's not. Expanding variables (swapping f1 for \x-x+1) isn't the evaluation mechanism in haskell, g and h really are semantically equivalent values and we can't do actual computation just by expanding variables. Whereas expanding a macro is equivalent to beta-reduction (evaluating a function application), which isn't required before passing something in Haskell. We pass functions, not just their results. Here's a trivial example that does so: (\x - x) (\x - x) A lambda calculus classic that doesn't typecheck in Haskell: (\x - x x) (\x - x x) Feel free to try evaluating it! Thank you for your message. I tryed and this is what I've got: ERROR - cannot find show function for: *** Expression : (\x - x) (\x - x) *** Of type: a - a ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Fwd: Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 17:18:18 +0200, Philippa Cowderoy [EMAIL PROTECTED] wrote: The thing is I think that for a language to have first-class functions, it must be homoiconic if I understand the terms correctly. You're confusing functions with the terms that are used to define them. The terms aren't first-class, the functions are. This is intentional: the only way you can tell functions apart is if they give you different results for the same parameter. Otherwise, what you have isn't a function but a combination of a function and some extra structure. I also confuse numbers with the terms that are used to define them (like 1.2) I guess I have to study more about this. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functions are first class values in C
On Sat, 22 Dec 2007 17:25:34 +0200, Philippa Cowderoy [EMAIL PROTECTED] wrote: On Sat, 22 Dec 2007, Cristian Baboi wrote: On Sat, 22 Dec 2007 17:13:55 +0200, Philippa Cowderoy [EMAIL PROTECTED] wrote: Here's a trivial example that does so: (\x - x) (\x - x) A lambda calculus classic that doesn't typecheck in Haskell: (\x - x x) (\x - x x) Feel free to try evaluating it! Thank you for your message. I tryed and this is what I've got: ERROR - cannot find show function for: *** Expression : (\x - x) (\x - x) *** Of type: a - a Yep, that's because while it can evaluate it down to (\x - x) your interpreter doesn't know how to print the result. You can demonstrate that it works by then passing in something to that result though: ((\x -x) (\x - x)) 1 I know that. The reason the interpreter doesn't know how to print the result is because converting functions to strings doesn't make sense. Thank you. You'll have to evaluate the other one by hand. Don't spend too long with it though! Don't worry, I'm lazy too :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New to Haskell
On Tue, 18 Dec 2007 10:29:43 +0200, Miguel Mitrofanov
[EMAIL PROTECTED] wrote:
What I should have been told about upfront:
- the syntax for an expression
- the syntax for a block
Don't see your point.
The point is the syntax is introduced as transformation of layout form to
non layout form.
As a user, I just want to be able to spot the basic components of a source
file without thinking about transformation rules.
- the adhoc syntax rules (how to distinguish among a tuple and a
pharanthesized expression and how to find the start and end of a block
for
example )
Oh, that's pretty easy, parenthesized expression is not divided by a
comma.
Thanks! What is the end of a block ? What introduce new blocks ?
Is this legal (`plus`) x y ?
It's this a tuple ? ([a,b,c,d ]) ?
etc.
- what guarantees are made by the LANGUAGE that an IO action (such as
do
putStrLn Hello world ) is not performed twice
There are no such guarantees. If you write
a = putStrLn Hello world
main = do {a; a;}
then your putStrLn would be performed twice. IO actions are first-class
values, that's a feature, not a bug.
What guarantees that by running the main, the string Hello world will be
printed exactly twice ?
- the lambda expressions can be written (input) but cannot be printed
(output)
Yes, since two different lambda expressions can denote the same function.
I just want the sistem to be able to print one of these expressions !
Its this too much to ask ?
I find it very strange that I can write a lambda expresion, but the system
cannot.
No, pattern matching bounds variables; if you write case x of {aa -
...} then aa becomes a LOCAL variable for the case statement, and
shadows the global definition. The same applies to u and v in h, except
that in this case local variables shadow upper-level local variables.
Ok.
