### Re: [GHC] #851: Incomplete-pattern checking for n+k patterns is not implemented

#851: Incomplete-pattern checking for n+k patterns is not implemented ---+ Reporter: simonpj | Owner: Type: bug | Status: closed Priority: low | Milestone: _|_ Component: Compiler |Version: 6.4.2 Resolution: wontfix | Keywords: warnings Difficulty: Unknown | Os: Unknown/Multiple Testcase: ds061 | Architecture: Unknown/Multiple Failure: None/Unknown | ---+ Changes (by igloo): * status: new = closed * failure: = None/Unknown * resolution: = wontfix Comment: The !NoNPlusKPatterns proposal was accepted for H'2010, so I don't think it is worth spending time on this. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/851#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### Re: [GHC] #3382: Panic in tidyNPat when using N+k patterns with overloaded strings

#3382: Panic in tidyNPat when using N+k patterns with overloaded strings -+-- Reporter: batterseapower|Owner: Type: bug | Status: closed Priority: normal|Milestone: Component: Compiler | Version: 6.10.4 Severity: normal| Resolution: fixed Keywords:| Difficulty: Unknown Testcase: deSugar/should_run/T3382 | Os: Unknown/Multiple Architecture: Unknown/Multiple | -+-- Changes (by simonpj): * testcase: = deSugar/should_run/T3382 * difficulty: = Unknown * status: new = closed * resolution: = fixed Comment: Ah yes, excellent point. Fixed by {{{ Mon Jul 20 07:12:26 GMT Daylight Time 2009 simo...@microsoft.com * Fix Trac #3382: desugaring of NPats Max spotted that the short-cut rules for desugaring NPats (where we compare against a literal) were wrong now that we have overloaded strings. }}} thanks Simon -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/3382#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### [GHC] #3382: Panic in tidyNPat when using N+k patterns with overloaded strings

#3382: Panic in tidyNPat when using N+k patterns with overloaded strings -+-- Reporter: batterseapower| Owner: Type: bug | Status: new Priority: normal| Component: Compiler Version: 6.10.4| Severity: normal Keywords:| Testcase: Os: Unknown/Multiple | Architecture: Unknown/Multiple -+-- Spotted this while reading the source code to tidyNPat. It looked dubious to me, and indeed it is: {{{ {-# OPTIONS_GHC -XOverloadedStrings #-} module Main where import Data.String instance IsString Int where fromString x = 1337 f :: Int - String f hello = correct f _ = false main = do print $ f 1337 print $ f 1338 }}} Result: {{{ mbolingbr...@mb566 ~/Programming/Checkouts $ ghc --make Boog.hs [1 of 1] Compiling Main ( Boog.hs, Boog.o ) ghc: panic! (the 'impossible' happened) (GHC version 6.10.3 for i386-apple-darwin): tidyNPat }}} -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/3382 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### RE: [Haskell-cafe] (n+k) patterns

Richard O'Keefe wrote: The thing is that it really seems bizarre to see this one feature singled out for non-implementation. If I can do the equivalent of n+k patterns by programming in the *type system*, why *not* in a pattern? Do you mean by something like the following? data Z = Z data S n = S n type Plus2 a = S (S a) minus2 :: Plus2 a - a minus2 _ = undefined -- or actually use the values, or whatever If so, I'd say that n+k patterns go well beyond this kind of pattern aliases, particularly since they operate on arbitrary Nums, not just an inductively defined natural number type. Cheers, Ganesh === Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html === ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [GHC] #851: Incomplete-pattern checking for n+k patterns is not implemented

#851: Incomplete-pattern checking for n+k patterns is not implemented --+- Reporter: simonpj | Owner: Type: bug | Status: new Priority: low | Milestone: _|_ Component: Compiler |Version: 6.4.2 Severity: normal| Resolution: Keywords: warnings | Difficulty: Unknown Testcase: ds061 | Architecture: Unknown Os: Unknown | --+- Changes (by igloo): * keywords: = warnings * testcase: = ds061 -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/851 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### Re: [GHC] #851: Incomplete-pattern checking for n+k patterns is not implemented

#851: Incomplete-pattern checking for n+k patterns is not implemented ---+ Reporter: simonpj | Owner: Type: bug | Status: new Priority: low | Milestone: _|_ Component: Compiler |Version: 6.4.2 Severity: normal| Resolution: Keywords:| Os: Unknown Difficulty: Unknown | Architecture: Unknown ---+ Changes (by simonpj): * milestone: = _|_ * priority: normal = low Comment: See Task #595 -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/851 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### Erroneous warning with n+k patterns

