Re: [Haskell-cafe] [ANNOUNCE] Fmark markup language
The problem with that is that some people DO end some headings with a full stop; for them your special syntax is not natural. Markdown/ReST is already using the no syntax idea (e.g. compared to pre-wiki markup such a LaTeX or Texinfo), so he's simply trying to push this idea further. I suspect it'll be difficult. What if I want to use indentation to express quotation instead? I think this one is solvable: a paragraph that's more indented than the previous heading can be considered a quote. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Darcs fetches too little files
Albert Einstein said: Insanity: doing the same thing over and over again and expecting different results. I repeated the command today and it worked! So, did you expect the result to be different, or did you re-try just to confirm that it doesn't work? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell development in Mac OS X after Gatekeeper
I think, Apple has made their stance quite clear by releasing the command line dev tools: I'm not sure what you mean by that, but looking at the history of Apple devices, especially the recent history with iPad, iPhone, etc... it's pretty clear to me where this is headed: keep as tight a control on the machine as possible without alienating too many users. As time passes, the average users become accustomed to the idea that they shouldn't actually control their own device, which allows tightening the control yet a bit more. Cryptography and formal methods will eventually make it provably difficult to use your own device in any way other than the one Apple intended. Stefan PS: Of course, Apple is not the only company aiming for that, it's just the one leading the way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Misleading MVar documentation
I'll try another writeup. Does anyone know where the original papers for MVars might be? In my mind, full/empty variables date back to dataflow machines. But I don't know where I got this idea. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What's the motivation for η rules?
One way to look at it is that β rules are the application of an eliminator (e.g. function application) to its corresponding constructor (the lambda expression), whereas η rules correspond to the application of a constructor to its corresponding eliminator. E.g. λ y . (x y)= x if x then True else False = x (π₁ x, π₂ x) = x IOW there's no need for a motivation: those rules just appear naturally. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: GHC 7.0.1 developer challenges
And if it actually freezes your machine (as opposed to making it go really slowly), that's a kernel bug. Very true, tho sometimes the difference between a real freeze and just going very slow is pretty subtle. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Strict Core?
That said, I've been told that UHC's core language uses the ideas from Strict Core, and they have/had a student at Utretch (Tom Lokhorst) who was working on implementing optimisations like arity raising and deep unboxing for the language. Many/most implementations of ML-ish languages use a similar approach. E.g. SML/NJ's Flint internal language is fairly similar to Strict Core (except for the lack of memoization of zero-arg functions, since that's not very often useful in the context of a strict language like SML) and uses the same kind of wrapper+inlinling to perform most of its optimizations. Note that in my experience working on SML/NJ's optimizer (using wrapper+inlining), such an approach to code transformation doesn't care about types at all, and indeed most/all of what tacc-hs09.pdf presents is mostly unrelated to whether the language is typed: e.g. the uncurrying of the zipWith higher-order argument is performed by the inliner (when inlining the various wrappers) without paying any attention to types. So types do not enable the optimization of higher-order functions: it's the wrapper+inlining strategy that enables it. Admittedly, the use of a statically typed intermediate language makes it easier to catch bugs in such optimization phases. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: A rant against the blurb on the Haskell front page
Good start, if only the advanced were replaced with something more characteristic, like lazy, or statically typed. Which, BTW, both do not appear in the whole blurb, even though they are *the* characteristics of Haskell, lazyness being even something that sets it apart from most other languages. AFAIK laziness is a property of the major implementations of Haskell, but not really of the language itself. All I see in the Haskell report points at it being applicative, call by name, but nowhere does the report seem to mandate a lazy strategy. It's just that being purely functional implies that the compiler is free to use laziness. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Chuch encoding of data structures in Haskell
churchedBool :: t - t - t Important detail: the precise type is ∀t. t → t → t. encodeBool x = \t e - if x then t else e So the type of encodeBool should be: Bool → ∀t. t → t → t whereas Haskell will infer it to be ∀t. Bool → t → t → t which means that a given object can only be eliminated to one type. I.e. to make such an encoding really usable, you need deep polymorphism (which GHC supports just fine, but which is not part of the Haskell standard). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: US Patent for the idea of using Haskell to implement UAX #9
revealed a link to a US Patent (7120900) for the idea of implementing the Unicode Bidirectional Algorithm (UAX #9 http://www.unicode.org/reports/tr9) in Haskell, making use, as far as I can tell, of nothing more than the normal approach any functional programmer would use, namely separation of concerns etc. In which case the patent should be null and void since obvious ideas aren't patentable, AFAIK. Doesn't matter: you'd need to pay lawyers to defend yourself to convince a court that it is null and void. So even if you may end up winning in the end (which is far from obvious), you'll have wasted a lot of time, effort, and money. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Embedded funcional programming?
As a side comment, I haven't noticed any reaction in the Haskell/iPhone community about Apple's recent policy change. The stricter they make it, the better, since it hopefully gets us closer to the point where people will see that they should stay the heel away from any such handcuffs, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: instance Eq (a - b)
Why isn't there an instance Eq (a - b) ? I guess it's because even for those cases where it can be written, it will rarely be what you want to do, so it's better to require the programmer to explicitly request a function-comparison than to risk silently using such a costly operation when the programmer intended no such thing. While we're here, I'd be more interested in a dirtyfast comparison operation which could look like: eq :: a - a - IO Bool where the semantics is if (eq x y) returns True, then x and y are the same object, else they may be different. Placing it in IO is not great since its behavior really depends on the compiler rather than on the external world, but at least it would be available. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: If the local variable can be changed ...
