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Re: Clunky parsing problem, looking for simple solution

by dws (Chancellor)
on Jun 16, 2002 at 06:38 UTC ( #174903=note: print w/replies, xml ) Need Help??

in reply to Clunky parsing problem, looking for simple solution

Interesting problem. A simple regexp-based solution that doesn't take lexemes into account will fail, at least on pathological cases like 1800 IF V$="THEN FOO ELSE IF BAR" THEN $K="FOO" I don't have the brain cells left tonight to work through a complete solution, though I have a dim recollection of having done something like this many years back using "fixups". 1800 IF V$="K" THEN A$="+K+" ELSE IF V$="R" THEN A$="?R?" ELSE IF V$="M" THEN A$="!M!":Z1=R1:Z2=R2 would translate first into
1800 IF V$="K" THEN GOTO {fixup:skip} 1800.1 GOTO {fixup:after-next-goto} 1800.2 A$="+K+" 1800.3 GOTO {fixup:end} 1800.4 IF V$="R" THEN GOTO {fixup:skip} 1800.5 GOTO {fixup:after-next-goto} 1800.6 A$="?R?" 1800.7 GOTO {fixup:end} 1800.8 IF V$="M" THEN GOTO {fixup:skip} 1800.9 GOTO {fixup:end} 1800.a A$="!M!" 1800.b Z1=R1 1800.c Z2=R2 1800.d REM
The second pass would peform the fixups.

  • {fixup:skip} becomes the number of the 2nd following line in the sequence
  • {fixup:end} becomes the final line number of the sequence
  • {fixup:after-next-goto} becomes the line number after the next goto in the sequence. (This works because conditionals cannot be nested within a line.)

By using a {fixup:skip} fixup (rather than calculating the target line number as you're generating the sequence, you allow for the possibility of doing peephole optimizations. In the sequence above,

1800.8 IF V$="M" THEN GOTO {fixup:skip} 1800.9 GOTO {fixup:end} 1800.a
could be optimized to
1800.8 IF V$<>"M" THEN GOTO {fixup:end} 1800.9
This "renumbers" lines within the sequence, but since target line number calculation/assignment has been deferred, not GOTOs are broken.

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Re: Re: Clunky parsing problem, looking for simple solution
by clintp (Curate) on Jun 16, 2002 at 14:25 UTC
    If it were a simple problem, I don't think I'd have posted it here. :)

    For now I suppose you can ignore the truly pathological case where there's a IF-like statement within double-quotes. I'm looking for a 90% solution and if some hand-porting is necessary then so be it.

    Note: I found just such a statement in my test suite. It was in a tutorial on GW-BASIC written in GW-BASIC.

      I don't think an IF-statement within double quotes should be looked at at all by your parser. It's just a string, after all. It's not a statement.

      Lur: "But if this cape shrinks, consider your species extinct!"

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