arc> an-unknown-variable
  Error: "reference to undefined identifier: __an-unknown-variable"
                                             ^
                                             |
                                         This one

But this should be supported. For instance, in C++, you can redefine the "standard function" operator+ :

  Foo something(Foo a, Foo b){
      return a + b;
  }

  a = Foo.new()
  a.each{ |i|
    puts i
  }

> Were you thinking about a multimethod dispatch such that of CLOS?

IMO the difference is big enough for CLOS Way != Ruby Way. ^^

Still, I wonder - how does CLOS implement this? How about for variadic functions?

> but I think that applying it to functions as basic as car would kill performance without a really good optimizer.

I think that for every method with the same name an hash table indexed on the types of the argument is created, and when the method is called, a lookup is made on the table.

> Hmm. I've been trying to grok through dynamic dispatching speedup techniques - the Sun Self papers are pretty good, Sun's index doesn't have the papers themselves but you can ask Google for them.

Use a "Polymorphic Inline Cache" (PIC). Basically if a piece of code could call several different methods, we figure out which one it is, then we create a copy of the calling function which does the type checking at the top and specialize all method calls to that type:

  (defm method ((t n my-type))
    (my-type::method n))
  (defm method ((t n your-type))
    (your-type::method n))
  (defm other-meth ((t n my-type))
    (my-type::other-meth n))
  (defm other-meth ((t n your-type))
    (your-type::other-meth n))

  (def general-function (n)
    (+ (method n) (other-meth n)))

  (general-function (your-type-creator 42))
  ===>
    (do
      (def general-function (n)
        (if (isa n 'your-type)
          (+ (your-type::method n) (your-type::other-meth n))
          (+ (method n) (other-meth n))))
      (general-function (your-type-creator 42)))

Edit:

Okay, I've been thinking. Basically the call* table is about specializing on 'apply, and we might think of 'defcall as:

  (defcall type (val . params)
    (code))
  ==>
  (defm apply ((t val type) . params)
    (code))

  (general-function (your-type-creator 42))

(general-function (your-type-creator 42))

(set your-type-creator another-type-creator)

(general-function (your-type-creator 42))

  (with (PIC (table) ;init empty table
         num-calls 0
         orig-fn
         (fn (n)
           (+ (method n) (other-meth n))))
    (def general-function (n)
      ((aif
         ; don't infer type: just look at the runtime type
         (PIC:type n)
            it
         (is optimization-trigger-level** (++ num-calls))
            (do (= num-calls 0)
                (= (PIC:type n)
                   (optimize* orig-fn (type n))))
            orig-fn)
        n)))

I don't think CLOS lets you check types on &rest, &optional, or &key parameters. So you couldn't use CLOS for the current behavior of '+.

Also note that CLOS only works on methods with "congruent lambda lists", that is, methods with the same number of required, optional, and keyword arguments. So you can't have

  (defmethod foo ((a type-a)) ...)
  (defmethod foo ((b type-b) &optional (c ...)) ...)

Hmm. This certainly seems easier to hack into arc. We could have each method start with a generic function whose lambda list we have to match.

Yes. But there's a problem: how do you define the table for, say, the variadic function '+ ?

One way to do this is to use settable-fn.arc and have some sort of attachment for a table of argument signatures.

HOWEVER, we will need to formalize on what part of the argument signature to dispatch from.

For example we can consider nex3's 'defm syntax:

  (defm foo ((t arg type) (t arg2 type2))
     ...)

  (defm foo (arg (o arg something))
    ...)

  (defm coerce ((t arg our-type) target)
    (case target
      int    (our-type-to-int arg)
      string (our-type-to-string arg)
      ...))

  (def coerce (obj typ . rest)
    (apply <base>coerce obj (annotate typ nil) rest))

  (defm <base>coerce ((t obj my-type) (t _ string))
    (convert-my-type-to-string obj))

                                     ; edit: corrected type
  (defm <base>coerce ((t obj string) (t _ my-type))
    (convert-string-to-my-type obj))

  (def + rest
    (reduce <base>+ rest))

  (defm <base>+ ((t x string) (t y string))
    (join x y))

  arc> (annotate 'foo (annotate 'bar obj))
  #3(tagged foo #3(tagged bar #3(tagged mac #<procedure>)))
  arc> (annotate 'foo obj)
  #3(tagged foo #3(tagged mac #<procedure>))
  arc> (annotate 'foo (annotate 'foo obj))
  #3(tagged foo #3(tagged mac #<procedure>))

  (is (type (annotate t r)) t)
  (is (rep  (annotate t r)) r)

  (define (ar-tag type rep)
    (cond ((eqv? (ar-type rep) type) rep)
          (#t (vector 'tagged type rep))))

  (xdef 'annotate ar-tag)

2) It should be, and it isn't.

  (defmemo fac (n)
    ((afn (n a)
       (if (> n 1)
           (self (- n 1) (* a n))
           a))
     n 1))

Yes. Yes it is. http://arclanguage.com/item?id=7288

Actually I think it might be better if we had a spec which says "A Good Arc Implementation (TM) includes the following functions when you (require "lib/math.arc"): ...." Then the programmer doesn't even have to care about "scheme functions" or "java functions" or "c functions" or "machine language functions" or "SKI functions" - the implementation imports it by whatever means it wants.

LOL. The 'maybe type looks awfully familiar... ^^

> If I come across somewhere in my Arc code where I use them (I haven't written too much new Arc recently) I'll report out.

  (= foo (fn ...))

    (def accumulate (combiner null-value term a next b)
        (def iter (a result)
    ...

What you want to do is:

    (def accumulate (combiner null-value term a next b)
        (let iter (fn (a result)
            ....

Arc:

    arc> (def fn2() (def i() (prn "fn2-i")) (i))
    #<procedure: fn2>
    arc> (fn2)
    fn2-i
    "fn2-i"
    arc> (def fn1() (def i() (prn-"fn1-i")) (fn2) (i))
    #<procedure: fn1>
    arc> (fn1)
    *** redefining i
    *** redefining i
    fn2-i
    fn2-i
    "fn2-i"
    

    > (define (fn2) (define (i) (print "fn2-i")) (i))
    > (fn2)
       "fn2-i"> (define (fn1) (define (i) (print "fn1-i")) (fn2) (i)) 
    > (fn1)
       "fn2-i""fn1-i">

http://arclanguage.com/item?id=3595 Suggest PG: Settable function objects

http://arclanguage.com/item?id=3698 Create your own collection in Arc: settable functions now implemented on arc-wiki.git

http://arclanguage.com/item?id=3762 Create your own collection: use directories as if they were tables with file-table

http://arclanguage.com/item?id=3858 Create your own collection: bidirectional tables

http://arclanguage.com/item?id=5254 Create your own collection: cached-table