So if the local variable can be changed, then we can use loop, etc. same as imperative languages. For example, for (i=0; i100; i++) where `i` is a local variable in function. It is true that a pure language could support such things (some pure languages do, e.g. Sisal). The experience of people using SML and Haskell seems to indicate that this is not very important in those languages: it's easy for the programmer to turn his variable-assignments into something pure (using new variables instead), and it's reasonably easy as well for the compiler to recognize loops and handle them just as efficiently as if they were while/for loops in an imperative language. OTOH, allowing modification of variables makes the whole language impure, because of variables caught in closures, e.g.: let count = 0 in \() - count := count + 1; count so it has very far reaching consequences. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: GPL answers from the SFLC
Note that this is a safety measure for the submitter: If the code is, indeed, released to the public, it is (dual licesed) GPL, anyway, even if that might not have been the intent. No. If the submitter did not explicitly release his code under the GPL, then it is not licensed under the GPL, even if is a derivative of GPL code. Instead, it is a breach of the GPL license and the submitter is exposing himself to a civil suit. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: GPL answers from the SFLC
The next question that comes to mind is thus: What if a new library X' released under BSD or MIT license implements the X API (making possible to compile Y against it)? Can such a new library X' be licensed under something else than the GPL (we guess Yes because we don't think it is possible to license the API itself)? Yes. Why should the existence of X' make any difference for the author of Y? Because the existence of X' makes it possible to use Y without using X. The order in which X and X' come to exist doesn't matter. This exact scenario took place for the GMP library, whose API was reimplemented as fgmp, specifically so that a user of the GMP library could release their code under a different library than the GPL. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: GPL answers from the SFLC
The thing is that the new X' library can provide the same API while not being very useful (bug, performance, whatever). And in this case, it is trivial to make that new X'. So I don't understand why the answer was no in the first place. The law is not a set of mathematical rules. It all needs to be interpreted, compared to the underlying intentions etc... So while you can say that it's pointless if you push the idea to its limit, that doesn't mean that it's meaningless in the context of the law. All it might mean is that in some cases, the interpretation is not clear. It's those cqases where a court needs to decide which interpretation should be favored. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Non-termination of type-checking
And I'm pretty sure that there's no way to convince Agda that F = R, or something similar, because, despite the fact that Agda has injective type constructors like GHC (R x = R y = x = y), it doesn't let you make the inference R Unit = F Unit = R = F. Of course, in Agda, one could arguably say that it's true, because Agda has no type case, so there's (I'm pretty sure) no way to write an F such that R T = F T, but R U /= F U, for some U /= T. It's easy to construct an F that is different from R but agrees with R for the case of Unit: F = λ _ - R Unit So there's a good reason why Agda doesn't let F and R unify: it would really be completely wrong. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ssh ports for monk and nun?
Trying to get ssh working via putty from behind my company firewall. My recommendation is to get access to an outside machine where you run an OpenVPN server on port 80 or 443. This will solve it once and for all. But first, please complain loudly and repeatedly about the firewall being closed to port 22. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why?
This is a matter that I genuinely at the present time do not grasp and I am hoping that some of you who are more familiar with the Haskell language may be able to help enlighten me. I feel the question to be an important one. What material benefit does Haskell derive from being a pure functional language as opposed to an impure one? Please provide examples as I require instruction. I think the STM monad is one of the greatest examples of why Haskell's approach to side-effects is a resounding success. Without having to change anything to the language and compiler, i.e. as a mere library addition (more or less), you get a concurrency system with optimistic synchronization, where all the needed invariants are trivially enforced by the type-system: - no prints or other un-revertable side effects in transactions. - all accesses to shared variables are protected by a transaction. - ... Monads aren't always perfect when it comes to side-effects, but in the context of STM, they really shine. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Status of TypeDirectedNameResolution proposal?
You know, another solution to the records problem, which is not quite
as convenient but much simpler (and has other applications) is to
allow local modules.
module Foo where
module Bar where
data Bar = Bar { x :: Int, y :: Int }
module Baz where
data Baz = Baz { x :: Int, y :: Int }
f a b = Bar.x a + Baz.y b
For someone coming from an SML background, that makes a lot of sense.
You could also add an automatic lightweight module, like Agda does,
where
data Baz = Node { x :: Int, y :: Int }
implicitly defines a local module Baz with record selection functions
Baz.x and Baz.y and even a Baz.Node constructor.
Stefan
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[Haskell-cafe] Daniel Schüssler anotheraddr...@gmx.de
this is rather trivial, but maybe someone else finds these useful: darcs get http://code.haskell.org/~daniels/haskell-snippets/ Since Emacs already comes bundled with several template systems (at least skeleton.el and tempo.el, where the first seems to be marginally more canonical), I think it would be better to define your templates using skeleton and thus remove the dependency on yasnippet. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: cvs.haskell.org down? haskell-mode abandoned?
In any case, I need it fixed before I can work on haskell-mode. Preferably by migrating haskell-mode over to cvs. :-) You mean migrating to DaRCS? That would be appreciated, yes. Um, yes. I can get the revision history into darcs pretty easily, such as it is, assuming cvs.haskell.org comes back up. Then I'll put the repository somewhere you can get at it. That fine? Yes, that would be wonderful. Ideally, it will eventually make it into something like darcs.haskell.org. who *is* the current maintainer? Last I heard, it was me. But if someone wants to take over, he's welcome. You're saying you don't have time to maintain it? Pretty much, yes. At least, I don't think I've done a good job maintaining it over the last couple years. Still, there needs to be a new release, Yes, urgently. and I've got a lot of improved haskell-mode code (written by other people) that isn't currently in the repository. Can you show me the patches? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: cvs.haskell.org down? haskell-mode abandoned?
In any case, I need it fixed before I can work on haskell-mode. Preferably by migrating haskell-mode over to cvs. :-) You mean migrating to DaRCS? That would be appreciated, yes. who *is* the current maintainer? Last I heard, it was me. But if someone wants to take over, he's welcome. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Best Editor In Windows
Real programmers use butterflies!! In Emacs-23, this is available as M-x butterfly C-M-c Too bad it wasn't around when I was writing my thesis, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Best Editor In Windows
The only thing I haven't figured out is how to do tab-completion of words in the ghci buffer. Do I need to use a different key combination? I couldn't find that in the documentation. I think it's just a missing feature. OK! Let me know if there's anything I can do to help. I noticed that other modes, e.g. ESS for R, do have tab-completion, but just skimming the source code I couldn't find how exactly they do that. You could try to just send the TAB directly to the underlying process and let ghci do the completion. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Best Editor In Windows
I'm very new at Haskell, i'm reading a book and starting, but i want to know which is the best editor for development under Windows, because now i'm using Notepad++(That i use to develop in C++). The best editor for development is Emacs, of course. http://www.gnu.org/software/emacs Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Best Editor In Windows
The only thing I haven't figured out is how to do tab-completion of words in the ghci buffer. Do I need to use a different key combination? I couldn't find that in the documentation. I think it's just a missing feature. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
I might also point out that 90% of all desktop computers run Windows, and yet every single C library binding on Hackage fails to compile on Windows. That really needs to be fixed. Luckily, this is being fixed ... by the Free Software movement. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: accessible layout proposal?
http://www.haskell.org/haskellwiki/Accessible_layout_proposal
I see two main problems with such a proposal (other than the particular
details of the syntax chosen for it):
- layout is not very well supported by most of the text editors,
contrary to parens/brackets/braces.
