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4 points by pg 4614 days ago | link | parent

Interesting, but does it make programs that much shorter? Can you give me an example of a piece of code that gets dramatically cleaner if you have something like you're suggesting?


4 points by absz 4613 days ago | link

The biggest win, which actually results from a semantic change, is that it becomes possible to cleanly apply a macro to a variable number of arguments. Consider

  (apply and (acons x) (car x) lst)
as opposed to

  (and (acons x) (car x) @lst)
. The first version is broken: it will be forced to evaluate (acons x), (car x), and lst before applying and to them, which breaks the semantics of and: (acons x) no longer guards against (car x) trying to take the car of a non-list, as everything is evaluated before being passed to apply.

On the other hand, the second version works: the list is spliced in and then the and macro is run as normal. The semantics are preserved here because @, like a macro, expands the code first; nothing is evaluated, and then the and macro runs on its arguments as usual, short-circuiting if (acons x) fails. The variable being spliced is still evaluated, but that is the point of this notation.

As a concrete example (though I think this is generally useful/powerful), partial application then becomes merely

  (def par (fn . args)
    (fn newargs (fn @args @newargs)))
. If apply were used instead of the splicing @, this would actually break on certain macros as noted above, both causing the abstraction to leak and preventing someone from partially applying and, or, etc.

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2 points by bogomipz 4605 days ago | link

The more I think about this, the more mind boggling I find it. What exactly would this expression expand to?

  (and (acons x) (car x) @lst)
The actual lst is only available at runtime, while the 'and macro expands at compile time.

  Edit:
After looking at the definition of 'and, it is clear that the above could never work. Without performing the splice it would expand to

  (if (acons x) (if (car x) @lst))
For each additional argument 'and must add another 'if to the code, so with lst being spliced in at runtime, we have an impossible problem.

It seems @ can only ever work reliably in quasiquote. I guess longtime lispers already knew, otherwise we would probably have had this feature for several decades already.

If you insist, it could still be done for functions. To avoid inefficient code, it better be smart about how it builds the list;

  (foo 'a @lst)     -> (apply foo (cons 'a lst))
  (foo 'a 'b @lst)  -> (apply foo (append (list 'a 'b) lst))
  (foo @lst 'c 'd)  -> (apply foo (append lst (list 'c 'd)))
  (foo @lst 4 @bar) -> (apply foo (append lst (cons 4 bar)))
and so on.

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1 point by absz 4602 days ago | link

I had assumed that (and (acons x) (car x) @lst) would expand to (and (acons x) (car x) lst.0 lst.1 ... lst.N), and only then would it expand to (if (acons x) (if (car x) (if lst.0 ...))). In other words, @s are expanded as though they were the outermost macro, not the innermost. That should remove the problem of 'and having to be psychic. Your point about efficiency, however, is a very good one.

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2 points by bogomipz 4600 days ago | link

As mentioned, the macro expands at compile time, but the value of lst is not available until runtime.

In order to expand a call to 'and with 6 sub expressions into (if e0 (if e1 (if e2 (if e3 (if e4 e5))))), all 6 expressions must be visible at compile time.

Splicing in a list of arguments to a macro will simply never work (except by compiling the code from scratch each time you run it, i.e. limiting yourself to the most inefficient of interpreter techniques)

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1 point by absz 4600 days ago | link

Aha! Oh, I see. I should have realized that. Yes, that's a problem :) I'm not convinced it's insurmountable, but this syntax certainly won't work. Thank you.

I still, however, think it might be nice for functions, but that's merely a difference in appearance, not functionality.

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4 points by brett 4614 days ago | link

Off the top of my head it's the 5 characters you save on calls to apply. Also, as opposed to apply, you keep the function in functional position which seems cleaner to me.

I'll keep thinking about common cases where it's a bigger win. A more elaborate call to apply where you have to compose the list comes to mind:

  (apply func (join lst '(a)))
vs

  (func @lst 'a)

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1 point by pg 4612 days ago | link

(apply func (+ lst '(a)))

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1 point by lojic 4611 days ago | link

Yours is 71% longer. That does seem significant. Or the auto-expand is 42% shorter. Wow, what are the chances that it'd be 42% shorter when I just watched Hitchhiker's Guide on TiVo the other night?

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1 point by nex3 4611 days ago | link

Perhaps... one in forty-two?

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