Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.forth > #56948

Re: Recognizers & EVALUATE recursion

Show all headers | View raw

On 4/8/2017 15:17, Anton Ertl wrote:
> Alex <alex@rivadpm.com> writes:
>> Using EVALUATE or allowing an implicit use of EVALUATE in the
>> execution path of a recognizer may cause problems. In my Forth
>> (edited for clarity):
>> 
>> : interpreter ( -- ) begin parse-name dup while default-recognize
>> execute repeat 2drop ;
>> 
>> : evaluate ( addr len -- ) ['] interpreter execute-parsing ;
>> 
>> A recognizer using EVALUATE re-drives the interpreter, and hence
>> the recognizer is potentially recursed into.
>> 
>> Not every parse will cause issues, but there are certain
>> pathological cases where the recogniser may infinitely recurse. In
>> my case it was an EVALUATE for :NAME causing an EVALUATE for :NAME
>> ... and so on.
> 
> Recursion is just like loops: You have to make at least a little bit 
> of progress before recursing, or you will get an endless recursion
> (at least with recursion that usually shows up as a return stack
> overflow rather than just a hanging program).

Yes. I documented this because it initially caught me by surprise, and
stack overflow is pretty disastrous in outcome & difficult to debug in
my system. It took a little while to realize that EVALUATE was the culprit.

> 
> In general EVALUATE looks like a much too coarse tool for the 
> implementation of a recognizer: it can do many things out of your 
> control, based on stuff like STATE, BASE, the search order, and the 
> contents of the individual wordlists.

They are known and in the user's control, since the evaluation is being 
done while the word is being recognized, not later.

> A slightly less coarse approach is to call the recognizer stack again
> (at least STATE should not influence that), but of course the issue
> of requiring progress remains.

The intent is to explore the feasibility of words of the form ([ ] is 
meta) w1:[w2[:w3]].p as object w1 [with public object w2 [etc]] 
supporting public symbol p.

-- 
Alex

Back to comp.lang.forth | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Recognizers & EVALUATE recursion Alex <alex@rivadpm.com> - 2017-04-07 14:59 +0100
  Re: Recognizers & EVALUATE recursion Matthias Trute <matthias.trute@gmail.com> - 2017-04-07 08:12 -0700
    Re: Recognizers & EVALUATE recursion Alex <alex@rivadpm.com> - 2017-04-09 22:39 +0100
  Re: Recognizers & EVALUATE recursion anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-04-08 14:17 +0000
    Re: Recognizers & EVALUATE recursion Alex <alex@rivadpm.com> - 2017-04-09 22:37 +0100

csiph-web