Hi all, This function: g :: Int - Int g (x+1) = 1 g _ = 0 (erroneously) gives this warning: Warning: Pattern match(es) are overlapped In the definition of `g': g _ = ... I'm not sure if this is covered by the first item on http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html but if so (well, probably even if not; I've grown quite attached to -Wall -Werror) then I'd like to add my voice to those who would like to see the mentioned overhaul done. Thanks Ian ___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### Re: bugs from n+k patterns (was: Re: Preventing/handling space leaks)

L.S., On Thu, 11 Dec 2003 06:23:51 -0800, Iavor S. Diatchki [EMAIL PROTECTED] wrote: : : Henk-Jan van Tuyl wrote: 1) It takes no effort, once you are use to it, to code without n+k patterns; this does not seem like a good argument. there are many other features like that in haskell. for example, going by that we could remove lambda abstractions (i am not saying we should) This is, of course, as apposed to the effort it takes to solve problems that might occur when using n+k patterns. on the other hand, when you often use these patterns, you might spend hours debugging an endless looping program. how do n+k patterns lead to looping programs? I can't remember exactly, it was a complicated story and I already decided not to use these patterns (it had something to do with pattern bindings; I don't know where I read it). If you are working under high pressure in a large project, chances are, that the testing departement will find your bug and write a bug report (or worse, the customer might find it). Report handling and bug solving costs an enormous amount of money. This has resulted in the clean room approach for software design: prevent bugs rather than solve them. See also Finnagle's Law. i find this reasoning backward. i agree (of course!) that we should write programs without bugs. i find it strange that people often motivate that, by telling me that bugs cost a lot of money for some company. if companies happened to make money out of bugs (and some do), would it then be ok to write buggy software? i guess it all comes down to what one takes as primary -- writing good software, or making money. It is almost impossible to write large programs without bugs; it takes a lot of effort to prevent/solve bugs, and a lot of people don't seem to care much about this. I have often heard remarks like: - We don't have time to do things right the first time; our software will be too expensive if we try to do that. - It's easier to solve bugs afterwards. (How will you know you found every bug?) - It takes too much time to think about a structured solution. - If I use a goto, I have less typing to do. You can write perfect programs using gotos. - After explaining the first few steps of the V-model to a new project leader: Whenever will you get to the real work if you do all that?! - (At the yearly performance evaluation:) You asked me to write down the specs of the program that you were to design: you shouldn't stick to formalism when we have so little time. - We have to release version 3.0 before our competitor (or: we have to stick to the release date); we will solve the rest of the bugs in 3.1 (as we had only one month to solve a lot of bugs and add the missing pieces and it wasn't properly tested, release 3.1.1 was necessary to get the product accepted by our customers). - The client doesn't mind a few bugs. If you look up statistics in software engineering books, you will find that only one or two percent of the software is bugfree and according to customer specifications, at the first release. An amazing percentage of projects is even never finished (I believe it was twenty percent). To get back to the topic of this mailing list: in my search for reliable software for a reasonable amount of work, I found that Lockheed uses a functional language in the specification phase of their projects, to create testable function specifications. That's why I am now studying Haskell, to use either as a specification language or for coding the final product. 2) It is likely, that n+k patterns dissapear in the next Haskell standard. If you don't like to rewrite, test and debug all your software every few years, don't use any language/compiler features that are likely to dissapear. This is another thing that might cost companies a lot of money. i didn't know anyone is working on a next haskell standard. have n+k patterns been made obsolete? See http://www.haskell.org/development/ or search the web for Haskell 2 or Haskell II. -iavor -- Best regards, Henk-Jan van Tuyl ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe

### bugs from n+k patterns (was: Re: Preventing/handling space leaks)

hi, first here is why i think n+k patterns are problematic; 1) they lead to some parsing awkwardness (e.g. when n+k pattern bindings are involved, but those don'treally make much sense anyways) 2) in haskell as it is, patterns are associated with algebraic datatypes, and n+k patterns may erronously suggest that all numbers are such for the rest, apologies if i appear to be ranting :-) Henk-Jan van Tuyl wrote: 1) It takes no effort, once you are use to it, to code without n+k patterns; this does not seem like a good argument. there are many other features like that in haskell. for example, going by that we could remove lambda abstractions (i am not saying we should) on the other hand, when you often use these patterns, you might spend hours debugging an endless looping program. how do n+k patterns lead to looping programs? If you are working under high pressure in a large project, chances are, that the testing departement will find your bug and write a bug report (or worse, the customer might find it). Report handling and bug solving costs an enormous amount of money. This has resulted in the clean room approach for software design: prevent bugs rather than solve them. See also Finnagle's Law. i find this reasoning backward. i agree (of course!) that we should write programs without bugs. i find it strange that people often motivate that, by telling me that bugs cost a lot of money for some company. if companies happened to make money out of bugs (and some do), would it then be ok to write buggy software? i guess it all comes down to what one takes as primary -- writing good software, or making money. 2) It is likely, that n+k patterns dissapear in the next Haskell standard. If you don't like to rewrite, test and debug all your software every few years, don't use any language/compiler features that are likely to dissapear. This is another thing that might cost companies a lot of money. i didn't know anyone is working on a next haskell standard. have n+k patterns been made obsolete? -iavor -- == | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | == ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: bugs from n+k patterns (was: Re: Preventing/handling space leaks)

On Thu, 11 Dec 2003 06:23:51 -0800 Iavor S. Diatchki [EMAIL PROTECTED] wrote: ... Henk-Jan van Tuyl wrote: ... 2) It is likely, that n+k patterns dissapear in the next Haskell standard. If you don't like to rewrite, test and debug all your software every few years, don't use any language/compiler features that are likely to dissapear. This is another thing that might cost companies a lot of money. i didn't know anyone is working on a next haskell standard. have n+k patterns been made obsolete? I don't know of any other feature that has had this, Many people feel that n+k patterns should not be used. These patterns may be removed or changed in future versions of Haskell., said about it in the (online) Report (3.17.2). Obviously, the second sentence is vacuous; anything can be removed or changed in future versions of Haskell, but the intent seems clear (and is somewhat self-fulfilling). ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: bugs from n+k patterns (was: Re: Preventing/handling space leaks)

The n+k pattern issue inspired endless debates on the Haskell committee and this feature was considered for removal in nearly every iteration of the Haskell report. We all agreed that n+k is extremely ad-hoc but that certain programs can be expressed slightly more elegantly using them. Unfortunately n+k doesn't match against negative numbers, so let n+1 = ... in n ... is not the same as let n = in (n-1) ... Of course n+k was designed for natural numbers but these are not a separate numeric type so you get a certain amount of confusion. One proposal was to make naturals a distinct type and restrict n+k to only naturals. The syntactic issues surrounding n+k are truely awful and I still have to look at the report to remember what happens with these: n+1 = 2 (n+1) = 2 A lot of people would have been happy to replace n+k by some view-like mechanism that gives the user control over the meaning of n+k but we never managed to get views into the report. John ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe

### RE: n + k patterns

Good point. Fixed in the head, but not in 6.0 Thanks Simon | -Original Message- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On | Behalf Of Ralf Hinze | Sent: 28 May 2003 20:40 | To: GHC bugs | Subject: n + k patterns | | GHCi infers for | | fac 0 = 1 | fac (n + 1) = (n + 1) * fac n | | the following type | | / _ \ /\ /\/ __(_) | / /_\// /_/ / / | | GHC Interactive, version 5.04.2, for Haskell 98. | / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | \/\/ /_/\/|_| Type :? for help. | | Loading package base ... linking ... done. | Loading package haskell98 ... linking ... done. | Compiling Main ( Fac.lhs, interpreted ) | Ok, modules loaded: Main. | *Main :t fac | forall a. (Num a, Ord a) = a - a | | The Report, however, states that | | 3.17.2 Informal Semantics of Pattern Matching | ... | An n+k pattern can only be matched against a value in the class Integral. | | Cheers, Ralf | | ___ | Glasgow-haskell-bugs mailing list | [EMAIL PROTECTED] | http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### n + k patterns

GHCi infers for fac 0 = 1 fac (n + 1) = (n + 1) * fac n the following type / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.04.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \/\/ /_/\/|_| Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Compiling Main ( Fac.lhs, interpreted ) Ok, modules loaded: Main. *Main :t fac forall a. (Num a, Ord a) = a - a The Report, however, states that 3.17.2 Informal Semantics of Pattern Matching ... An n+k pattern can only be matched against a value in the class Integral. Cheers, Ralf ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

