August 2007 - Haskell-prime

hello
by Tim- tigre11 30 Aug '07

30 Aug '07

hello thanks for my new subscription, do I need to pay a fee or is it free to join ?

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RE: Re: Remember the future
by Benjamin Franksen 24 Aug '07

24 Aug '07

Simon Peyton-Jones wrote: > | It is unfortunate that the [ghc] manual does not give the translation rules, or at > | least the translation for the given example. > > Hmm. OK. I've improved the manual with a URL to the main paper > http://citeseer.ist.psu.edu/erk02recursive.html > which is highly readable. And I've given the translation for the example as you suggest Cool, thanks. BTW, the Haskell' wiki says its adoption status is 'probably no' which I find unfortunate. IMHO recursive do is a /very/ useful and practical feature and the cons listed on http://hackage.haskell.org/trac/haskell-prime/wiki/RecursiveDo don't weigh enough against that. Ok, just my (relatively uninformed) 2 cents. Cheers Ben

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template haskell syntax
by Christian Maeder 24 Aug '07

24 Aug '07

Hi, for haskell prime I suggest to change the stolen syntax "[e|", "[p|", "[d|", "[t|" to [|<letter>| in order to avoid the confusion with list comprehensions. Cheers Christian

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Re: Qualified identifiers opinion
by Isaac Dupree 20 Aug '07

20 Aug '07

Simon Marlow wrote: > I believe the solution we adopted for GHC 6.8.1 (and I proposed for > Haskell') strikes the right balance. > > M.where is lexed as an identifier. This doesn't require adding any > exceptions or corner cases to either the implementation or the > specification of the grammar. It is much easier to implement than the > existing Haskell 98 rule (I deleted 30 lines of code from GHC's lexer to > implement it). It's easy to understand. It removes an opportunity for > obfuscation. It must be the right thing! Now I've found the h'-wiki page http://hackage.haskell.org/trac/haskell-prime/wiki/QualifiedIdentifiers I _think_ the change to lexical syntax on that page is the one Simon mentions? and is also the same as what I am supporting? (I am terribly confused about "Foo.f = " though, since I thought I _used_ some code that qualified its definitions that way, and thought it was odd. Maybe it was just referring to the things it defined by e.g. Foo.f (without importing itself), and I was confused, and further confused that definitions then COULDN'T be qualified that way? oh dear...) Isaac

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Re: Qualified identifiers opinion
by Isaac Dupree 18 Aug '07

18 Aug '07

Christian Maeder wrote: | 3. I'm against qualified identifiers, with the unqualified part being a | keyword like "Foo.where". (The choice of qualification should be left to | the user, usually one is not forced to used qualified names.) Okay, here's a thought experiment... one may follow along, and agree or not as one likes (I'm not sure how much I agree with it myself, though it might be an interesting way to structure a compiler) > {-# LANGUAGE ForeignFunctionInterface #-} > module Foo where Suppose all modules have an implicit, unavoidable > import ":SpecialSyntax" (module, where, let, [], -- ... > , foreign --because that extension is enabled > ) Now let's import some imaginary already-existing modules that use "keywords" > import A (foreign) > import B (mdo) This turns up a problem already: explicitly naming things in an import or export list might not work unambiguously, because keywords are sometimes used to mean special things there. Going on... maybe we imported the whole modules. According to standard Haskell import rules, there is no conflict until the ambiguous word is used. Either "f" here works fine, because ":SpecialSyntax" in this module did not import "mdo": > f = mdo > f = B.mdo Whereas the possibilities with "foreign"... > g = foreign --error, ambiguous!!!! > foreign import ccall ........ --error, ambiguous!!!! > g = A.foreign --okay, unambiguous > ":SpecialSyntax".foreign import ccall .... -- can't write in Haskell! Now, if we want to avoid the understandably undesirable matter of imports interfering with keywords (after all, keywords can appear before the import list is finished, such as "module" "where" and "import"), we could tweak the import-conflict rules for this special case. In this module where "foreign" is imported from ":SpecialSyntax", the mere phrase "import A" could raise an error, as if all imported syntax were used (unqualified, as always) in the module. Whereas, "import qualified A" would not. (and what about "import A hiding ..."?) By the way, we are lucky that pragmas have their own namespace {-# NAME arguments #-}. But as I mentioned, we lack a namespace for extensions that have a semantic impact on the annotated code. Certain preprocessors invent things like {-! !-} ... or add template-haskell syntax, or some arbitrary other keywords syntax like "proc...do"... or even steal large portions of existing comment syntax (Haddock, which isn't even a semantic impact on the code!) BTW #2: The simpler and less variable the lexer is, the easier it is to scan for LANGUAGE pragmas. That search doesn't need to deal with keywords at all. (although it may be popular not to use the usual lexer in order to search for those pragmas, I don't know) Isaac