- more importantly: it introduces a new syntax that doesn't replace the
old one, so it makes the language more complex. Compare that with the
layout for `do', `let', `where' and `case' which is fine because it's
used 99% of the time and the alternative syntax is the same for each
one of them (simply add {;;;}).
So I'd be OK with a switch from
data foo = Toto | Tata Int | Titi
to
data foo = { Toto ; Tata Int ; Titi }
and
data foo =
Toto
Tata Int
Titi
and similarly for module import lists, but only if it comes with
a deprecation of the current syntax.
Stefan
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[Haskell-cafe] Re: Why is Bool no instance of Num and Bits?
[...] unsafe [PerformIO ...] looks safer to me. Hmmm Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Darcs as undo/redo system?
I have thought about applications of patch theory like this as well. I could imagine applying it to the undo stack in GIMP. Allowing you to undo things on different layers. I think one of the things you Don't know about the GIMP, but in Emacs, you can select a region of text and undo just the changes in that region. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Is curryfication practical with dynamic scope?
PS: In a meta interpreter, lexical scope seems to be actually easier to implement than dynamic scope. Depends on whether your meta-language is lexically or dynamically scoped. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell/JS -- better through typeclasses?
There's a tool for converting SML to JavaScript: Such tools converting between different languages, are usually called compilers, by the way. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
I think the non-applicable to code observation is very likely true – we'd like to be able to write nice descriptive variable names. In doing this, we probably want them to be more than the 1 or 2 characters that Haskellers traditionally use, maybe of the order of 5-10. Given this, it would seem a shame to only be able to fit 6-13 litterals on a line, that sounds like we'll quickly be having to wrap lines with deffinititions of any significance on them. I really like the 80-columns rule. And I also agree that long identifiers can be useful. All this means is that you use up more lines. Note that this can be a good thing: indentation is information, so by using more lines, you give more indentation information. My personal preference with Haskell is to ignore the 78 character limit, but only when layout otherwise becomes horrible otherwise. I consider screen real-estate a very valuable resource (and I'm appaled by the fact that current 21 displays are limited to 1600x1200 when they could go up to 1800x1400 ten years ago), and in this light the 80 columns limit tends to work fairly well: using significantly less (like 50) makes the code really difficult to write, whereas using more tends to waste a lot of space because, while some lines will make good use of the extra columns, most of them won't. Finally, I find that the indentation-pressure imposed by the 80-columns limit forces me to write better code: when code indentation grows too high, I'm forced to move it to a separate function, making the code more readable at the same time (by being forced to choose a name for the function and to choose appropriate arguments and return values). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Issues with running Ghci from emacs
It is looking for the closest ancestor directory with a file ending in .cabal, and that's where cd goes. Probably the simplest way to trick it without chaniging any setting is to create an empty dummy.cabal in the directory of your program. The haskell-mode code in the CVS repository has a fix for that problem. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Amazing
So IMO static typing is good, but it's only with functional programming that it really shines. You can go one step further: if you start using dependent types, you'll see that it gets yet harder to get your program to type-check, and once it does, you don't even bother to run it since it's so blindingly obvious that it's correct. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Verifying Haskell Programs
State of the art is translating subsets of Haskell to Isabelle, and verifying them. Using model checkers to verify subsets, or extracting Haskell from Agda or Coq. Another state of the art is to use type classes, GADTs, and/or type functions, to specify and prove the properties you want about your program. Basically using similar techniques as used in dependently typed languages. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: WYSIWYG literate programming
http://haskell.org/haskellwiki/Emacs#Unicodifying_symbols_.28Pretty_Lambda_for_Haskell-mode.29 I'm pretty sure this text wasn't there last time I looked, yet last time I looked was already long after Haskell-mode integrated such a feature. In any case I've added a note to mention that all you need to do is (setq haskell-font-lock-symbols t). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Dr Dobbs: Time to get good at functional programming
http://www.ddj.com/development-tools/212201710;jsessionid=3MQLTTYJRPL3CQSNDLRSKH0CJUNN2JVN Do they purposefully obfuscate names? I mean who are those Martin Obersky and Don Sype? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Gluing pipes
\ x y - f (g x) (h y) [...] f $. g ~ h ~ id I keep help wonder: other than a 5 chars, what is it we have gained? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What *not* to use Haskell for
So I should clarify I'm not a troll and do see the Haskell light. But one thing I can never answer when preaching to others is what does Haskell not do well? The most obvious cases where Haskell does not do well, for me: - When you feed it Java code. Incidentally, the same holds when you feed it C code. - When you try to write a malloc library. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why 'round' does not just round numbers ?
2.4x - x That's supposed to be 2.4x - 2, of course. Ah, damn it. I was hoping for a long discussion on just what math would look like with rounding like that ;-) I think it has a name... modulo maybe? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What I wish someone had told me...
The instance selection for an interface is done at run-time and this is inherently necessary. The instance (in a different sense) selection for type classes is almost always resolvable statically. In Haskell 98 In both cases, the dispatch is inherently dynamic, and in both cases, most dispatches can be resolved at compile-time with sufficient effort. The actual percentage may be quite different, tho. Implementation techniques are also fairly different, and the resulting coding style is also very different, but the two concepts are fundamentally very close to each other. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What I wish someone had told me...
leaders and pioneers: if you know one language, picking up others should be easy, they just differ in syntax. I have heard it, and I used to believe it. Now I think it's only true provided the one language you know is suitably advanced (and currently non-existent, I think). It all depends on what you consider as knowing a language. After all, a Real Programmer can write Fortran in any programming language, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: What I wish someone had told me...
Would you please explain this a bit more: the various unfortunate consequences of type erasure in Java are avoided by the fact that Haskell types lack constructors, so the user never expects to be able to conjure up a value of an unknown type. Even if Haskell had Java-style constructors, it wouldn't be a problem, since type classes exist independently from any object, so the code that needs the constructor will simply receive it in the corresponding dictionary. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: cabal upgrade
It's something to consider in the future, although a change-aware filesystem (git, say? It's fast) would probably be better. ^^^ You misspelled darcs. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Hmm, what license to use?
I am not allowed to use such an interpretation. The (expensive and very carefully researched) legal advice used to shape the use of Open Source code at my employer has resulted in a no LGPL under any circumstances whatsoever policy. [...] That still leaves anyone free to use LGPL if they want to, but please don't assume that it allows commercial use by all potential users. It *does* allow commercial use. Your example just shows that some people may decide not to take advantage of it, based not on problematic restrictions but just on paranoia. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Hmm, what license to use?
