Sorry to post this again, I'll try keep it brief.
I have a choice statement inside a recursive parser "Encode" implemented
in the Prolog-inspired relational style of CTM, ch9.
Basically, what I need is that if the first choice succeeds at any level
of the parse, the second choice must not be attempted at that level. (I
know this is not concurrent or anything, but I'm just trying to get the
basics here! If you want to demonstrate the concurrent
approach/solution too, I won't complain!!!
To implement this I have used a Cell ("FoundWord", marked *** below),
assigned initially to "false", which is set to "true", if the first
branch succeeds, hoping that it could be used as a guard, to prevent the
second choice.
Trouble is, it doesn't work. All the rest of the parser works fine, but
"FoundWord" always seems to be "false" when accessed in the second
choice, even if it certain that it has been previously assigned true.
Which makes me realise that I have completely failed to understand
something, probably to do with the scoping, but my logic might be wrong,
too, who knows...
Here's the basic code:
proc {Encode NumberStr Words PrevIsNum}
First Rest Word WList CurrIsNum
FoundWord = {NewCell false} % <---- ***
in
choice
NumberStr = nil
Words = nil
[]
Words = Word|WList
{Splitter First Rest NumberStr} % NonDet - Works like a kind of
backward append/3
choice
%% NonDet - "Corresp" is based on member/2 and checks a
dict
{Corresp First Word}
CurrIsNum = false
{Assign FoundWord true} % <---- ***
[]
% there was no word, use the single digit
{Length First 1}
% check that we are not following a digit
PrevIsNum = false
{Access FoundWord false} % <---- ***
First = Word
CurrIsNum = true
end
{Encode Rest WList CurrIsNum} % Recursive Call
end
end
It is used, as expected by Solns =
{SearchAll proc{$ Ws} {Encode NumberStr Ws false} end}
For anyone more interested (the problem was originally a copmuting
challenge which I thought would look elegant in Oz) see my previous post
of 22/8/05 or
http://www.mozart-oz.org/pipermail/mozart-users/2005/006708.html
The program code is here: http://aegean.pwp.blueyonder.co.uk/store/tel.oz
Background:
http://web.archive.org/web/20030204013211/http://alvin.jpl.nasa.gov/gat/ftp/study.txt
TIA
Alex
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