How would you parse a standard if - else if - else statement?

[jonhue]

I tried quite a few things and still did not find a way to do this.

It seems to me as if the parser desperately tries to follow one path and when it fails, instead of looking for another, it just stops.

Here is my current implementation of a parser:

        @self.pg.production('file : ')
        @self.pg.production('file : expression_seq')

        @self.pg.production('block : INDENT expression_seq DEDENT')

        @self.pg.production('expression_seq : expression')
        @self.pg.production('expression_seq : expression NEWLINE expression_seq')

        @self.pg.production('else_clause : else NEWLINE block')

        @self.pg.production('else_if_clause : else_if expression NEWLINE block')

        @self.pg.production('else_if_clause_seq : else_if_clause')
        @self.pg.production('else_if_clause_seq : else_if_clause NEWLINE else_if_clause_seq')

        @self.pg.production('expression : if expression NEWLINE block')
        @self.pg.production('expression : if expression NEWLINE block NEWLINE else_if_clause_seq')
        @self.pg.production('expression : if expression NEWLINE block NEWLINE else_clause')
        @self.pg.production('expression : if expression NEWLINE block NEWLINE else_if_clause_seq NEWLINE else_clause')

        @self.pg.production('expression : INTEGER')

        @self.pg.production('expression : false')
        @self.pg.production('expression : true')

Here is the grammar:

file = [ expression_seq ] ;
expression_seq = expression , { NEWLINE , expression } ;
block = INDENT , expression_seq , DEDENT ;
expression = if | INTEGER | 'false' | 'true' ;
if = 'if' , expression , NEWLINE , block , { NEWLINE , else_if_clause_seq } , [ NEWLINE , else_clause ] ;
else_clause = 'else' , block ;
else_if_clause = 'else if' , expression , NEWLINE , block ;
else_if_clause_seq = else_if_clause , { NEWLINE , else_if_clause } ;

Is there something wrong with my rules? How would you implement such a (common) grammar?

[jonhue]

So as of now, the parser parses:

if true
  1
else
  1

true

but not:

if true
  1

true
=> rply.errors.ParsingError: (None, SourcePosition(idx=13, lineno=4, colno=1))

or

if true
  1
else if true
  1
else
  1

true
=> rply.errors.ParsingError: (None, SourcePosition(idx=29, lineno=5, colno=1))

[shellbit32]

Have you managed to figure this out? I'm facing a similar problem

[jonhue]

@FernandoLGL https://stackoverflow.com/questions/54205626/how-would-you-parse-a-standard-if-else-if-else-statement-with-rply

[shellbit32]

Thanks! My problem wasn't really a shift/reduce problem, but this rep https://github.com/joshsharp/python-braid helped me. I'm just making a simple SQL-like language but was having a similar issue when defining SELECT