When I compare GPL and MIT/BSD licenses, I do a simple reasoning. Suppose a doctor in a battle field meet a badly injuried enemy. Should he help the enemy? My answer would be that he indeed should, at the condition that the patient will switch side. Oh wait, that's just what the GPL says. Stefan Analogies are broken ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Line noise
qsort [] = [] qsort (x:xs) = qsort (filter (x ) xs) ++ [x] ++ qsort (filter (x =) xs) Note that you can help the reader by making the precedence a bit more obvious: qsort [] = [] qsort (x:xs) = qsort (filter (x ) xs) ++ [x] ++ qsort (filter (x =) xs) I find that my students can understand this much better than the 1½ liner. Regarding the mapM_ example: mapM_ (\(n,v) - putStrLn $ [ ++ show n ++ ] = ++ show v) (zip [0..] vs) To somebody familiar with Haskell, that is as clear as day. I guess I'm not familiar with Haskell, because I find the paren-counting to be difficult to do in my head (and god knows that I'm familiar with parenthese). Also, I tend to shy away from ($) and (.) and to only use them when I have a list of functions to compose/apply and want to avoid growing the depth of my AST with the length of the list. So I'd probably write the mapM_ example as: mapM_ (\(n,v) - putStrLn ([ ++ show n ++ ] = ++ show v)) (zip [0..] vs) which I find a lot easier to read. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Line noise
I believe this was voiced as an SML issue more than an OCaml issue, though honestly I don't know enough of the differences to distinguish them. Before I mentioned that function/prefix application always binds tighter than operator/infix application, he was using many redundant parentheses, thanks to defensive programming against whichever dialect was at fault. It was apparently a programmer issue rather than a language issue: SML's precedence rules are pretty much identical to Haskell's in this regard. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell Weekly News: Issue 85 - September 13, 2008
A more difficult question is: how do I know that the formal specification I've written for my program is the right one? Tools can fairly easily check that your programs conform to a given specification, but they cannot (to my knowledge) check that your specification says exactly what you want it to say. The key is *redundancy*: as long as your property is sufficiently different (in structure, in authorship, etc...) you can hope that if the spec has a bug, the code will not have a corresponding bug and vice versa. It's only a hope, tho. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Bug in Emacs Haskell Mode
Emacs Haskell Mode has the following useful feature: when Haskell - Load File is used to load a file into GHCi from Emacs, Haskell Mode automatically looks for a *.cabal file in an attempt to find the project directory. I'm truly sorry about the fact that I only release new versions rather rarely. I recommend you use the CVS version, where this bug was fixed a few months ago. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] do and ifthenelse indentation in GHC
http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse says (and my memory seems to agree) that GHC has been changed to accept things like readnums f n = do eof - hIsEOF f if eof then return n else do l - hGetLine f readnums f (n + read l) where the `then' and `else' are aligned with the `if' rather than being slightly more indented as the Haskell 98 standard requires. Yet, when I try it with GHCi 6.8.2 I get an error: Prelude :load /home/monnier/tmp/foo.hs [1 of 1] Compiling Main ( /home/monnier/tmp/foo.hs, interpreted ) /home/monnier/tmp/foo.hs:6:18: parse error (possibly incorrect indentation) Failed, modules loaded: none. Prelude Does anybody know what's up with that? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: type families and type signatures
Id is an operation over types yielding a type, as such it doesn't make much sense to me to have (Id a - Id a) but rather something like (a - Id a). Actually, it can make perfect sense: e.g. if the `Id' function acts as a constraint. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Implementing fixed-sized vectors (using datatype algebra?)
You seem to write 12 as 1 :+ 2 instead of () :+ 1 :+ 2. But I think, the latter representation should probably be prefered. With it, :+ always has a number as its left argument and a digit as its right. Without the () :+ we get ugly exceptional cases. You can see this, for example, in the instance declarations for Compare. With the second representation, we could reduce the number of instances dramatically. We would define a comparison of digits (verbose) and than a comparison of numbers based on the digit comparison (not verbose). Even if () would be preferred from the programmers point of view (I'm not sure how much we could reduce the number of instances though), it makes the representation less attractive on the user-side. Anyone using the library would find it annoying and would wonder why is it neccessary. I wouldn’t wonder. Leaving out the () :* part just works because our type-level “values” are not typed, i.e., there aren’t different kinds Digit and Number but only kind *. If :+ would be a data constructor (on the value level), it would take a number and a digit argument which would forbid using a digit as its left argument. So I consider using a digit on the left as “unclean”. It’s similar to using a number as the second part of a cons cell in LISP. How 'bout treating :+ as similar to `append' rather than similar to `cons'? Basically treat :+ as taking 2 numbers (rather than a number and a digit). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: anybody can tell me the pronuncation of haskell?
*did* disagree with me was also named Jeremy. How confusing. Are both Jeremys pronounced identically? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: The programming language market (was Re: Why functional programming matters
* Say computers are cheap but programmers are expensive whenever explaining a correctness or productivity feature. This is true only if talking to people in high-income nations. Is it? Maybe you're right. But historically, computers have been available at all kinds of price ranges, so people chose the price point that fit them. So, for the last 15 years or so already computers have been chosen (in the wealthy countries) to be cheaper than programmers. Is there any reason to think that the same forces aren't at play in lower-income nations? After all, cheap (typically second hand) computers are easy to come by. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Newbie question
How does caller choose which particular instance of Num they want? By passing the type they want. That's what the Num a = thingy does. In object-oriented language If function return type is an interface it means that it can return any implementation of this interface, but caller can't choose which particular inplementation they want. The full type of f you've given is: forall a . (Num a) = Integer - a where the forall a . is normally not written. What you describe (a function that returns something where the type can be chosen by the function itself) would have type: Integer - (exists a . (Num a) = a) I.e. the a is not passed as a (type) argument, but instead it's returned by the function. What the difference between haskell class and interface in object-oriented languge such Java or C#? From a low-level point of view, the difference is that the vtable is manipulated separately from the objects. The Num a basically stands for the type of the vtable (which is called dictionary in Haskell). To bundle an object with its vtable as is traditionally done in OO languages, you need to create an existential package, e.g. something of type (exists a . (Num a) = a). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why purely in haskell?