### RE: n+k patterns

| hbc is on the Integral side, if that counts. :-) | Just because ghc doesn't follow the spec isn't a good reason | to change the spec. :-) I absolutely didn't say that! All I'm saying is * Two of the four impls have to change regardless * The change is non-de-stabilising on the rest of the report * So we should think what the best answer is I argued that (Num a, Ord a) makes most sense to me. You argued that (Integral a) was a conscious choice (something I don't remember but I'm sure you're right), and is the right one anyway. I'd be interested to know what others think. If there's any doubt, we'll stay with Integral. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

I argued that (Num a, Ord a) makes most sense to me. You argued that (Integral a) was a conscious choice (something I don't remember but I'm sure you're right), and is the right one anyway. I'd be interested to know what others think. If there's any doubt, we'll stay with Integral. My view is that (n+k) patterns are evil, so it doesn't really matter what we decide. :-) No, seriously, I'm a little worried about widening the range of numeric types for which (n+k) patterns are supposed to work. I can (just about) imagine wanting to use Rationals in an (n+k) pattern, but Float and Double? I'm not convinced that would be useful. f :: Double - Double f (n+1) = n Main print (f 3.0) 2.0 Main print (f 2.0001) 1.00010002 Main print (f 1.01) 1.00082740371e-10 Regards, Malcolm ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

I argued that (Num a, Ord a) makes most sense to me. You argued that (Integral a) was a conscious choice (something I don't remember but I'm sure you're right), and is the right one anyway. I'd be interested to know what others think. If there's any doubt, we'll stay with Integral. My view is that (n+k) patterns are evil, so it doesn't really matter what we decide. :-) No, seriously, I'm a little worried about widening the range of numeric types for which (n+k) patterns are supposed to work. I can (just about) imagine wanting to use Rationals in an (n+k) pattern, but Float and Double? I dimly remember that the justification for having n+k was to allow inductive definitions, which only applies to Integral. I'd vote for keeping it as it is, too. Jón ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

I strongly disapprove of n+k patterns from a whole-language taste perspective, so I am most unkeen to broaden their scope. Because they are such a language kludge already it simply doesn't make sense to try to reason rationally about what the best answer for them is. It's like putting lipstick on a chicken. If anything, we should have restricted them to the very simplest case covered in the early textbooks, i.e. just Int. John | hbc is on the Integral side, if that counts. :-) | Just because ghc doesn't follow the spec isn't a good reason | to change the spec. :-) I absolutely didn't say that! All I'm saying is * Two of the four impls have to change regardless * The change is non-de-stabilising on the rest of the report * So we should think what the best answer is I argued that (Num a, Ord a) makes most sense to me. You argued that (Integral a) was a conscious choice (something I don't remember but I'm sure you're right), and is the right one anyway. I'd be interested to know what others think. If there's any doubt, we'll stay with Integral. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

I too am against broadening the scope of n+k patterns, for reasons that others have already given. In particular, I am absolutely against allowing n+k patterns to be used for Float/Double. If n+k patterns are to be meaningful at all, you want matching y against x+1, you want a unique x such that (x+1)=y. Yet for floats there may not be such an x (y = positive zero), or there may be more than one (y=2^n with n chosen so that 2^n+1 is not exactly representable but 2^n-1 is, then x could be 2^n or 2^n-1). ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

At 03:27 30-01-02 -0800, Simon Peyton-Jones wrote: | hbc is on the Integral side, if that counts. :-) | Just because ghc doesn't follow the spec isn't a good reason | to change the spec. :-) I absolutely didn't say that! All I'm saying is * Two of the four impls have to change regardless * The change is non-de-stabilising on the rest of the report * So we should think what the best answer is I argued that (Num a, Ord a) makes most sense to me. You argued that (Integral a) was a conscious choice (something I don't remember but I'm sure you're right), and is the right one anyway. I'd be interested to know what others think. If there's any doubt, we'll stay with Integral. Personally I vote for keeping Integral. The strongest reason for my choice is that if we want to be sure the pattern is really correct, we need a bijection. For Integral, we have + and - to form one, but we can't construct one for Float and Double, though by this change they would be allowed in the pattern. Rijk-Jan van Haaften ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

