[REBOL] Re: More dialects than you think, validating function input.
From: greggirwin:mindspring at: 13-Sep-2002 12:52
Hi Brett,
<< As for the expecting bit. I don't think there is a general solution.
Having
to tell the user "error...expecting money" adds an application requirement
making a bigger application. Each "expecting X" becomes an alternative rule
to the rule that is attempting a match. So your rules bloat out quite a bit
if you want to report on every grammar term. For some apps maybe it is
enough to say "Recognised two stock orders. Could not recognise the rest of
the instructions." >>
I think I didn't say that I thought more than I said. :)
What I was *thinking*, but didn't write in my message, was how many parsers
provide context information when a syntax error occurs and that PARSE might
be able to make available that kind of information so we don't have to do it
all ourselves at the individual rule level. F'rinstance:
>> add 1
** Script Error: add expected value2 argument of type: number pair char
money date time tuple
** Near: add 1
REBOL has rules about how to parse code based on the spec block (the rule)
for a function, no? So, based on the rule(s) that have succeeded, you know
what state the parser was in when it hit a rule that failed, and what that
rule (or rules) was.
I like the way it works today, not popping an error directly to the user,
but you have no idea exactly where the parse failed unless, as you point
out, you add stuff to all your rules to track exactly what's going on. Maybe
that ends up being a viable alternative in some cases, but if PARSE gave us
some more clues, it might be useful.
Another problem I have is that I start thinking way ahead of myself about
what could be
. I can see great potential for higher level dialects driving
PARSE; generating rules dynamically (including specific constraints) based
on data from a database, previous messages in a conversation, etc.; and
tools to help visualize and debug dialects. I just about jump out of my seat
when I think about some of the possibilities. :)
--Gregg
