May 2001 - Haskell - Haskell.org

Re: Templates in FPL?
by cwitty＠newtonlabs.com 24 May '01

24 May '01

Jerzy Karczmarczuk <karczma(a)info.unicaen.fr> writes: > We know that a good part of "top-down" polymorphism (don't ask me what > do I mean by that...) in C++ is emulated using templates. > > Always when somebody mentions templates in presence of a True Functionalist > Sectarian, the reaction is "What!? Abomination!!". > > Now the question: WHY? > > Why so many people say "the C++ templates are *wrong*" (and at the same time > so many people use it every day...) I agree with Marcin about the bad points of templates. (I'd like to point out, though, that the idea that "adding new code can't change the meaning of a program" no longer holds in Haskell extended with overlapping type classes.) > Is it absolutely senseless to make a functional language with templates? > Or it is just out of fashion, and difficult to implement? There are some very cool things about C++ templates; they are more powerful than you might suspect. (Note: the following summarizes and paraphrases several papers I've read on the topic; let me know if you want more information and I'll look up references for you.) Basically, templates let you extend the compiler. You get a primitive functional programming language, with which you can do basically arbitrary computations on types and on integers; this language can be used to generate special-purpose code. One way to look at this is as a form of partial evaluation (although there's nothing which automatically decides which computation to do at compile time and which at run time). In the functional programming language, you have conditionals, the usual arithmetic operations, pairing, recursion, etc.; basically, it's a real programming language (although a very annoying one -- the syntax is atrocious). Here are a few examples of the kind of thing you can do with C++ templates: * Compute log base 2 at compile time. You can write a template such that if n is a compile-time constant, Log2<n>::val is its log base 2. * Generate FFT routines. You can write a template such that if n is a compile-time constant which is a power of 2, then FFT<n>::do_fft() is a routine which computes an FFT on an array of size n, as a single chunk of straight-line code (no loops). (Not necessarily useful -- it generates a lot of code for reasonable-sized arrays, and cache effects probably mean that something with loops is more efficient -- but cool nonetheless.) * Implement lambda (over a subset of full C++). You can make lambda(X, a*(X+b)) do "the right thing" (basically by arranging for the expression a*(X+b) to have a type like Times<Num, Plus<PlaceholderX, Num> > and then writing a function using recursion over this type). These techniques are useful when you're going for speed and generality; I would certainly imagine that such things could be useful in a functional programming language as well. In fact, over the past few weeks, people on the Haskell mailing lists have been trying to do vaguely similar kinds of compile-time computation at the type level, using type classes and functional dependencies. On the other hand, C++ templates have huge flaws for this kind of thing. As I mentioned, the syntax is atrocious; also, there are lots of things you would like to do which are apparently just out of reach (the template language is not quite powerful enough). I'd like to see somebody make a serious effort to add this kind of compile-time processing to Haskell. It might not look anything like templates; something like OpenC++ and OpenJava might be better. (These are C++ and Java compilers which are extensible; you can write compiler extension modules using a standard API, and the compiler dynamically loads your extensions.) There is a big reason why C++ templates (or any other form of compile-time arbitrary computation on types) would be hard to add to Haskell: type inference and polymorphism. For C++ templates, you need to know the argument types before you can do the template processing, and you cannot know the type of the result until the processing is done. Probably this facility would only be useful for monomorphic parts of your program, which might mean it's not appropriate for Haskell at all. Carl Witty

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Cygwin Build Bug (Probably Affects other GNU GLIBC Builds)
by Brent Fulgham 23 May '01

23 May '01

For thread-safety reasons, the GNU C Library (2.2+) defines "errno" as a macro that calls a function to get the errno of the current thread. Consequently, the source file "src/runtime/Builtin/cIOExtras.c" needs to include the <errno.h> header file, rather than declaring an extern variable. Diff: ========================================================================== *** cIOExtras.old.c Wed May 23 14:54:58 2001 --- cIOExtras.c Wed May 23 14:26:54 2001 *************** *** 1,10 **** /* basic unsafe utilities, defined in IOExtras */ #include "cinterface.h" #include "mk.h" void performGC () { C_GC(0); } int unsafePtrEq (void* a, void* b) { return (a==b); } /* basic error handling via C's errno */ ! extern int errno; int getErrNo (void) { return errno; } --- 1,11 ---- /* basic unsafe utilities, defined in IOExtras */ #include "cinterface.h" #include "mk.h" + #include "errno.h" void performGC () { C_GC(0); } int unsafePtrEq (void* a, void* b) { return (a==b); } /* basic error handling via C's errno */ ! /*extern int errno;*/ int getErrNo (void) { return errno; }

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creating xml doc with haxml
by Taesch, Luc 23 May '01

23 May '01

i understand now how to filter existing xml doc with haxml. but what about creation from scratch ? if i have myCFel = mkElem "tag" [(literal "value")] myel is a CFilter, not an element so i i want to create a effective element ( to pass to putStrLn . render $ ppContent for instance), i should give it something to filter, like emptyContent = CString False "" to finally do myel = mylit emptyContent . are there any other way to use combinator for creating a structure ( as opposed to "operate on an existing structure")? thanks Luc This message is for the named person's use only. It may contain confidential, proprietary or legally privileged information. No confidentiality or privilege is waived or lost by any mistransmission. If you receive this message in error, please immediately delete it and all copies of it from your system, destroy any hard copies of it and notify the sender. You must not, directly or indirectly, use, disclose, distribute, print, or copy any part of this message if you are not the intended recipient. CREDIT SUISSE GROUP and each of its subsidiaries each reserve the right to monitor all e-mail communications through its networks. Any views expressed in this message are those of the individual sender, except where the message states otherwise and the sender is authorised to state them to be the views of any such entity. Unless otherwise stated, any pricing information given in this message is indicative only, is subject to change and does not constitute an offer to deal at any price quoted. Any reference to the terms of executed transactions should be treated as preliminary only and subject to our formal written confirmation.

