October 2002 - Haskell

class Function ?
by Lloyd Allison 29 Oct '02

29 Oct '02

Almost certainly this is either . easy and obvious or . unnecessary or . impossible for some a well-known reason. Which is it please? ... I would like to have a ``class Function'' which has the operators ``$'', ``.'', etc. and *most* particularly ``'', so that one can define sub-classes of Function (e.g. functions having inverses, say) that can still be applied in the usual way, i.e. ``f x''. Lloyd PS. [How] can one make ``->'' an instance of some new class? -- Lloyd ALLISON, CSSE, Monash University, Victoria, Australia 3168. web: http://www.csse.monash.edu.au/~lloyd/ tel: +61 3 9905 5205 -- This catches the spirit but falls short at ``f x'' :- module Main where -- ------------------------------------------10/2002--L.A.--CSSE--Monash--.au-- -- Would like there to be a ``class Function'' having -- an apply operator, why not ($), and perhaps others such as (.), -- with ``->'' being an instance of class Function (as it is of Show 6.1.6) -- (come to that, how do you make ``->'' an instance of anything new?), -- and would like to define new instances and subclasses of class Function -- along the lines of... class Function fnType where -- would like Function to be in Prelude and ($) :: (fnType t u) -> t -> u -- rather use Prelude's ($) or is it "" ? apply :: (fnType t u) -> t -> u f $ x = apply f x -- and then would like to write f x apply f x = f Main.$ x data Arrow t u = FN (t->u) -- i.e. ``->'' instance Function Arrow where -- ? in Prelude ? apply (FN f) x = f x class (Function fnType) => Invertible fnType where -- a subclass, i.e. inverse :: fnType t u -> fnType u t -- invertible Functions data IArrow t u = IFN (t->u) (u->t) instance Function IArrow where apply (IFN f i) x = f x instance Invertible IArrow where inverse (IFN f i) = IFN i f successor = IFN (\x -> x+1) (\x -> x-1) -- e.g. an Invertible Function linRec p f x = -- e.g. yer typical linear recursive schema let x0 = x -- slightly contrived (OK, a toy) up x = if p x then dn x else x : (up (apply f x)) dn x = x : if x==x0 then [] else dn (apply (inverse f) x) in up x0 main = print "L.A., CSSE, Monash, 10/2002: Re a hypothetical class Function" >> print( successor `apply` 6 ) -- prefer successor 6 >> print( successor Main.$ 6 ) -- prefer successor $ 6 >> print( (inverse successor) `apply` 6 ) -- prefer (inverse successor) 6 >> print( linRec ((<=) 4) successor 1 ) >> print( linRec (\_->True) successor 1 ) -- ----------------------------------------------------------------------------

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Re: class Function ?
by Ashley Yakeley 29 Oct '02

29 Oct '02

At 2002-10-29 00:34, Josef Svenningsson wrote: >Overloading "blank space" or function application has been one of my >favourite haskell jokes for some time. I'm pretty sure it's not possible... -- Ashley Yakeley, Seattle WA

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FW: Active Haskell development and libraries
by Simon Peyton-Jones 29 Oct '02