Yes, this is a doctrinal problem. Since *any* concrete reaction, e.g., an error message is a kind of answer, the only - unusable as it is - way of not providing it is to fail the termination... You can just disallow the call, using the type system. Not that it's always easy or practical either, mind you. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell-mode 2.4
Johan == Johan Tibell [EMAIL PROTECTED] writes: Emacs is completely frozen until I press C-g and then it goes back to normal (without loading the file). Here's the back trace: Debugger entered--Lisp error: (quit) accept-process-output(#process haskell) (and (not (re-search-forward comint-prompt-regexp nil t)) (accept-process-output proc)) So it seems to be waiting for the prompt but can't find it. If you look at the buffer containing the interactive process, is there a prompt there? If not, can you try and figure out why not? If yes, can you try and figure out why it is not recognized by the comint-prompt-regexp? I had the following in my .ghci to make GHCi's prompt less verbose: :set prompt Removing that solves the problem. You can also add the following to your .emacs: (add-hook 'inferior-haskell-mode-hook (lambda () (set (make-local-variable 'comint-prompt-regexp) ^ ))) Or some fancier regexp (the default is ^\\*?[A-Z][\\._a-zA-Z0-9]*\\( \\*?[A-Z][\\._a-zA-Z0-9]*\\)* ). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: viewing HS files in Firefox
My problem is when viewing plain darcs repositories (like mine on
darcs.johantibell.com which I recently fixed with the above mime type
hack.)
Please complain to your browser('s authors): most browsers only provide
*one* way to view a given mime-type, which is stupid. It's not specific
to .hs files.
They could at least provide a way to override the provided mime-type, so
you can say display this application/octet-stream file as
a text/plain. Similarly they should allow you to choose (via
a context-menu, for example) to open a pdf file in the pdf plugin or in
a separate application.
Stefan
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Re: [Haskell-cafe] Haskell-mode 2.4
First of all, thanks. My Aqua Emacs (on OS X Leopard) hangs when I send a command like C-c C-l or C-c C-t to the interpreter (I can quit the command using C-g) . Starting the interpreter and using :l from inside it works fine. This is using GHC 6.8 (and I think I also tried 6.6.) What could be the problem? I also tried to eval this but it didn't make a difference: (setq inferior-haskell-find-project-root nil) It looks like this is a problem with 6.8.2 as haskell-mode-2.3 has the same problem? Seems like the commands being sent to the haskell process never reaches it. Try can you try to look into the nature of the hang? Is Emacs completely frozen? Does C-g get you back to something usable? If so, can you set Options = Enter Debugger on Quit, then reproduce the problem than hit C-g and show me the backtrace? If not, can you run Emacs under a debugger and give me a backtrace of when Emacs is frozen? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell-mode 2.4
First of all, thanks. My Aqua Emacs (on OS X Leopard) hangs when I send a command like C-c C-l or C-c C-t to the interpreter (I can quit the command using C-g) . Starting the interpreter and using :l from inside it works fine. This is using GHC 6.8 (and I think I also tried 6.6.) What could be the problem? I also tried to eval this but it didn't make a difference: (setq inferior-haskell-find-project-root nil) It looks like this is a problem with 6.8.2 as haskell-mode-2.3 has the same problem? Seems like the commands being sent to the haskell process never reaches it. Try can you try to look into the nature of the hang? Is Emacs completely frozen? Does C-g get you back to something usable? If so, can you set Options = Enter Debugger on Quit, then reproduce the problem than hit C-g and show me the backtrace? If not, can you run Emacs under a debugger and give me a backtrace of when Emacs is frozen? Emacs is completely frozen until I press C-g and then it goes back to normal (without loading the file). Here's the back trace: Debugger entered--Lisp error: (quit) accept-process-output(#process haskell) (and (not (re-search-forward comint-prompt-regexp nil t)) (accept-process-output proc)) So it seems to be waiting for the prompt but can't find it. If you look at the buffer containing the interactive process, is there a prompt there? If not, can you try and figure out why not? If yes, can you try and figure out why it is not recognized by the comint-prompt-regexp? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell-mode 2.4
Here's for Xmas, version 2.4 of the Haskell-mode package. Haskell-mode is an Elisp package to support editing Haskell in Emacs. See the home page at http://www.iro.umontreal.ca/~monnier/elisp/ -- Stefan Changes since 2.3: * Update license to GPLv3. * New derived major mode for .hsc files. * Removed the C-c C-r binding to reload a file. You can still call inferior-haskell-reload-file (and/or bind it to your favorite key, including C-c C-r) or you can now use C-u C-c C-l. * C-c C-d looks up the symbol at point in the Haddock docs. * Haddock comments are highlighted with font-lock-doc-face if it exists. * Use `tex' rather than `latex' for haskell-literate. * inf-haskell.el tries to find the root of the module hierarchy to determine the root of a project (either by looking for a Cabal file or relying on the `module' declaration line). If all works well, this will make C-c C-l automatically switch to the root dir, so that dependencies in other directories are automatically found. If it doesn't, complain and/or set inferior-haskell-find-project-root to nil. * The new command haskell-hoogle helps you query Hoogle from Emacs. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Monad.Reader 8: Haskell, the new C++
PS: And, no, you won't be able to set breakpoints in type-level programs... Yet. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Functional dependencies conflict between instance declarations
Never mind, that GHC compiler was again more clever than me, sigh. That's really frustrating about Haskell: the compiler captures so many errors at compile time, that newbies hardly get anything done, it's a constant battle against the errors. But once it compiles, it usually works at runtime :-) This is what I love about Haskell: If it typechecks, it probably does the thing you meant it to. I've never seen any other language like it. It's amazing! Next stop: Coq, where the fight with the type checker is so much more difficult that when the code finally type checks you don't even need to run it at all. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Using haskell with emacs
C-c C-b ... when pressed for the first time this will start an interpreter (ghci or hugs most of the time), when pressed with a running interpreter it'll switch to that buffer. C-c C-l ... Load the current file into the editor. There is no function-wise compilation. More generally, in Emacs C-h m will give you a list of keybindings for the current major mode. Also many major modes offer a menu in the menu-bar where the main special features are advertized. Don't forget C-c C-r to reload the file in the buffer. I've removed it from the next release of haskell-mode, so if you need it, please explain why. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Using haskell with emacs
I've removed it from the next release of haskell-mode, so if you need it, please explain why. So C-c C-l is the preferred method for re-loading? Yes. I don't know enough about the details to know if :l and :r do exactly the same things in ghci. I don't either. But experimentation suggests that the only difference is that :r doesn't need an argument (which in the case of haskell-mode doesn't make any difference since the arg is provided by haskell-mode anyway). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Newbie question: multi-methods in Haskell
Remember that type classes do not provide object-oriented functionality. The dispatch is static, not dynamic. I beg to disagree. map (\n. n + n) calls different (+) operations depending on the (type of the) argument list. That's why dictionaries are passed around (they are called vtables in many OO languages) by several Haskell implementations. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Indentation woes