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BAL paper available
by S.D.Mechveliani 23 May '01

23 May '01

Paper announcement ------------------ The file http://www.botik.ru/pub/local/Mechveliani/basAlgPropos/haskellInCA1.ps.zip contains more expanded explanations on the BAL (Basic Algebra Library) project (previous variant was haskellInCA.ps.zip). My real intention in whole this line of business was always not just to propose a standard but rather to discuss and to find, what may be an appropriate way to program mathematics in Haskell. The matter was always in parametric domains ... Whoever tried to program real CA in Haskell, would agree that such a problem exists. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Tutor for Haskell is Wanted
by Jbkim1078＠aol.com 23 May '01

23 May '01

Hello, I am Yoon, a student of UNSW in Sydney,Australia. I am learning Haskell but need a lot of help for the course. Is there any way that I can get a private tutor? Thanks very much.

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RE: Haskell & .NET CLR
by Simon Peyton-Jones 23 May '01

23 May '01

| Are any of the various Haskell implm., e.g. ghc, being | targeted for the .NET CLR? Yes, we are, here in Cambridge. We've run some Haskell programs on the CLR already, but it's not nearly ready for release. Simon

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Re: Haskell & .NET CLR
by Bill Halchin 23 May '01

23 May '01

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Haskell & .NET CLR
by Bill Halchin 23 May '01

23 May '01

Hello, Are any of the various Haskell implm., e.g. ghc, being targeted for the .NET CLR? Also can anybody point in the direction of any projects to implement the .NET CLR on anything other than Win32 platforms, e.g. that evil Free Software Foundation :^)?? Let's make .NET open source! hehehehhee Regards, Bill Halchin _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com

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Re: Templates in FPL?
by cwitty＠newtonlabs.com 23 May '01

23 May '01

"D. Tweed" <tweed(a)cs.bris.ac.uk> writes: > In my experience the C++ idiom `you only pay for what you use' (==> > templates are essentially type-checked macros) and the fact most compilers > are evolved from C compilers makes working with templates a real pain in > practice. I'm not sure what you mean by type-checked here. Templates are not type-checked at definition time, but are type-checked when they are used; the same is true of ordinary macros. Carl Witty

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RE: Recursive types?
by David Bakin 22 May '01

22 May '01

Thank you. Now, on seeing your Tree example I tried to use it by defining height using structural recursion. But I couldn't find a valid syntax to pattern match - or otherwise define the function - on the type CT. Is there one? Or, maybe, are constructor classes the only way to use these types? If that's the case - is that inherent in these types or just the way Haskell does it? Because I was thinking that although you don't know much about these types (e.g., the operations you can perform on k) you should be able to compute height (& size) and leaves :: Tree c k e -> [e] and even replMin on the information that's there in the type. -- Dave -----Original Message----- From: Tom Pledger [mailto:Tom.Pledger@peace.com] Sent: Monday, May 21, 2001 7:25 PM To: haskell(a)haskell.org Subject: Recursive types? David Bakin writes: | I'm having trouble understanding recursive types (e.g., as described in | Functional Programming with Overloading and Higher-Order Polymorphism by | Jones. | | He gives as an example | | | > data Mu f = In (f (Mu f)) | | > data NatF s = Zero | Succ s | > type Nat = Mu NatF | | Among the things I don't get are: | | 1. Comparing this to the non-higher-order type: | | > data NatX = Zero | Succ NatX | | What are the advantages of the higher-order type? What are the | disadvantages (incl. runtime costs)? | | 2. How are values of type Nat represented? (It helps me to think about | these things concretely.) _|_ In _|_ In Zero In (Succ _|_) In (Succ (In _|_)) In (Succ (In Zero)) In (Succ (In (Succ _|_))) In (Succ (In (Succ (In _|_)))) In (Succ (In (Succ (In Zero)))) and so on. If you never care about distinguishing In _|_ from _|_, you can save space and time by declaring `newtype Mu ...' instead of `data Mu ...', in which case Nat's run-time representation is the same size as NatX's. One advantage of such higher-order types is reusability. For example, this type CT c k e = c k (Tree c k e) -- Contained Tree, container, key, element data Tree c k e = Empty | Node (CT c k e) e (CT c k e) can be used with no frills newtype Plain k t = Plain t ... :: Tree Plain () e or with every subtree labelled ... :: Tree (,) String e or with every subtree inside the ST monad import ST ... :: Tree STRef s e etc. I don't know whether this is a shining example of an advantage, and am keen to see other comments. Regards, Tom _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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