It would be equivalent to:

  (zap cdr foo)

  (scdr foo nil)

  (= foo '(1 2 3 4))

  (del (car foo)) -> (2 3 4)

  (del (cdr foo)) -> (2)

  (= foo '(1 2 3 4))

  (del (foo 2)) -> (1 2 4)

  (= foo '(1 2 3 4 5 6 7))

  (del (foo 1 4)) -> (1 6 7)

"Then how do you usually invoke a method on an object? I guess what I mean is, what's the shortcut for (w/self bar (bar<-x ...))?"

No shortcut yet, but the idea is that hopefully you won't need to most of the time. The point of defining methods like `call` is that you don't call them directly:

  (= foo (object call (fn () ...)))

  (foo ...)

What's the advantage 'del gives me? Wouldn't my programs have fewer tokens if I directly used (zap cdr foo) instead of (del car.foo)?

Right now 'del looks like a smorgasbord of special-case utilities which could all have shorter tailor-made alternatives. Ideally what I like to see is a utility with an abstract enough description that I can build new tools upon it and straightforwardly extend it to make all my tools handle a new case. (Basically I have the same indifference to 'del as I have to 'coerce. ^_^ )

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"The point of defining methods like `call` is that you don't call them directly"

Okay, I thought it might be something like that. ^_^ But in that case, it reminds me of Racket's structure type properties again. It's up to you to decide whether that's a flattering comparison or not, but you didn't seem to like them the last time.

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"One idea would be that get-attribute would return a binded form, so you could just call (foo<-x ...) directly, without needing w/self[...]"

That's what I was intuitively thinking would happen. ^_^ I usually wouldn't say that, though. Perhaps I've been working in JavaScript for too long now. :-p

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"[...]but still allow w/self to override it."

Er, even my currently weird intuition doesn't like that kind of inconsistency. >.>

What about making 'get-attribute a metafn? That is, ((get-attribute a b) ...) would expand into something like (invoke-attribute a b ...) the same way ((compose a b c) ...) expands into (a (b (c ...))).

Then when you want to call foo's x from bar, you say something like this:

  (w/self bar
    ((do foo<-x) ...))

This would take a bit of language-core-hacking. Metafns are special-cased like special forms in official Arc--and even in your fork of ar, even though you're using gensyms for special forms. I would have tried to "fix" that in Arc a long time ago, but my new design for custom syntax was so different from Arc's that I considered it part of Penknife instead. The most straightforward way to get started in ar would probably be to design metafns as macros which expanded into literal macros, and then to let literal macros expand even when they're not in functional position.

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Abstraction and readability. Let me answer your question with a question: why do we need (= (car foo) ...)? you can just use scar! Or why do we need (= (foo 'a) ...)? We can just use things like sref!

For the same reasons that overloading = to understand things is nice, overloading del to understand things is nice. So, if you feel that using (scar foo ...) is better than (= (car foo) ...), then by all means use (zap cdr foo), but I personally like using the = forms, and so I like del for the same reasons.

I'll note that there is a defdel, which lets you create new "del" stuff, similar to defset. And you can extend dref as well, just like extending sref. It's basically the exact same idea as =, except it's applying to deletion rather than assignment.

And let me say one more thing. How would you easily and concisely implement this, which deletes a range of elements:

  (= foo '(1 2 3 4))

  (del (foo 1 2)) -> (1 4)

  (= foo '(1 2 3 4 5 6 7))

  (del (foo 0 nil 2)) -> (1 3 5 7)

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"It's up to you to decide whether that's a flattering comparison or not, but you didn't seem to like them the last time."

I didn't like how they automatically created stuff like exn-message, rather than letting you do (foo 'message). That's different from disliking the overall idea.

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"Perhaps I've been working in JavaScript for too long now. :-p"

JavaScript does have bind... :P

  foo.bind(this)

"This would take a bit of language-core-hacking."

Or I could just use dynamic variables, which is what I'm doing. It's quite simple: if self is bound, use that, otherwise use the object. Basically, self defaults to the current object, while letting you override it with w/self.

This is actually very similar to JavaScript:

  foo.method(...)
  foo.method.call(bar, ...)