I wish to be able to indent my code like so: longFunctionName various and sundry arguments | guard1 = body1 | guard2 = body2 | ... where declarations That is, with guards and where clauses indented to the same level as the function name. Sounds like a generalization of the idea of allowing indentation like if foo then bar else baz in `do' notation. It might probably be obtained similarly by just adding a few optional semi-colons at the right place in the BNF rules. whether those optional semi-colons will render the grammar significantly more complex, I don't know. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell mode for emacs - some questions
I finally got emacs using Haskell-mode working. It seems that the latest Sounds like it was difficult. Could you describe what you tried, what didn't work, and what did work in the end? Hopefully I can then improve the doc. - How can I just compile and run in one go without having to type ghc --make main.hs as arguments for the compile... command and then typing main.exe for shell command...? This is what you do all the time when using Visual Studio, Eclipse, etc: just hit the F5 key which builds all the dependencies and runs one or more generated executables. Easy. Visual Haskell does this (in the cases it does not hang for some unknown reason). Of course I could dig into ELISP, but somehow I feel this must be supported somehow. I never use GHC in this way, I always use GHCi instead. Furthermore, I tend to work on only parts of a program, so there isn't necessarily a main function. I'd be happy to add support for your usage pattern, but since I'm not familiar with it, I'm not sure what to add. Another problem is that unless your project is tiny, it'll have several files and currently Emacs doesn't know which file is the main one. I added very-preliminary support for Cabal in the CVS code of haskell-mode which should allow haskell-mode (at some point in the future) to figure out what's the main file an how to compile it. Currently all it does (other than font-lock the cabal file itself) is look for the Cabal file to figure out the root of the project, so that C-c C-l first does a cd to the root, which should allow dependencies in other directories to work more seemlessly. Patches (or precise feature requests) are very welcome. E.g. it should be fairly easy to add an F5 binding like you describe. The main issue is how to inform Emacs of what should be done. In VS or Eclipse, what do you have to do in order for F5 to work? Is opening some random source file enough, or do you have to select a Cabal file or what? Use C-c C-l to load the file into GHCi. This is better than just compiling it: you get an interactive environment in which to play around with, test out functions, etc. You can still 'run' your program by typing 'main' in GHCi. We could also add a binding which sends main to GHCi. - There seems to be support for Haskell Font Lock Symbols, which should convert \, - and maybe other symbols to good looking Unicode fonts. I can't get the correct fonts working on Windows. I never use Windows so I can't really help you there. Maybe ask on gnu.emacs.help how to get those chars displayed. I'm pretty sure Windows has the needed fonts, so all that's missing is some way to help Emacs make use of them. If you figure it out, please send me a note about what you had to do, so I can add it to the documentation. Now as I understood this is not really supported because of indentation problems, as underneed the single Unicode arrow character is converted into -? You slightly misunderstood: this hack is fully supported. I just added some warnings to the docstring to make sure the user doesn't blame me when he gets bitten. This is unfortunate, because that's one of the things I really like about Sun's Fortress: the usage of Unicode symbols makes the text look much more mathematically, versus the half-century-old ASCII text. Furthermore in almost all Haskell books the richer font symbols are used for clarity. Any hints on this topic? I use this hack all the time and haven't been bitten yet. Some important aspects: - the λ should use a 1-char-wide lambda (i.e. the unicode one). There's also a JIS lambda used if the unicode one can't be found, but this one is 2-char wide, so it risks introducing mis-indentation. - the → and ← OTOH should use the 2-char-wide JIS arrow, rather than the unicode arrows for the same reasons. - ... I wouldn't be surprised if this code had bit-rotted, or if there never has been Windows support. I use it all the time, so it definitely hasn't bit-rotted (actually, it's one of the most often updated part of the code). There shouldn't need to be anything special for Windows, but indeed I've never tried it on Windows (and don't intend to either). It's a corner function, used by few and therefore not that polished. By all means, try it out, and if it doesn't work, feel free to submit patches, but I doubt it'll get changed any time soon by a haskell-mode developer :) You'd be surprised, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Tutorial on Haskell
No we don't! At least not anywhere near as much of an exaggeration as that statement would be about an strongly typed imperative language (sequencing can't be type checked*, imperative programs are mostly sequencing, thus imperative programs are mostly unchecked). Actually, side effects *can* be type checked, but it's a lot more painful (the type of an assignment is not nearly as simple as the one you're used to). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: A wish for relaxed layout syntax
list := '[' item* ';'? ']' Indeed, it's the same idea as the 'else' in do-blocks. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: literate Haskell newbie question
cat foo.lhs | grep -e ^ | sed s/^ //
Running for the Useless Use of cat Award?
A simpler version would be:
sed -n -e 's/^//p foo.lhs
I don't guarantee that this will work correctly, tho. Maybe
sed -n -e 's/^ //p foo.lhs
will work better? This said, the OP talked about TeX, so presumably he uses
the other format, so maybe a better sed would be:
sed -n -e '/^\\begin{code}/,/\\end{code}/{/^\\begin{code}/d;/^\\end{code}/d;p}'
foo.lhs
-- Stefan
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[Haskell-cafe] Re: Church Encoding Function
I'm pretty sure you can define a catamorphism for any regular algebraic data type. Actually, so-called negative occurrences in (regular) data types cause problems. Try to define the catamorphism of data Exp = Num Int | Lam (Exp - Exp) | App Exp Exp to see the problem, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: LGPL libraries
Gtk2Hs and HDBC are both LGPL licensed, but aren't they always static linked? Is there a way to use them in closed-source programs? Of course, just read the LGPL again. It doesn't prevent use of static-linking. It's a bit long and verbose with odd-sounding rationales, but all it really says is that you should make sure that the LGPL'd part of your application can be modified by the end user. Dynamic linking is the easiest way to get that result, but it's not the only one. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: seq does not preclude parametricity (Re: IO is not a monad)
FYI, don't try to run this in GHC, because it gives the simplifier fits. You mean it triggers a bug in the inliner? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: GHC performance of 64-bit
I noticed that GHC generates slower code on an Linux amd64 bit platform than the 32-bit version on a cheaper 32-bit machine. CPUTime for running sieve of Erathostenes to generate 10,000 primes: Athlon XP 2800 (32-bit): 7.98 secs Athlon 64 3800 (64-bit): 10.29 secs This is using GHC 6.6 on the 64-bit machine and 6.4.1 on the 32-bit one. I googled around and could not find any information regarding degraded performance of ghc/haskell on 64-bit machines. Any ideas? Could it be the effect of doubling the size of pretty much everything, leading to poorer memory-hierarchy performance? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Simple GADT parser for the eval example
...No surprise there, since there is no way to fail in the event of a maltyped Expr. The next thing to try is a type class solution... Think about it: you need to do type checking. For our little language we used Template Haskell to do your Expr - Term a translation, thus the type-checking of our input programs is actually done by Haskell's type checker ;-) See our PLPV paper http://www.iro.umontreal.ca/~monnier/tct.pdf Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is Haskell not homoiconic?
