RE: n + k patterns

2003-05-30 Thread Simon Peyton-Jones
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
| 
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RE: n+k patterns

2002-01-30 Thread Simon Peyton-Jones

| 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

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RE: n+k patterns

2002-01-30 Thread Malcolm Wallace

 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

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Re: n+k patterns

2002-01-30 Thread Jon Fairbairn

  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




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Re: n+k patterns

2002-01-30 Thread John Launchbury

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
 
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RE: n+k patterns

2002-01-30 Thread George Russell

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).

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RE: n+k patterns

2002-01-30 Thread Rijk J. C. van Haaften

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


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RE: n+k patterns

2002-01-30 Thread Simon Peyton-Jones

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
| 
| 

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Re: n+k patterns

2002-01-30 Thread Lennart Augustsson

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



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RE: n+k patterns

2002-01-30 Thread carlos . scheidegger

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


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Re: n+k patterns

2002-01-29 Thread Ross Paterson

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.

 In any case,  I propose to change Integral to Ord and Num.

Without this change, the relevant rule of the formal semantics of patterns
wouldn't preserve the static semantics.

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Re: n+k patterns

2002-01-29 Thread Olaf Chitil


 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

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RE: n+k patterns

2002-01-29 Thread Simon Peyton-Jones

| 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

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RE: n+k patterns

2002-01-29 Thread Mark P Jones

| 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


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RE: n+k patterns

2002-01-29 Thread Simon Peyton-Jones

| 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

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Re: n+k patterns

2002-01-29 Thread Lennart Augustsson

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

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Re: n+k patterns

2002-01-29 Thread Lennart Augustsson

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



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Re: n+k patterns, etc.

1993-06-01 Thread hudak-paul


  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.

1993-05-20 Thread Simon L Peyton Jones



|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

1993-05-19 Thread marc



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: n+k patterns, etc.

1993-05-19 Thread Kent Karlsson


Some more questions concerning some constructs in Haskell:

   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?

   By the way, what about other special syntax constructs, like [...]
for lists and (...,...,...) for tuples. Are these still available even
if the standard prelude is not imported?  Apparently lists and tuples
cannot be selectively hidden, but the entire standard prelude can be
hidden.  Or, to rephrase the question, are such special syntax constructs
reexported by the unseen and unhidable intermediary module mentioned
in the previous paragraph?  If so, then I think that the unhidable
intermediary module for special syntax constructs should be made
explicit (but, of course, still not codable in Haskell itself, due
to the special syntax).

Asking strange questions
/kent k

P.S. (Apropos Lennarts quiz)
I think that the function names in "funlhs"es (in each let/where)
should not be allowed to be rebound by some pattern whithin one of
the "funlhs"es.  I.e.  id id = id should not be a definition of the identity
function (while still allowing the definition id = \id.id).  This would
just naturally extend the "linearity restriction" in section 4.4.2, which
disallows e.g. f x x = ... and f = \x x-... but allows f = \x-\x-... .
(I don't dare go as far as proposing to ban "shadowing" completely.)

/kn





Re: n+k patterns

1993-05-18 Thread kff



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

1993-05-18 Thread smk


Phil wrote:

   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

The Report tries to handle this by "always implicitly importing"
PreludeCore.  This is not sufficient to prevent rebinding, e.g. if
somebody calls a formal parameter (=).


There is a related story w.r.t. the Definition of Standard ML.
In its appendix, the SML report defines the meaning of
some so-called "derived forms" in form of rewrite rules (e.g. it
rewrites if-then-else into case-expressions and case-expressions into
function application).  These rewrite rules also use predefined
identifiers (like true and false), which means that SML has exactly
the same problems as Haskell.

I took it for granted that the SML definition "meant" to refer to
predefined identifiers, and all the standard implementations agreed on
that.  However, when I had an email-dispute with somebody who was
involved in designing the SML definition about some of the problems
relating to this, I was very surprised to find out that he was
willing to understand the definitions literally and leave it to the user
to modify the meaning of if-then-else by redefining true
if s/he chooses to do so.
I was even more surprised to hear from Dave Berry who works on
Harlequin's SML compiler that their prototype indeed implemented
these rules literally [well, almost: they did not get the "identifier
status" bit right, a problem that does not exist in Haskell];
I am not sure whether their final release will still contain this feature,
at least I discouraged them.

Moral: do not take anything for granted, you can end up with somebody's
implementation violating your hidden assumptions.

--
Stefan Kahrs

P.S.
I join Lennart's "Ban n+k pattern"-movement.




Re: n+k patterns

1993-05-18 Thread wadler


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

1993-05-18 Thread Ken Sailor


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





Re: n+k patterns

1993-05-18 Thread wadler


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?


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Re: n+k patterns

1993-05-18 Thread Lennart Augustsson



 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

1993-05-18 Thread Joe Fasel


|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




Re: n+k patterns

1993-05-18 Thread hudak-paul


The Haskell Committee was well aware of the problem of name
capture in syntactic expansions.  We discussed various solutions
and opted for the simplest (if not most formal):  whenever a 

name freshly appears on the right side of a translation, add
a comment that says where it came from.  If you look at the
translations for expressions in Section 3 of the Report, you
will see many instances of the phrase "as defined in the Standard
Prelude".  It was an oversight not to include such a phrase in
the translation of patterns given in Section 3.14, the source of
Lennart's original question.

As for n+k patterns, I tried to get them out of Haskell on several
occasions, even before the Report was released, but failed!  Now
that I've programmed in Haskell for a few years, I'm convinced more
than ever that they should be removed.  So add me to the movement!

  -Paul