Re: n+k patterns

[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: Successor patterns in bindings and n+k patterns

[kh]

 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

[hudak-paul]

  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

[smk]

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

[Joe Fasel]

| 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