Homiconic means that the primary representation of programs is also a data structure in a primitive type of the language itself The main reason is that Haskell is designed as a compiled language, so the source of the programme can safely disappear at runtime. So there's no need to have a representation of it beyond the source code. I'm not sure it's relevant. In syntactically scoped Lisps, the code is mostly manipulated at compile-time by macros, rather than at run-time. And indeed, Template Haskell makes Haskell pretty much homoiconic. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What's going on in our courses?
Last Spring my Functional Programming class implemented a Genetic Algorithm with Neural Networks that learned to play Nim. The students had a really good time--they also learned lots about Functional Programming with Haskell. Part of the final exam was a tournament. This Fall in AI we'll be doing GA's again and Genetic Programming. Is there a list or forum that talks about teaching our students about Haskell? Indeed, I'd be interested as well. I'm actually looking for a good textbook for a concepts of programming language course. All the books I can find tend to emphasize OO or imperative programming too much for my taste. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell performance (again)!
Assuming your machine architecture supports something like condition codes. On, e.g., the MIPS you would need to test for and == separately. And even if your machine supports condition codes, you'll need one test plus two conditional jumps. Not much better than MIPS's 2 independent tests plus 2 conditional jumps. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [newbie] How to test this function?
First, how do I fix the identation of the if then else? getList = find 5 where find 0 = return [] find n = do ch - getChar if ch `elem` ['a'..'e'] then do tl - find (n-1) return (ch : tl) else find n OK. Thanks. I didn't find that one because it's not offered as an identation option in emacs haskell mode. It's not just a missing option, it's a bug. I'll hopefully get around to fixing it soon. Emacs is evil! Of course, that's why we like it so much. It also inserts random tab characters into your code just to save a few space bytes. Actually, it has nothing to do with saving space: it only uses TABs because many people *want* to use tabs. Of course, the rest of the users *doesn't want* to use tabs, so we get to be flogged both ways. BTW, when was the last time you tried haskell-mode? Haskell-mode (at least since version 2.0) makes it difficult for the user to insert TABs by mistake. Tends to completely trash indentation e.g. when pasting code into mails etc. Huh? You mean your mail reader doesn't display tabs in the same way as 8 spaces? TABs are evil for many reasons, but I've never seen this one. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [newbie] How to test this function?
For that one, if it doesn't get mended for long enough, Haskell' might accept the present layout. http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse Hmm... the bug in haskell-mode has been known for almost a year now. So you're saying that I should just wait even more and it'll get fixed by Haskell'? IOW Haskell isn't just lazy but it even rewards laziness? Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Monad laws
Are you really sure Haskell compilers do that optimization? I would regard a compiler that does optimizations that are justified by laws that the compiler cannot check as broken. You mean like the non-aliasing law in Fortran? Stefan ;-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: does the compiler optimize repeated calls?
Purely functional does give you some performance benefits, though. Nice theory. People don't use functional languages for their performance, mind you. All the optimizations that are supposedly made possible by having a pure functional language tend to be either not quite doable (because of non-termination making the language a bit less pure) or simply too hard: the difficulty being to decide when/where the optimization is indeed going to improve performance rather than worsen it. It's much too difficult for a compiler to figure out which functions might benefit from memoization (and with which specific form of memoization). Especially compared with how easy it is for the programmer to do the memoization explicitly. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: how do you debug programs?
I simply can't let this pass without comment. It's irrelevant if you're using a functional or imperative language, debuggers are invariably a waste of time. The only reason to use a debugger is because you need to inspect the contents of a processes address-space; That's a very narrow definition of debugger. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: map (-2) [1..5]
I'd have thought it would have been simpler to just make the rule that -2 (no spaces between '-' and '2') would be a single lexeme, But then x-2 won't mean subtract 2 from x but call x with arg -2. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why Not Haskell?
Well I understand the free as in free speech not free beer motto, but suppose person A is talented at writing software but prefers a peaceful existence and lacks the contacts/refs/desire/energy etc to be a consultant or contractor, and has had the bad experience of being forced to work extremely long hours with low pay while in an employed position, and person B is outgoing, ebullient, and talented at marketing and advertising. Now person A spends some years quietly writing some code, which uses a GPL library and is therefore GPL'd, and sells it, as is his/her right under the GPL to person B. If person A really worked for years using a GPL'd library and hoping to make money selling the resulting program (rather than services around that program), he's a complete and total idiot. In any case, making a living by selling a program (as opposed to services around that program) is a difficult business. Except when it's a program written on-demand for a customer who pays you directly to write it (in which case the GPL probably won't get in way, BTW). I can't entirely dismiss GNU/FSF/GPL but it poses a fundamental conflict with the only way I can see of earning a living so it's like a continuous background problem which drains some of my energy and enthusiasm hence the length of my rambling post where I made another attempt to understand my relation to it. Maybe you should thank the FSF for making you doubt: you should really think very hard about how you're going to make a living off of selling a program, even if that program hasn't been anywhere near any GPL'd code. In all likelihood it'll be much easier to earn your money by selling services around your program than just the program itself. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why Not Haskell? (sidenote on licensing)
Sorry, I didn't mean to offend anybody, or be misleading. I like GPL, but I also like the disease metaphor (although is not as much being sneezed at as having sex with somebody :-). Then you should think twice before using such metaphors: you end up propagating hate for something which you like. And it's really not as easy to control as you suggest: If you ever take in a single patch under the GPL, Any patch or outside piece of code you choose to include in your code should be checked to see if its licence allows you to use it like you intend. That's true for any license, not just for the GPL. And don't forget: the default license is no licence at all (i.e. basically just what the copyright's fair use says, which seems to be asymptotically moving towards the empty set as time goes). or even implement a new feature in an obvious way that has been implemented by somebody else under the GPL, you are in trouble. Doesn't sound credible. You're free to write and sell a program whose source code is exactly the same as Emacs's (or PowerPoint for that matter) as long as you can show it was pure accident (or if you like a more classic example url:http://en.wikipedia.org/wiki/Pierre_Menard_(fictional_character)) AFAIK The problem you talk about only comes with patents and is unrelated to copyright/licenses/GPL. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANN: System.FilePath 0.9
Trying to design a consistent naming system, it helps if we all agree on what the various parts of a filepath are called, this is my draft of that: http://www-users.cs.york.ac.uk/~ndm/temp/filepath.png For what it's worth, the FSF's coding conventions uses path only for lists of directories (typically used for a search, e.g. MANPATH, PATH, LD_LIBRARY_PATH, load-path, ...), not for file names. In their convention, /foo/bar/baz is a file name, where bar is a file name element IIRC. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why does Haskell have the if-then-else syntax?