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Re: Qualified identifiers opinion
by Isaac Dupree 18 Aug '07

18 Aug '07

Christian Maeder wrote: > Hi Isaac, > > just to give you a reply at all, see below. I reply > glasgow-haskell-users(a)haskell.org since I'm not subscribed to > haskell-prime. And I don't want to subscribe, because I'm more > interested that Haskell becomes more stable (and standard). Then maybe you can join haskell-prime and provide the energy that rounds up all the little fixes and tries to actually produce the thing! Drastic changes are not intended to go in. Haskell' should bring more stability and standardness (as long as it doesn't diverge too much from Haskell98, which would decrease stability and standardness) > So here is > my opinion: > > 1. The lexer should recognize keywords. > > 2. I would not mind if Haskel98 rejected all keywords that are also > rejected by extensions, so that the lexer is extension independent. > (Starting with Haskell98, removing conflicting identifiers as soon as I > switch on valuable extensions does not make sense.) Trouble is, extensions are just that: extensions, and more with their own keywords may be added in the future! unless we want an internet-standard-like "x-keywordname" - but that doesn't solve this problem: standardized new keyword names clogging up the general namespace, as long as they don't have a symbol (like Objective-C has @class, @whatever...). > 3. I'm against qualified identifiers, with the unqualified part being a > keyword like "Foo.where". (The choice of qualification should be left to > the user, usually one is not forced to used qualified names.) > > 4. However, "Foo.where" should always be rejected and not changed to > "Foo.wher e"! (Longest matching, aka "maximal munch", must not consider > keywords!) > > (see end of: http://www.haskell.org/onlinelibrary/lexemes.html#sect2.4) > > I would not mind if a name "F. " is plainly rejected. It only makes > sense, when a data constructor is the first argument of the composition > operator "(.)" I wouldn't mind if that was banned either. That case needs to be considered for implementing my lexer. In fact, banning that and qualified keywords allows the lexer proper not to know keywords and nevertheless ban qualified keywords (a bit of a hack). But... while I wouldn't _recommend_ using qualified keywords, and compilers could give a warning even for haskell98 code that uses known extension-keyword-names at all, it seems best to me, to _allow_ them, in the interests of allowing code to remain fairly stable with the potential of extensions being developed (especially thinking of the BangPatterns that had an effect on existing definitions of (!) ). > > Maybe "." and "$" as operators should require white spaces on both > sides, since "$(" also indicates template haskell. but it's so convenient as it is... plenty of code uses (.) without spaces, and I don't like the way template-haskell steals "$(" and "$id" (from the point of view of a person who has never tried to use template-haskell). I think haskell is more stable by allowing existing code e.g. (f = fix (\rec -> .... rec ....) --'rec' is arrow-sugar keyword than banning some bunch of new keyword names. And allowing interim interoperability with old code that exports those names, like the unfortunate (!) or (.) (I know, those aren't exactly ever keywords/syms) seems like a good idea when it removes complexity rather than adding it. I don't want Haskell98 to become a language that has difficulty interoperating with libraries and using-applications that use newer Haskells. from other comments: >> What's wrong with the status quo? Our current lexical rules *seem* >> complicated to newbies, but just like everything else in Haskell it carries >> a deep simplicity; having only one rule (maximal-munch) gives a certain >> elegance that the proposals all lack. >> >> I'd hate to see Haskell become complex all the way down just to fix a few >> corner cases; I see this pattern of simplicity degerating through >> well-intentioned attempts to fix things all over the language... > > I agree with Stefan, for the reasons he stated and for one additional > reason: There would be a multitude of unintended behavior changes. Well, GHC doesn't implement aforementioned maximal-munch re: keywords. I don't think it's good (compositional?) design for the set of keywords to be part of the lexer rather than a pass after it, when keywords behave so similarly to other words, and also when there are non-keywords like "as" and "qualified" and sometimes "forall" (whose non-reserved status I support). lex --> keywords --> layout --> parse Besides, I don't think any of the above proposals will generate behavior changes in real code. Some cause more errors (adding more keywords; banning adjacent '.' or '$') and some allow a few more things that were errors before. f = Just.let x = x in id --a.k.a. f = Just would break in my proposal, but it also breaks according to Haskell98... Isaac