29 Oct '02

I thought this message (originally on haskell-cafe) was worth sending to the main Haskell list. Mark makes good points. I've appended Manuel's reply Simon -----Original Message----- From: Mark Carroll [mailto:mark@chaos.x-philes.com] Sent: 28 October 2002 21:28 To: haskell-cafe(a)haskell.org Subject: Active Haskell development and libraries The Libraries and Tools For Haskell page has quite a list of things, but a few broken links, and links to very many projects that seem to have started off as interesting research projects that, as the page itself says, were more proofs of concept and are no longer maintained. We can't help but be slightly worried by this when we're thinking about what language(s) try out for production code, given that when we look at things like Objective Caml there are nice things like the Caml Humps at http://caml.inria.fr/humps/ that shows a lot of recent work, and even a new O'Caml O'Reilly book coming out (well, mostly a translation of the French book, I think). The Haskell Communities and Activities Report is a great start for showing some recent Haskell activity, and the Libraries and Tools page at least separates out the unsupported GUI-libraries, and there is certainly some good stuff for Haskell - for instance, we like HUnit and Haddock, and I'm thinking that Parsec might come in handy. Still, at our end we're wanting to stop and think: I'm looking forward to reading the next communities and activities report. In particular, I want to reassure myself that there is a lot of useful actively-maintained stuff we can use, and more coming. Although I'm very happy that the libraries and tools page exists at all, I wonder if there's any possibility of the Haskell stuff being listed in a nice table too, including information as when the latest release was and with what versions of which Haskell compilers each thing was last found to be working. Maybe there is some existing free software that can make such a site easily maintained? Obviously I'm not demanding that all you guys write lots of extra libraries and tools for Haskell for free for me. (-: (Of course, stuff is already being written and released by posters here.) Partly I'm suggesting that, to avoid putting people off using Haskell, maybe the active and currently-working things should be highlighted and easy to find relative to the other things, right on the libraries and tools page, given that currently there are a few broken links and projects that are practically stalled, and partly I'm wanting to see what other people think about where Haskell is now relative to things like O'Caml from the point of view of commercial instead of research users. At least, am I right in thinking that more industrial Haskell users are wanted, and that it's not meant to be solely an interesting piece of research? To take an example, right now we're not exactly sure what the recommended way is to have Java code call our Haskell functions, for instance. (Background: I have reasonable influence in a tiny company and would like us to use Haskell to create some of our core products - I'm a big believer that use of rich, strong, static typing and a functional style will bring us benefits - at least, I'd like to exploit the fruits of programming language research in recent decades. However, although I'm fairly optimistic about Haskell's future, I've jumped the wrong way before, and I'm wanting to make sure that the momentum in the Haskell community is in the right direction. We would be very happy to contribute stuff ourselves if we needed to to bring things the final few inches toward what we need, as we're generally opensource fans, but we're too small to fund a whole lot of general stuff right now.) Apologies if this seems to be a reiteration of previous vocal worrying I've been doing. (-: Really, I'm just hoping that some changes on the libraries and tools page can help users of Haskell and encourage more people to join the community - especially given that much of the legwork is already being done by Claus anyway - and to help us be sure that Haskell and its associated things is, strategically, a good technology for us to rely on, from the point of view of there being a vibrant and growing repository of useful libraries. -- Mark -----Original Message----- From: Manuel M T Chakravarty [mailto:chak@cse.unsw.edu.au] Sent: 29 October 2002 05:57 To: mark(a)chaos.x-philes.com Cc: haskell-cafe(a)haskell.org Subject: Re: Active Haskell development and libraries Mark Carroll <mark(a)chaos.x-philes.com> wrote, > We can't help but be slightly worried by this when we're thinking about > what language(s) try out for production code, given that when we look at You surely have a point here. IMHO what is needed is a system that allows library, tool, etc maintainers to add, edit, and delete entries themselves. If these entries are annotated with the date of the last update, it also becomes easier to gauge whether a project is still alive. An automatic system might even test for stale links and remove or mark them. An alternative to set up our own system would be to use an existing one; eg, freshmeat.com. There is a "Haskell" sub-category under programming languages: http://freshmeat.net/browse/834/?topic_id=834 which currently contains only 14 entries. At the very least, this sub-category could be linked from the "Libraries and Tools page" (Hello Olaf & John!) and people could be encouraged to submit their work to freshmeat. The nice side effect of this is that Haskell software, then, also pops up in general language-independent searches on freshmeat. Manuel

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Re: class Function ?
by Ashley Yakeley 29 Oct '02

29 Oct '02

At 2002-10-28 22:41, Nick Name wrote: >The only thing that should be done is to invent a special name for the >juxtaposition operator (or just to use $), and to let (->) become an >instance of the "Function" class. I think function application has to be primitive and not an operator. Otherwise you would end up with this: f a is syntactic sugar for f `juxtapose` a is syntactic sugar for (juxtapose f) a is syntactic sugar for (juxtapose ((juxtapose juxtapose) f)) a is syntactic sugar for (juxtapose ((juxtapose juxtapose) ((juxtapose ((juxtapose juxtapose) juxtapose)) f))) a etc. -- Ashley Yakeley, Seattle WA

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Re: Haskell Online Bibliography
by Benderjgdefault＠aol.com 29 Oct '02

29 Oct '02

In a message dated 10/28/2002 5:55:05 AM Central Standard Time, franka(a)cs.uu.nl writes: > An Online Bibliography of Haskell Research > http://haskell.readscheme.org/index.html > I appreciate the kind remarks regarding my site! I would like to mention that tonight I have uploaded a new version of the site which provides a simple search facility. By default, multiple words are "and"-ed together. As well the keywords +AND+ and +OR+ are supported. A space binds most strongly as an "and"; +AND+ is next most "strong"; "+OR+" is weakest. As well, there is a (relatively) new Mini-Bibliography on Module Systems for Functional Programming Languages: http://readscheme.org/modules/ Stay tuned-- there is an update to the Haskell bibliography which will add over a hundred new citations to the site! This should occur by the end of November. Jim

