Arc Forumnew | comments | leaders | submitlogin
1 point by conanite 5981 days ago | link | parent

how do you use a scanner? Is it

  (car my-scanner)
or

  (my-scanner 'car)
? I've been browsing arki source for examples but I'm completely out of my depth there ...

One aspect of the token generator is that sometimes it recognises two tokens simultaneously. In other words, when it sees the right-paren in

  ... foo)
, it recognises "foo" and right-paren all at once. Perhaps this is the wrong way to do it, and I should be using 'peekc instead. But I suppose I can do this with a scanner:

  (scanner 'car "foo"
           'cdr (scanner 'car right-paren
                         'cdr (tokenize-scanner etc ...)))
I had previously tried modelling each state as a separate sub-function as you suggest, but couldn't get it to work. But that was before I noticed the

  (with ((a b c d) nil)
      (= a (fn () ...))
      (= b (fn () ...))
      (= c (fn () ...))
   ...)
idiom. Gotta try again ...


4 points by almkglor 5981 days ago | link

> I noticed the

  > (with ((a b c d) nil)
  >     (= a (fn () ...))
  >     (= b (fn () ...))
  >     (= c (fn () ...))
  >  ...)
> idiom.

If it's an idiom, probably needs a macro for it then ^^

  (mac with-r (vars . body)
    (let vars (pair vars)
      `(let ,(map car vars) nil
         ,@(map [let (var val) _ `(= ,var ,val)] vars)
         ,@body)))
edit: as an aside, peekc doesn't seem to work properly sometimes ^^

-----

1 point by absz 5935 days ago | link

Nice macro, but why the -? withr seems more consistent with things like withs.

-----

2 points by almkglor 5934 days ago | link

I agree. Should indeed be 'withr, and probably also define a 'givenr just for consistency

-----

2 points by almkglor 5981 days ago | link

How to use a scanner:

  (car my-scanner)
How to construct a scanner:

  (scanner 'car (your-expression)
           'cdr (your-expression))
Note that the expressions in the 'scanner form are delayed, i.e. they are not evaluated until a 'car or 'cdr is performed on your scanner, and they are evaluated only once.

edit: an important note: scanners have the exact read semantics of lists. So simply zapping cdr at a scanner will not advance the scanner, it will only advance the place you are zapping.

There's no need to use 'peekc or similar: all you need is to use stuff like 'cadr, 'caddr.

Because scanners have the exact read semantics of lists, you can use such things as 'each, 'map, etc. Just don't write using scar, scdr, or sref.

If you wanted to emulate lists, you can do something like:

  (def my-cons (a d)
    (scanner 'car a
             'cdr d))
Of course, since a and d are just var references, there's little point in delaying their execution.

edit2: Here's how you might make a generator:

  (def generator (f v)
    (scanner 'car v
             'cdr (generator f (f v))))

  (= b (generator [+ _ 1] 0))
  (car b)
  => 0
  (cadr b)
  => 1
  (cadr:cdr b)
  => 2
'map, 'keep, and a few other functions become lazy when applied on scanners, so you can use an infinite-series generator on them safely

-----