  (foo<-method ...)
  (w/self bar (foo<-method ...))

I was originally going to use that as an example. ^_^

Arc's notion of assignment follows the abstract, informal contract that (= (foo ...) bar) changes whatever (foo ...) returns. This means we can have utilities like 'zap and 'swap that treat something as both a "get"-like expression and a "set"-like expression. I don't see 'del having a contract like that, so I don't see why "it's nice to be able to bundle them up."

I'd implement your range deletion examples like this...

  (mac del (place start (o howmany 1) (o step 1))
    ...)

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"I didn't like how they automatically created stuff like exn-message, rather than letting you do (foo 'message). That's different from disliking the overall idea."

Ah, okay. That works for me.

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"JavaScript does have bind... :P"

I'm already fully aware a JavaScript programmer can control the value of "this" using call, apply, or bind. ^_^

All I'm talking about is the way that foo.bar( baz ) calls the value of foo.bar with foo as "this," but if you instead define idfn() and say idfn( foo.bar )( baz ), then "this" is the root object. That's not the most elegant semantics in the world, but I'm saying my intuition is somehow in line with it right now.

(The "root object" aspect isn't part of my intuition, though. I'm fine with idfn( foo.bar )( baz ) using anything for "this", including treating it as a dynamic variable like you are.)

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"Or I could just use dynamic variables, which is what I'm doing."

Again, I was already taking that for granted and not arguing with it. Instead, I was talking about a way for (foo<-x ...) to automatically use (w/self foo ...), the way JavaScript foo.x( ... ) is like foo.x.call( foo, ... ).

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"It's quite simple: if self is bound, use that, otherwise use the object."

That's not reassuring....

I'm particularly concerned about the case where we're using (foo<-method ...) from inside another object's method. I'm afraid the self variable will already be bound then, so instead of calling the method on foo, we'll end up calling it on the current object. As a workaround, we'll have to say (w/self foo (foo<-method ...)).

del's contract is that it zaps the variable to whatever dref returns. It is more analogous to = than you think. In fact, it's exactly the same idea, only the name is different, and it's purpose is different.

So, my point still stands. If you think that (= (car foo) ...) is good, then why is (del (car foo)) bad? You may think, "well, I don't think that's useful". Fine. There's plenty of Arc functions I don't think are useful. Just don't use del, then.

But in the same way that = being general-purpose is obviously useful, del being general purpose is obviously useful. For instance, how do you delete an attribute?

  (del foo<-bar)

  (del-attribute foo 'bar)

Did you define new functions "get-foo", "set-foo" and "del-foo"? Well, nobody has to know about "del-foo" because you can just use defdel, in the same way that nobody needs to know about scar, because you can just use (= (car ...) ...)

And nobody has to know about set-foo, because you can use defset... and nobody has to know about get-foo, because you can use defcall. Notice the pattern, here?

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"The specific case I'm interested in is the one where we're using (foo<-method ...) from inside another object's method. I'm afraid the self variable will already be bound then, so instead of calling the method on foo, we'll end up calling it on the current object."

If the name and purpose are different what's left? :p

Slices and steps don't currently work with '=. So they feel bolted on to del.

Of course. I don't recall ever conflating the two, and I think we should allow for both assigning to slices, and deleting slices. In fact, then deletion could use assignment, so this:

  (del (foo 0 1))

  (= (foo 0 1) nil)

  (= (foo 0 1) nil)

(FYI: The ..< I'm using is Groovy's notation for half-inclusive ranges, inspired by Ruby's ... notation but less ambiguous with Groovy's other syntax. I also prefer it for its obvious asymmetry.)

It would probably make more sense to use (slice foo ...) to get a slice and (= (slice foo ...) ...) to put it back.

