11 Jun
2015
11 Jun
'15
9:30 a.m.
That sounds really cool and useful extension.
Regarding the furthest failure, the core of the problem is the distinction
between an error and a failure. Error reports on a problem in the input,
while failure is information for choice parser combinator. In general, the
furthest failure is a better approximation of an error than the last
failure, so we use it.
I am not sure what exactly is the problem in case of PrattParser. I guess
the last failure gives better results for a user? One has to consider a
pratt parser included in the normal parser, e. g. Expressions parsed by
pratt in a Java Grammar. Depending where an error occurs, different
strategy for choosing the proper failure is necessary :-/
Regarding tokenization, there is a message token, that returns
PPTokenParser, which transforms a parsed input into the PPToken object.
Perhaps this might be helpful?
Cheers Jan
On Wed, Jun 10, 2015, 20:52 Richard Sargent <
richard.sargent(a)gemtalksystems.com> wrote:
camille teruel wrote
context
> On 10 Jun 2015, at 19:11, Chris Cunningham
<
cunningham.cb@
> wrote:
>
> Inteteresting....
>
> On Wed, Jun 10, 2015 at 9:36 AM, Camille <
camille.teruel@
<mailto:
camille.teruel@
>> wrote:
for
Hello Pharoers and Moosers,
I did a Pratt parser extension for PetitParser.
<snip>
> @PP Devs:
> I had trouble with the PPContext furthestFailure that is taken into
> account instead of the failures I return, so I had to redefine
> #parseWithContext: to return the failures I want. The results given by
> furthestFailure were not very meaningful in my case (the same is true
PPExpressionParser btw).
But I guess it was introduced because it gives good results in other
cases.
So would it be possible to change this behavior to let the parser decide
if it returns the furthestFailure or the original failure?
The intent behind the furthestFailure is that it give the failure that
gets the furthest into the source stream. It is most useful when there
are embedded choice operators in the parser - the original/pre furthest
behaviour would return the last failure, which depending on the incoming
stream and the order of the choice options could be significantly not
useful.
I ran into this when working with the sql parser, which started off with
the outer choice of (by memory):
^ selectStatement / insertStatement / updateStatement /
deleteStatement
If I was trying to part a select statement that had an error at the very
end of the statement, the parser would return an error talking about how
the incoming stream failed in deleteStatement. Not useful.
I would be saddened if this further failure was not available.
Yes in that case returning the furthest failure gives better results.
However, this don’t give meaningful messages in all cases.
For exemple with the calculator I gave in my previous message, if I parse
‘1+’ I want to get ‘expression expected at: 2’ but instead it returns ‘$-
expected at 2'.
I’m not proposing to remove this feature but to let parsers decide to use
it or not.
Something like (changes in bold):
PPParser>>parseWithContext: context
| result |
context initializeFor: self.
result := self parseOn: context.
"Return the furthest failure, it gives better results than the last
failure"
(result isPetitFailure and: [ self wantsFurthestFailure and: [ furthestFailure notNil ] ])
ifTrue: [ ^ context furthestFailure ].
^ result
This screams at me. Why not just delegate to the context and use a context
that returns the preferred failure? e.g. end with:
^context preferredResultFor: result.
PPParser>>wantsFurthestFailure
^ true
Like this, one can return the failures he wants.
PPPrattParser>>wantsFurthestFailure
^ false
Camille
>
> -cbc
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