Confusingly, if c then t else f Also works, although no-one really knows why. Actually, it doesn't work inside a `do' layout, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: help with MPTC for type proofs?
I'm thinking that either the functional dependency constraint is weaker than I thought, or that somehow GADTs aren't interacting with FDs as I'd like, but I'm not sure which. Or, of course, it may be that my recursive instance is not doing what I like. Or I may be just plain confused, as is pretty clearly the case... The interaction between FD and GADTs is not very good, in our experience. Hopefully this will be fixed at some point. But in the mean time, what we ended up doing is to use GADTs instead of classes and FDs: data Eq a b where refl_eq :: Eq a a data Commute a b c d where various axioms defining how D can result form A, B, and C -- Lemma that says that D is uniquely defined by A, B, and C. Comm_unique :: Commute a b c d - Commute a b c d' - Eq d d' -- Proof. Comm_unique p1 p2 = ... The problem is that in your case it seems that you do not want to explain to the type checker how D depends on A B C: you just want to say that it's uniquely defined. But maybe you can get away with: data Eq a b where refl_eq :: Eq a a data Commute a b c d -- No axioms provided to Haskell. -- Lemma that says that D is uniquely defined by A, B, and C. Comm_unique :: Commute a b c d - Commute a b c d' - Eq d d' -- The proof is not given either. Comm_unique p1 p2 = undefined When you'll do a case on Comm_unique a b (which will tell the type checker that D and D' are one and the same) you'll just want to make sure that Haskell doesn't try to look at the `refl_eq' value. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: lambda evaluator in GADT
The introduction to GADT usually starts with a little expression evaluator. So I gave it a try, but there are some troubles. Actually, the generalization is not necessarily trivial at all, depending on what you need to do with your ASTs. data E a where Lit :: a - E a App :: E (a - b) - E a - E b Lam :: Var a - E b - E (a - b) Val :: Var a - E a data Var a = Var String You're using a first-order abstract syntax. Each GADT branch corresponds to one of the typing rule of your language, and when you introduce variables, your typing rules end up needing an extra environment (which maps each var to its type), which you also need to add here: E env a. Building up `env' is left as an exercise. An alternative is to use higher-order abstract syntax, which correspond to using hypothetic judgments in your typing rules instead of an extra environment. I.e. something like: data E a where Lit :: a - E a App :: E (a - b) - E a - E b Lam :: (E a - E b) - E (a - b) eval (Lit x) = x eval (App f x) = (eval f) (eval x) eval (Lam f) x = f (Lit x) Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: haskell-mode indentation
Being an emacs-addicted person, I'm using haskell-mode for editing haskell code. But its default indentation rule doesn't seem to be correct. I mean when i type something like the code below, i've got the following indentation: doSomeStuff a b c = do somefunc a anotherfunc b onemorefunc c Whereas the correct indentation (in my opinion) is as follows: doSomeStuff a b c = do somefunc a anotherfunc b onemorefunc c Yup, there's clearly a bug here (or at least misfeature). But at least doSomeStuff a b c = do somefunc a anotherfunc b onemorefunc c would make sense as well, if somefunc can take 5 arguments (and haskell-mode doesn't do any kind of type-inference, so it should conservatively allow this indentation as well). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Code completion? (IDE)?
vim7 has introduced omni-completion... So I'm interested wether there are any projects which support any kind of completion.? I have been working on some code completion support for EclipseFP. It is right now in a really infant stage, but it at least is something. Just take a look at the latest integration build that you are able to find at http://eclipsefp.sourceforge.net/download Could you describe the general approach taken? I'd like to see something like that added to Emacs's haskell-mode as well, and I expect a fair bit of the work can be done on the Haskell side and thus shared among editors. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
The trouble with monad comprehensions was that it became far too easy to write ambiguous programs, even when you thought you were just working with lists. One solution was already suggested: to make the comprehension syntax be pure syntactic sugar whose semantics depends on the semantics of the identifiers the syntactic sugar expands into. So you could keep the current list-only comprehension as default, and allow monad comprehension by providing a library (which the users need to import so as to hide the Prelude's definition). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Announcing Djinn, version 2004-12-11, a coding wizard
I've written a small program that takes a (Haskell) type and gives you back a function of that type if one exists. It's kind of fun, so I thought I'd share it. Doh! It seems your code takes a string representing the type and returns a string representing the code, whereas I expected at first you were doing some funky type class molestation so you can use djinn in your code and let Haskell fill it in. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Indenting issue with Haskell mode
I believe the following is not indented correctly. It only happens with (x:xs) when you use nested cases, I think. case foo of [] - case bar of [] - return () (x:xs) - Thanks. The indentation indeed works particularly poorly here in ways that I hadn't seen yet. I don't have time to look into right now, but I've added it to the indent.hs file in the mean time. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: layout problem
Indeed! I always use braces and semicolons with do-notation. You are free to do so too! Nothing requires you to use layout. Indeed, you can freely mix the two. I would not recommend braces and semicolons, because these allow a bad layout (easy to parse for a compiler, but hard to read for a human), unless you invest the time to make a tidy layout despite the braces and semicolons. (So why not only make a tidy layout?) Unless you use a simplistic text editor, the braces and semi-colons allow the text editor to do the layout for you. While I find the layout notation attractively clean, I find the redundancy of autolayout+braces+semicolons to save me from a lot of trouble. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
