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Re: Recognizers & EVALUATE recursion

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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).

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.  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.

- anton
-- 
M. Anton Ertl  http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
     New standard: http://www.forth200x.org/forth200x.html
   EuroForth 2016: http://www.euroforth.org/ef16/

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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

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