OK, OK, I give in! Integral it remains. I repent. Simon | -Original Message- | From: Rijk J. C. van Haaften [mailto:[EMAIL PROTECTED]] | Sent: 30 January 2002 17:00 | To: Simon Peyton-Jones | Cc: [EMAIL PROTECTED] | Subject: RE: n+k patterns | | | At 03:27 30-01-02 -0800, Simon Peyton-Jones wrote: | | hbc is on the Integral side, if that counts. :-) | | Just because ghc doesn't follow the spec isn't a good reason to | | change the spec. :-) | | I absolutely didn't say that! All I'm saying is | | * Two of the four impls have to change regardless | * The change is non-de-stabilising on the rest of the report | * So we should think what the best answer is | | I argued that (Num a, Ord a) makes most sense to me. | You argued that (Integral a) was a conscious choice | (something I don't | remember but I'm sure you're right), and is the right one anyway. | | I'd be interested to know what others think. If there's any doubt, | we'll stay with Integral. | | Personally I vote for keeping Integral. The strongest reason | for my choice is that if we want to be sure the pattern is | really correct, we need a bijection. For Integral, we have + | and - to form one, but we can't construct one for Float and | Double, though by this change they would be allowed in the pattern. | | Rijk-Jan van Haaften | | ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

Simon Peyton-Jones wrote: | hbc is on the Integral side, if that counts. :-) | Just because ghc doesn't follow the spec isn't a good reason | to change the spec. :-) I absolutely didn't say that! All I'm saying is * Two of the four impls have to change regardless Only because two of the implementation teams didn't read the report. :-) -- Lennart ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

On 30 Jan 2002, at 12:01, [EMAIL PROTECTED] wrote: Yet for floats there may not be such an x (y = positive zero), or there may be more than one (y=2^n with n chosen so that 2^n+1 is not exactly representable but 2^n-1 is, then x could be 2^n or 2^n-1). Well, I am just a newbie in Haskell, but this reason, together with the fact that n+k patterns were designed to be used in inductive definitions do make me strongly favor the restriction of n+k patterns to the class Integral (Malcolm Wallace pointed out that there may be a use for Rational n+k patterns, but I think it would only be advisable in the case that Rational numbers were implemented explicitly with numerators and denominators). Besides that, what about deprecating n+k patterns in future Haskell reports? :) just my $0.02, Carlos Eduardo Scheidegger ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

In any case, I propose to change Integral to Ord and Num. I agree. And nhc98 seems to actually implement this. Nonetheless I find using n+k patterns for floating point numbers pretty horrible. And it raises the question why k cannot be a rational ... But then n+k patterns are a wart anyway. Btw., in 3.17.2 Informal Semantics of Pattern Matching the end of the following sentence should be changed: Matching a non-_|_ value x against a pattern of the form n+k (where n is a variable and k is a positive integer literal) succeeds if x=k, resulting in the binding of n to x-k, and fails if xk. There is no guarantee that if x=k == False then x k == True. So the sentence should end ..., and fails otherwise. Olaf -- OLAF CHITIL, Dept. of Computer Science, The University of York, York YO10 5DD, UK. URL: http://www.cs.york.ac.uk/~olaf/ Tel: +44 1904 434756; Fax: +44 1904 432767 ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

| Btw., in 3.17.2 Informal Semantics of Pattern Matching | the end of the following sentence should be changed: Actually I've changed the entire wording of that section in consultation with Ross who has read it rather carefully. You can find the current draft of the expressions chapter at http://research.microsoft.com/~simonpj/tmp/exps.html The numbered items in the Informal Semantics of Pattern Matching are substantially changed and IHMO much clearer. | Nonetheless I find using n+k patterns for floating point | numbers pretty horrible. And it raises the question why k | cannot be a rational ... But then n+k patterns are a wart anyway. Actually, you can only use integer 'k's. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

| On Tue, Jan 29, 2002 at 07:36:56AM -0800, Simon Peyton-Jones wrote: | The Haskell Report says of n+k patterns: | | A n+k pattern can only be matched against a value in | the class Integral. | | This seems far too strong. All that is needed are Ord (for the =) | and Num (for - and fromInteger), and indeed that's what GHC requires. | Do Hugs or nhc actually require Integral? | | Hugs demands Integral and the assumption that only one class is involved | seems deeply entwined in the code. Hugs demands Integral because that's what it was told to do to follow the report. So in that sense, yes, the code depends on having only one class. But it would be easy for someone to change that. Then again, if we're following the rules of minimal change for Haskell 98, then I wouldn't have thought this was up for grabs. (I'm thinking, for example, of the unnecessary same context restriction on mutually recursive binding groups, which has more practical impact, is very clearly a bug, and has not (AFAIK) been fixed in Haskell 98. Then there's David Wakeling's generalized gap proposal, and ...) All the best, Mark ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### RE: n+k patterns