This isn't ideal, as far as I'm concerned, 'cause it means we don't always get what we put in:

  arc> (= foo '(a b c d e f g))
  (a b c d e f g)
  arc> (= (slice foo 1 3) '(x y))
  (x y)
  arc> foo
  (a x y e f g)
  arc> (slice foo 1 3)
  (x y e)

  arc> (= foo (table))
  #hash()
  arc> (= (foo 'a 1) nil)
  nil
  arc> foo
  #hash()
  arc> (foo 'a 1)
  1

  (= (foo 0 1)   '(3 4))   ; splice a list into the first element
  (= (foo 0 nil) '(1 2 3)) ; replace the list with a different one

  (= (cut foo 0 1) '(3 4))

In tables, (foo 0 1) already means the same thing as (or foo.0 1). But hmm, maybe you're not interested in slicing tables. That's fine, then. ^_^

By the way, newLISP does something like that with (0 1 foo): http://www.newlisp.org/downloads/newlisp_manual.html#implici...

The other issue was that I think at some point you were using (foo 0) to mean a one-element slice, and that would definitely be ambiguous with regular old indexing.

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"Hm... why not use cut rather than slice?"

What would a table slice even mean? They're not even ordered. The closest thing would be a table that has a subset of the keys of another table, but that wouldn't use numeric indexing.

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"The other issue was that I think at some point you were using (foo 0) to mean a one-element slice, and that would definitely be ambiguous with regular old indexing."

I was? (foo 0) should always be an element, not a slice. Ah well, it doesn't matter. If I did use that, I was incorrect.

I'll note that (del (foo 0)) is equivalent to (= (foo 0 1) nil) so perhaps that confused you.

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"Arc's 'cut accepts start and stop positions, and your slicing examples instead use a start position and a length."

They did? I always thought of it as being a start/end position, like cut. So for instance:

  (= foo '(1 2 3 4 5 6))

  (= (foo 2 5) nil)

  foo -> (1 2 6)

  (= foo '(1 2 3 4 5 6))

  (cut foo 2 5) -> (3 4 5)

Hence my "but hmm." ^_^

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"I'll note that (del (foo 0)) is equivalent to (= (foo 0 1) nil) so perhaps that confused you."

Yeah, that's probably it.

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"I always thought of it as being a start/end position, like cut."

It's hard to tell because you use 0 as the start pretty often, but here are the two examples I was going off of:

From http://arclanguage.org/item?id=14656:

  (= foo '(1 2 3 4 5 6 7))

  (del (foo 1 4)) -> (1 6 7)

  (= foo '(1 2 3 4))

  (del (foo 1 2)) -> (1 4)

  (subset x 'foo 'bar 'qux)

  (x 0 1)

  (foo 'a 'b 'c)

By the way, sref has a terrible argument signature:

  (sref foo value key)

  (sref foo key value)

  (= (foo 0 1) 'bar)

  (sref foo 'bar 0)

  (sref foo 'bar 0 1)

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Yeah, I think I noticed that at one point a long time ago, and I consider it to be a bug. (I promptly forgot about it. XD;; ) I never rely on that behavior, but then I don't extend things in the Arc core much either, for cross-implementation compatibility's sake.

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"To be fair, having the key as the last argument is great for slices"

I think that's the reason it is the way it is. I've ended up designing some of my utilities the same way. For instance, I usually give failcall the signature (func fail . args), which is very similar to sref's (func new-value . args). I'd probably call 'sref "setcall" if I were to reinvent it.

  (= (sig 'msec nil))

  (= (sig 'msec))

  (= (sig 'seconds nil))

  (= (sig 'seconds))

That was my first reaction as well. I see one general new idea here, though: to extend indexing to return not just places but slices like in python and Go.

  (= l '(1 2 3 4))
  (l 1) ; 2
  (l 0 2) ; '(1 2)
  (= (l 0 2) '(3)) ; l is now '(3 3 4)

  (= l '(1 2 3 4))
  (= l.0 nil) ; l is now (nil 2 3 4)
  (= (l 0 1) nil) ; l is now (2 3 4)

It is in Python. Would it be useful in Arc? We already have cut, which does the same thing:

  (cut '(1 2 3 4) 0 2) -> (1 2)

  foo = [1, 2, 3, 4]
  foo[0:2] = [0]
  foo -> [0, 3, 4]

  del foo[0:1]
  foo -> [3, 4]

  del foo[:]
  foo -> []

And del is identical to assigning to [].

  >>> foo = [0, 1, 2]
  >>> foo[0:1] = []
  >>> foo
  [1, 2]