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Re: Haskell Online Bibliography
by Benderjgdefault＠aol.com 29 Oct '02

29 Oct '02

In a message dated 10/28/02 5:55:05 AM, franka(a)cs.uu.nl writes: >An Online Bibliography of Haskell Research >http://haskell.readscheme.org/index.html I appreciate the kind remarks regarding my site! I would like to mention that tonight I have uploaded a new version of the site which provides a simple search facility. By default, multiple words are "and"-ed together. As well the keywords +AND+ and +OR+ are supported. A space binds most strongly as an "and"; +AND+ is next most "strong"; "+OR+" is weakest. As well, there is a (relatively) new Mini-Bibliography on Module Systems for Functional Programming Languages: http://readscheme.org/modules/ Stay tuned-- there is an update to the Haskell bibliography which will add over a hundred new citations to the cite! This should occur by the end of November. Jim

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Re: Haskell Online Bibliography
by Bill Halchin 28 Oct '02

28 Oct '02

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exception classifier functions
by Dean Herington 28 Oct '02

28 Oct '02

It's unfortunate that the exception classifier functions differ between GHC and Hugs, even where capabilities overlap. GHC has: ioErrors :: Exception -> Maybe IOError arithExceptions :: Exception -> Maybe ArithException errorCalls :: Exception -> Maybe String dynExceptions :: Exception -> Maybe Dynamic assertions :: Exception -> Maybe String asyncExceptions :: Exception -> Maybe AsyncException userErrors :: Exception -> Maybe String Hugs has: justIoErrors :: Exception -> Maybe IOError justHugsExceptions :: Exception -> Maybe HugsException Would greater commonality be possible? Also, I don't care for the naming scheme for these functions. While use of these functions with `catchJust` and `tryJust` reads very well, other use reads poorly. Names such as `asIOError`, `asArithException`, etc., would better described their interfaces. -- Dean

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RE: precedence bug with derived instances
by Simon Peyton-Jones 28 Oct '02

28 Oct '02

| (1) In the first section, in: | | instance (cx, cx') => Ci (T u1 ... uk) where { d } | | the use of "(cs, cs')" is a bit loose (that is, suggestive rather than | precise syntax). One can't (according to the report, though GHC seems to | allow it) have nested parentheses in a context, which would occur if cx | included parentheses. True --- but I think it's too late to improve this. I don't think it's misleading enough to matter. | (2) In section D.1, in: | | For example, False < _|_ is _|_, even though False is the first | constructor of the Bool type. | | change the operator from "<" to "<=". There is no reason to expect (False < | _|_) to be True. Good idea. | (3) Section D.4 gives the expected rule: | | head (readsPrec d (showsPrec d x "")) == (x,"") | | Why should this not be: | | readsPrec d (showsPrec d x "") == [(x,"")] I can't think why not, unless there is a shorter valid parse, which I suppose is conceivable. I'd rather not change this. | (4) I don't understand this paragraph in section D.4: | | readsPrec will parse any valid representation of the standard types | apart from lists, for which only the bracketed form [...] is accepted. See | Appendix A for full details. | | The statement seems to say that ((readsPrec 5 (show | "abc"))::[([Char],String)]) is invalid, but that can't be what it means to | say. Sigh. You are right. For strings, only "foo" is acceptable. (GHC accepts the ['f','o','o'] form too, but the Report does not. So I guess the wording should be: "readsPrec will parse any valid representation of the standard types apart from strings, for which only quoted strings are accepted, and other lists, for which only the bracketed form [...] is accepted. " | (5) The next paragraph of section D.4 says: | | Spaces and parentheses are only added where needed, ignoring | associativity. | | The phrase "only where needed" suggests that (show (1 :$ 2 :$ NT)) should | produce the string "1:$(2:$NT)" (no spaces) rather than the string "1 :$ (2 | :$ NT)", as claimed later in the section (and borne out by GHC). It needs | to be stated that an infix operator is preceded and followed by a space. Maybe I should remove "spaces and"; this is really only about parens. | (6) In the bullet that begins "The derived Read instance allows", change | "parenthese" to "parentheses". Thanks Simon

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Haskell Online Bibliography
by Frank Atanassow 28 Oct '02

28 Oct '02

An Online Bibliography of Haskell Research http://haskell.readscheme.org/index.html I didn't see this announced; sorry if I missed it and this is a duplicate. Kudos to Jim Bender for a nice site! -- Frank

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