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Re: [Haskell-cafe] Re: New to Haskell
On Tue, 18 Dec 2007 11:56:36 +0200, Jon Fairbairn
[EMAIL PROTECTED] wrote:
Cristian Baboi [EMAIL PROTECTED] writes:
- the syntax for a block
Not sure what you mean by block.
do a - [1..10]
b - [3,4]
return (a,b)
is an expression... you can write that same expression as
do {a - [1..10]; b - [3,4]; return (a,b)} too.
I mean anything that you can put between { }, and between ;
- the adhoc syntax rules (how to distinguish among a tuple
and a pharanthesized expression
a tuple has commas in it. I'll grant that (x) not being a
1-tuple is a little ad-hoc, but there really is very little
ad-hockery in Haskell (and a 1-tuple behaves very much like
a plain value after all).
Is this ([1 ,2 ,3 ,4]) a tuple or what ?
It has commas in it!
- the fact that lambda expressions are not the same thing
as algebraic data values
It might help to know why you think they might be the same;
the syntax is different and the name is different...
Ah, just a thought, nothing more.
Lambda expressions are values, which is just data, after all.
Even C can apply a function variable to an argument (function pointers).
- what guarantees are made by the LANGUAGE that an IO action
(such as do putStrLn Hello world ) is not performed
twice
As has been pointed out, «do putStrLn Hello world» is an
expression that you can bind to a variable and use as many
times as you like. Incidentally, it means the same as
«putStrLn Hello World»; do connects a sequence of bindings
and expressions, so you don't need it if there's nothing to
be connected to.
Yes, but that was not the question.
What make you so sure it will be printed the exact number of times you
intended ?
- the lambda expressions can be written (input) but cannot
be printed (output)
This is a fundamental property of the language. A lambda
expression is programme and at runtime the system doesn't
know one lambda expression from another (all it can do with
one is apply it to something).
Even C can apply a function variable to an argument (function pointers).
What make Haskell different beside the lazy evaluation and mutable
variables things ?
The biggest problem for me, so far, is the last one.
I can't see how your example illustrates that, I'm afraid.
In a very strange way. Nevermind.
Here is some strange example:
What I don't understand is why I'm forced to use guards like
x==aa in cc, when aa is clearly bounded (is 7) and why in
function h, the bounded u and v become free variables in
the case expression.
I would have liked the language design to have permitted
case to pattern match against variables too, but the
question is, what would the syntax be? There was a fair bit
of discussion about this when the language was designed (and
since), but no-one could come up with a good way of doing
it. One aspect of it is this: we want
f 0 = 42
f x = 3*x
to work, and we want all function definitions to be
translated into the core language in the same way,
so you get
f = \a - case a of
0 - 42
x - 3*x
and given that, you can't have a variable on the LHS of -
do anything other than get bound to the value of the
expression in the case (a in the example). It's not just a
the top level, either:
f Nothing = 0
f (Just n) = n+1
just means
f = \v - case v of
Nothing - 0
Just n - n+1
so you can't have variables inside constructors do anything
but get bound at that point.
Thank you very much!
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Re: [Haskell-cafe] New to Haskell
On Tue, 18 Dec 2007 12:25:18 +0200, Miguel Mitrofanov [EMAIL PROTECTED] wrote: - the syntax for an expression - the syntax for a block Don't see your point. The point is the syntax is introduced as transformation of layout form to non layout form. As a user, I just want to be able to spot the basic components of a source file without thinking about transformation rules. Well, most users are. Are what ? Able to spot or thinking about ... Have you asked them all ? Is this legal (`plus`) x y ? Never tried to write this myself, it looks stupid. What else haven't you tried to write by know ? It's a kind of mirror, you know . then your putStrLn would be performed twice. IO actions are first-class values, that's a feature, not a bug. What guarantees that by running the main, the string Hello world will be printed exactly twice ? What kind of guarantees do you want? Written in blood. I just want the sistem to be able to print one of these expressions ! Its this too much to ask ? Yes, 'cause it means you want to embed almost all source code into the compiled program. So ? Information from NOD32 This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
