Re: [Pyparsing] Conditional Suppress?

[June K. <jun...@gm...>]

Well, a much simpler solution just occurred to me:

class EmptySuppress(Suppress):  #suppress only empy tokens
    def postParse( self, instring, loc, tokenlist ):
        if len(tokenlist[0]): return tokenlist
        return []

cmpdstmt=EmptySuppress(Group(Literal('{').suppress()+
               ZeroOrMore(stmt)+
               Literal('}').suppress()))


2008/2/15, June Kim <jun...@gm...>:
> Hello,
>
> It's been a long while since I last used pyparsing(Hi Paul). I have
> good memories of pyparsing. Recently, I'm using it again, for parsing
> a subset of C and PL/I syntax.
>
> First, look at the following code please:
>
> from pyparsing import *
> from pprint import pprint
>
> t="""
> a=a+1;
> if (a>b) { //if (1==2) gogogo();
>    if (c>d) a=a+1;
>    else b=b+1;
> } else {
>    c=c+1;
>    if (3==4)
>        printf("abc");
>    else if (4==5)
>        dothis();
>    else
>        dothat();
>    if (a>b) {go();go();come();}
>    if (d>e) if (c>g) if(a>q) a=a+1;
> }
> """
>
> ifst=Forward()
> stmt=Forward()
>
> cmpdstmt=Group(Literal('{').suppress()+
>               ZeroOrMore(stmt)+
>               Literal('}').suppress())
>
> stmt << (Literal(';').suppress() |
>         cmpdstmt |
>         ifst |
>         (NotAny('}')+SkipTo(';',include=True)).suppress())
>
> ifst <<  Group(Keyword("if")+nestedExpr("(",")").suppress()+stmt+\
>         Optional(Keyword("else")+stmt))
>
> p=ifst.ignore(cStyleComment).setDebug(False)
> pprint (list(p.scanString(t))[0][0].asList(),width=2,indent=2)
>
> ###########################################
>
> What I am trying to do is get the minimal tree-structure of if
> statements. I am not interested anything other than if statements.
>
> The result is:
>
> [ [ 'if',
>    [ [ 'if',
>        'else']],
>    'else',
>    [ [ 'if',
>        'else',
>        [ 'if',
>          'else']],
>      [ 'if',
>        [ ]],
>      [ 'if',
>        [ 'if',
>          [ 'if']]]]]]
>
> I am fairly satisfied with the result, but would like the blank list
> removed. That is, I want the {go();go();come();} part not present(not
> even as an empty list) in the parseResult. However I can't totally
> suppress the cmpdstmt since some of them might include 'if'.
>
> I played with setParseAction but couldn't get what I wanted. I also
> branched a few new grammars for treating non-if-cmpdstmt as following:
>
> ifst=Forward()
> stmt=Forward()
> cmpdstmt=Forward()
>
>
> stmt << (Literal(';').suppress() |
>         cmpdstmt |
>         ifst |
>         (NotAny('}')+SkipTo(';',include=True)).suppress())
>
> restcmpdstmt=Group(Literal('{').suppress()+
>               ZeroOrMore(stmt)+
>               Literal('}').suppress())
>
> nonifstmt=Forward()
> nonifcmpdstmt=Group(Literal('{').suppress()+
>               ZeroOrMore(nonifstmt)+
>               Literal('}').suppress())
>
> # a statement that doesn't include if-statement
> nonifstmt << (Literal(';').suppress() |
>         nonifcmpdstmt |
>         (~oneOf('} if')+SkipTo(';',include=True)).suppress())
>
> cmpdstmt << (nonifcmpdstmt.suppress() | restcmpdstmt)
>
> ifst <<  Group(Keyword("if")+nestedExpr("(",")").suppress()+stmt+\
>         Optional(Keyword("else")+stmt))
>
>
> It returns the expected result.
>
> [ [ 'if',
>    [ [ 'if',
>        'else']],
>    'else',
>    [ [ 'if',
>        'else',
>        [ 'if',
>          'else']],
>      [ 'if'],
>      [ 'if',
>        [ 'if',
>          [ 'if']]]]]]
>
> Does the code look right?
>
> Yet, the problem is that the branched grammar version is too complex
> to write, read, and maintain.
>
> What alternatives or improvements do you recommend? (maybe
> post-processing the parseResult after the parsing's finished?)
>
>
> June Kim
>