Style Guide

At some point, especially when there are lots of semantic actions attached to various points, the grammar tends to be quite difficult to follow. In order to keep an easy-to-read, consistent and aesthetically pleasing look to the Spirit code, the following coding style guide is advised.

This coding style is adapted and extended from the ANTLR/PCCTS style and Boost Library Requirements and Guidelines and is the combined work of Joel de Guzman, Chris Uzdavinis, and Hartmut Kaiser.

Rule names use std C++ (Boost) convention. The rule name may be very long.
The '=' is neatly indented 4 spaces below. Like in Boost, use spaces instead of tabs.
Breaking the operands into separate lines puts the semantic actions neatly to the right.
Semicolon at the last line terminates the rule.
The adjacent parts of a sequence should be indented accordingly to have all, what belongs to one level, at one indentation level.

program
    =   program_heading [heading_action]
        >> block [block_action]
        >> '.'
    |   another_sequence
        >> etc
    ;

Prefer literals in the grammar instead of identifiers. e.g. "program" instead of PROGRAM, '>=' instead of GTE and '.' instead of DOT. This makes it much easier to read. If this isn't possible (for instance where the used tokens must be identified through integers) capitalized identifiers should be used instead.
Breaking the operands may not be needed for short expressions. e.g. *(',' >> file_identifier) as long as the line does not exceed 80 characters.
If a sequence fits on one line, put spaces inside the parentheses to clearly separate them from the rules.

program_heading
    =   no_case["program"]
        >> identifier
        >> '('
        >> file_identifier
        >> *( ',' >> file_identifier )
        >> ')'
        >> ';'
    ;

Nesting directives: If a rule does not fit on one line (80 characters) it should be continued on the next line intended by one level. The brackets of directives, semantic expressions (using Phoenix or LL lambda expressions) or parsers should be placed as follows.

identifier
    =   no_case
        [
            lexeme
            [
                alpha >> *(alnum | '_') [id_action]
            ]
        ]
   ;

Nesting unary operators (e.g.Kleene star): Unary rule operators (Kleene star, '!', '+' etc.) should be moved out one space before the corresponding indentation level, if this rule has a body or a sequence after it, which does not fit on on line. This makes the formatting more consistent and moves the rule 'body' at the same indentation level as the rule itself, highlighting the unary operator.

block
   =  *(   label_declaration_part
       |   constant_definition_part
       |   type_definition_part
       |   variable_declaration_part
       |   procedure_and_function_declaration_part
       )
       >> statement_part
   ;

Boost C++ Libraries

Style Guide