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Qualified identifiers opinion
by Isaac Dupree 16 Aug '07

16 Aug '07

Especially after writing a partial lexer for Haskell, I opine that this should be all legal: module Foo where --in case you didn't know, this is legal syntax: Foo.f = undefined Foo.mdo = undefined Foo.where = undefined x Foo.! y = undefined x Foo... y = undefined --remember ".." is reserved id, e.g. [2..5] {-# LANGUAGE RecursiveDo, BangPatterns #-} module Bar where import Foo hello !x = mdo { y <- Foo.mdo Foo... ({-Foo.-}f x y); return y } {- Haskell 98 -} module Baz where import Foo goodbye x = x ! 12 (Foo.where) lexing as (Foo.wher e) or (Foo . where) does not make me happy. (being a lexer error is a little less bad...) Especially not when the set of keywords is flexible. I don't see any good reason to forbid declaring keywords as identifiers/operators, since it is completely unambiguous, removes an extension-dependence from the lexer and simplifies it (at least the mental lexer); Also I hear that the Haskell98 lexing is (Foo.wher e), which I'm sure no one relies on... Well, that's my humble opinion on what should go into Haskell' on this issue. Isaac

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default for quotRem in terms of divMod?
by Isaac Dupree 08 Aug '07

08 Aug '07

(something very odd was going on with that reply-to! trying to send this in a sensible way...) Christian Maeder wrote: > > > Isaac Dupree wrote: >> In class Integral, divMod has a default in terms of quotRem. >> (quot,rem,div,mod all have defaults as the both-function they're part >> of.) I'm sure divMod is more natural than quotRem to implement for some >> types... so why doesn't quotRem have a default in terms of divMod? it >> has no default! Then the "minimal to implement" will change from >> (toInteger and quotRem) to (toInteger and (quotRem or divMod)). >> >> Isaac > > while I don't care if quotRem or divMod should be implemented. I oppose > to give both default implementations in terms of the other. > > Already for the class Eq either == or /= must be defined, with the > unpleasant effect that an empty instance like: > > instance Eq T > > leads to a loop (when == or /= is called on elements of type T). > > The empty instance does not even raise a warning about unimplemented > methods (since the default definition is used). > > I'd rather prefer to remove /= as method of Eq. I second that this is a problem. However, I think that compilers are perfectly justified in replacing nontermination with a runtime error(i.e. exception - the generalization of a call to "error"), and also generating warnings when it's unimplemented class methods, and we could solve it that way. I don't know how hard it is to detect automatically that two default definitions produce a useless loop... if that's too hard, possibly some explicit annotation in the code could be done (Already there is an informal convention of saying exactly what methods must be implemented, so that should be formalize-able...). I _think_ that for Eq and my Integral proposal, GHC's strictness analyser is quite up to the task. In general, what do we think about replacing certain nontermination with an exception, particularly by the compiler's detection? GHC already does it very unreliably with the <<loop>> low-level thunk something-or-other.... It might be interesting to see how many spurious warnings would be generated, if GHC also generated a warning for each - i.e. is nontermination ever intentionally used as a _|_ normally? Isaac

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default for quotRem in terms of divMod?
by Isaac Dupree 08 Aug '07

08 Aug '07

In class Integral, divMod has a default in terms of quotRem. (quot,rem,div,mod all have defaults as the both-function they're part of.) I'm sure divMod is more natural than quotRem to implement for some types... so why doesn't quotRem have a default in terms of divMod? it has no default! Then the "minimal to implement" will change from (toInteger and quotRem) to (toInteger and (quotRem or divMod)). Isaac

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