| Hugs demands Integral because that's what it was told to do | to follow the report. So in that sense, yes, the code | depends on having only one class. But it would be easy for | someone to change that. | | Then again, if we're following the rules of minimal change | for Haskell 98, then I wouldn't have thought this was up for | grabs. (I'm thinking, for example, of the unnecessary same | context restriction on mutually recursive binding groups, | which has more practical impact, is very clearly a bug, and | has not (AFAIK) been fixed in Haskell 98. Then there's David | Wakeling's generalized gap proposal, and ...) That is a fair point, and is exactly the reason I bother the Haskell list with these proposals rather than simply executing them. This is an unforced change, as you point out, but in fact GHC and NHC currently do one thing, and Hugs does another (i.e. follows the spec!). So some of us have to change our implementations. It clearly isn't a big deal, because no one has ever reported this as a practical problem. Other things being equal, it would be better to make the spec make as much sense as possible; hence my proposal. But if it's really hard to change Hugs, maybe we should leave the spec and change GHC and nhc. But you say it would be easy. I agree it's a moot point. I lean towards making the change, but I'm willing to be persuaded. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

Well, it was a deliberate decision to limit the n+k pattern to class Integral because people at that time felt that n+k was something that should only be used with integers. So it's not a fluke, it was quite deliberate. I kind of like Integral for this (as much as I can like anything about n+k :). Since this is not a typo, I don't see why it should be changed unless there is some really good reason. -- Lennart Simon Peyton-Jones wrote: Folks The Haskell Report says of n+k patterns: A n+k pattern can only be matched against a value in the class Integral. This seems far too strong. All that is needed are Ord (for the = and -) and Num (for fromInteger), and indeed that's what GHC requires. Do Hugs or nhc actually require Integral? In any case, I propose to change Integral to Ord and Num. Does anyone think this is the Wrong Thing? How this bug has lasted so long I don't know. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Re: n+k patterns

Simon Peyton-Jones wrote: | Hugs demands Integral because that's what it was told to do | to follow the report. So in that sense, yes, the code | depends on having only one class. But it would be easy for | someone to change that. | | Then again, if we're following the rules of minimal change | for Haskell 98, then I wouldn't have thought this was up for | grabs. (I'm thinking, for example, of the unnecessary same | context restriction on mutually recursive binding groups, | which has more practical impact, is very clearly a bug, and | has not (AFAIK) been fixed in Haskell 98. Then there's David | Wakeling's generalized gap proposal, and ...) That is a fair point, and is exactly the reason I bother the Haskell list with these proposals rather than simply executing them. This is an unforced change, as you point out, but in fact GHC and NHC currently do one thing, and Hugs does another (i.e. follows the spec!). So some of us have to change our implementations. hbc is on the Integral side, if that counts. :-) Just because ghc doesn't follow the spec isn't a good reason to change the spec. :-) -- Lennart ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell

### Haskell 1.3 (n+k patterns)

I feel the need to be inflamatory: I believe n+k should go. There are lots of good reasons why they should go, of course. The question is: are there any good reasons why they should stay? My understanding is that the only reason they are advocated is that they make teaching induction easier. I don't believe it. I teach an introductory FP course including induction. I introduce structural induction directly, and the students have no problem with it. When I have tried to talk to individuals about natural number induction using (n+k) patterns, then the problems start. Because they are so unlike the form of patterns they have become used to they find all sorts of difficulties. What if n is negative. Ah yes, well it can't be. Why not. It just can't. etc. Let's throw them out. John.

### Re: Haskell 1.3 (n+k patterns)

jl writes: I feel the need to be inflamatory: I believe n+k should go. Again, I agree completely. Let's get rid of this horrible wart once and for all. It's a special case that makes the language more difficult to explain and implement. I've hardly seen any programs using it so I don't think backwards compat is a problem. Anyone who thinks this change will cause them more than 10 minutes work, plese speak up. -- Lennart

### Re: n+k patterns, etc.

I think that we should try a different approach, forget about the importing mechanism, and make a single statement defining the intended semantics. Section 1.2 (The Haskell Kernel) is the place. I propose adding the following. The translations given, and the identities given for the semantics of case expressions, are not macros. A simple replacement of the right-hand-side for the left-hand-side with substitution of parameters does not give the intended semantics. The reason for this is that the translations make use of certain names defined in the standard prelude (see section 5.4), and macro substitution could result in the capture of these names by locally defined entities, or the use of a name in a context in which it is not defined at all because the part of the prelude in which it is defined has not been imported. The general rule is: the use of a name defined in the standard prelude in a translation intended to show the semantics of a construct always implies the definition in the standard prelude. Then, people can locally rebind as much as they want, but the constructs defined by translation will be unaffected. This sounds reasonable to me. -Paul

### Re: n+k patterns, etc.

|What if (the appropriate parts of) the standard prelude is | explicitly *not* imported: | | import Prelude () | or | import Prelude hiding(map) | | (see section 5.4.3). | |Are then the hidden parts of the standard prelude still available via | n+k patterns, list comprehensions etc.? (Via some unseen and unhidable | intermediary module.) Or are constructs that use hidden parts of the | standard prelude (according to their translations given in the report) | not available? Yes. No. Respectively. The Report is obviously not clear enough on this point. The wording given for translations (eg list comprehensions) that "map" refers to the Prelude "map" is meant to indicate that it refers to the Prelude "map" whether or not the latter is explicitly in scope. That's the consistent story for all special syntax, to answer the latter part of your message. Simon

### Re: n+k patterns

From my point of view (n+k)-patterns have a very special meaning. This natural numbers should be considered as a type like this: data Nat = Zero | Succ Nat Therefore a (n+k)-pattern is an abbreviation for Succ(Succ(...Zero...)). It's obvious that "+" in "(n+k)" doesn't mean a somewhere else defined (or locally rebound) function. If we keep this in mind there shouldn't be any problem. (Tell me if I'm wrong.) Greetings, Marc Rehmsmeier.

### Re: Successor patterns in bindings and n+k patterns

And now for a little quiz. What's the value of the following (legal) Haskell expression? (Don't try it with hbc, it fails.) let (+) + 1 + 1 = (+) in 1 + 1 Given infixl 6 + (since you can't change this without renaming!): (+) + 1 + 1 == lpat6 + pat7 var + int + pat so I'd expect the answer to be 0. glhc and gofer both agree with me. Kevin

### Re: Successor patterns in bindings and n+k patterns

And now for a little quiz. What's the value of the following (legal) Haskell expression? (Don't try it with hbc, it fails.) let (+) + 1 + 1 = (+) in 1 + 1 This is illegal syntax!! (+) and (the second) + are the same variable, thus violating the linearity constraint for left-hand-sides! It should be as illegal as writing something like: f f = 1 Isn't that OBVIOUS?? (:-) -Paul

### Re: Successor patterns in bindings and n+k patterns

Another strange thing about n+k patterns. Its definition uses = , but = is not part of the class Num. Does that mean that n+k patterns have to be instances of class Real? One could leave it class Num, if the translation were expressed in terms of "signum" rather than "=". Question: Can one misuse the feature of n+k-patterns to simulate n*k+k' patterns? [I am talking about weird user-defined instances of Num.] Stefan Kahrs

### Re: Successor patterns in bindings and n+k patterns

| Another strange thing about n+k patterns. | | Its definition uses = , but = is not part of the class Num. | Does that mean that n+k patterns have to be instances of class Real? Certainly. In fact, they're really meant to apply only to class Integral (and it would be natural numbers, if we had them). | One could leave it class Num, if the translation were expressed | in terms of "signum" rather than "=". Being able to match complex numbers (along the positive real axis only!) with n+k patterns would be a dubious advantage, IMHO. | Question: | Can one misuse the feature of n+k-patterns to simulate | n*k+k' patterns? [I am talking about weird user-defined | instances of Num.] quite possibly. | Stefan Kahrs --Joe Fasel

### Re: n+k patterns

Lennart Augustsson [EMAIL PROTECTED] writes: PS. I'd like to start the "Ban n+k patterns"-movement, any followers? Count on me! I guess this makes it an organized movement ... two persons (movement) that know about each other (organized). Cheers, Karl-Filip

### Re: n+k patterns

You are quite right. I'd forgotten about local rebinding, because I feel that all local rebinding should be disallowed. Anyone want to start a movement to eliminate local rebinding? (1/2 :-) Cheers, -- P - Begin Included Message - From [EMAIL PROTECTED] Tue May 18 14:56:37 1993 Date: Tue, 18 May 93 15:55:42 +0200 From: Lennart Augustsson [EMAIL PROTECTED] To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Subject: Re: n+k patterns Both (=) and (-) belong to classes defined in PreludeCore, and hence cannot be rebound. This was a deliberate decision, made in order to turn your point into a non-problem. It's true that things from PreludeCore cannot be rebound on the top level, but they can be rebound locally. So the problem I state does occur. OK, I think that (=), (-), etc. should refer to those from PreludeCore, but it doesn't say anywhere. Another question along the same lines: What if (+) has been rebound? Are n+k patterns still allowed? -- Lennart - End Included Message -

### Re: n+k patterns

I like the capability to redefine syntax. For example, I would like to be able to define syntax that looks like EBNF when writing parsers. I would like to be able to write E = T {(`+`|`-`) T} rather than e = concat1 (t,zeroOrMore (concat2 (alternative (lit '+',lit '-'),t))) Of course infix operators help, but what about nice multiple token symbols like { } ? So, minimally, I am in favor of the local redefinition of symbols like '+' and '=', and think it unfortunate that there is a clash between the redefinition and treatment of n+k patterns. This is a vote for dumping n+k patterns, and a wish for more flexible syntax not hampered by special cases. Ken

### n+k patterns

Phil Wadler says: You are quite right. I'd forgotten about local rebinding, because I feel that all local rebinding should be disallowed. Anyone want to start a movement to eliminate local rebinding? (1/2 :-) Cheers, -- P And *I* think we should stop using names altogether and program in terms of SKI combinators! Since these constructs have well-known, unchanging meanings there is little chance that a program will be misunderstood*. I don't know about the rest of the people reading this, but I find the notation S(S(S(KS)K))((S(S(KS)K))(KI)) = (S(S(KS)K))(S(S(KS)K))(KI) to be much more precise than the n+k equivalent, 1 + 1 = 2. The similarity of the latter notation to the informal numerals of everyday life is an open invitation to sloppy reasoning leading to widespread program bugs. Besides, anybody who can't handle needless complexity shouldn't be fooling around with dangerous machines such as computers. -- Dan Rabin ([EMAIL PROTECTED]) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) *in fact, there is little chance that a program will be understood, period.

### Re: n+k patterns

Both (=) and (-) belong to classes defined in PreludeCore, and hence cannot be rebound. This was a deliberate decision, made in order to turn your point into a non-problem. Long live (n+k)! -- P - Begin Included Message - From [EMAIL PROTECTED] Mon May 17 21:33:41 1993 From: Lennart Augustsson [EMAIL PROTECTED] Subject: n+k patterns Date: Mon, 17 May 93 22:25:03 +0200 To: [EMAIL PROTECTED] Could those in charge of the formal semantics of Haskell (yes, that's you folks in Glasgow!) tell me what the meaning of n+k patterns are? In the report it says that case e0 of { x+k - e; _ - e' } translates to if e0 = k then { let { x' = e0-k } in e[x'/x] else e' Which = and - does this refer to? What if they have been locally rebound? E.g. let x - y = x ++ f y where f 0 = [] f (n+1) = f n in [] - 0 Does the translated - still refer to the method in PreludeCore or to the - in scope? -- Lennart PS. I'd like to start the "Ban n+k patterns"-movement, any followers? - End Included Message -

### Re: n+k patterns

Both (=) and (-) belong to classes defined in PreludeCore, and hence cannot be rebound. This was a deliberate decision, made in order to turn your point into a non-problem. It's true that things from PreludeCore cannot be rebound on the top level, but they can be rebound locally. So the problem I state does occur. OK, I think that (=), (-), etc. should refer to those from PreludeCore, but it doesn't say anywhere. Another question along the same lines: What if (+) has been rebound? Are n+k patterns still allowed? -- Lennart

### Re: n+k patterns

|Another question along the same lines: What if (+) has been rebound? |Are n+k patterns still allowed? | |-- Lennart The answer should be that n+k patterns are still allowed, but (+), (-), and (=) from PreludeCore are used in the translation. --Joe

### n+k patterns

Could those in charge of the formal semantics of Haskell (yes, that's you folks in Glasgow!) tell me what the meaning of n+k patterns are? In the report it says that case e0 of { x+k - e; _ - e' } translates to if e0 = k then { let { x' = e0-k } in e[x'/x] else e' Which = and - does this refer to? What if they have been locally rebound? E.g. let x - y = x ++ f y where f 0 = [] f (n+1) = f n in [] - 0 Does the translated - still refer to the method in PreludeCore or to the - in scope? -- Lennart PS. I'd like to start the "Ban n+k patterns"-movement, any followers?