When you write a grammar for Semantic, it is important to carefully indicate the start nonterminals. Each one defines an entry point in the grammar, and after parsing its semantic value is returned to the back-end iterative engine. Consequently:
The semantic value of a start nonterminal must be a produced by a TAG like grammar macro.
Start nonterminals are declared by
%start statements. When
nothing is specified the first nonterminal that appears in the grammar
is the start nonterminal.
Generally, the following nonterminals must be declared as start symbols:
These grammar macros recursively parse a part of input data, based on rules of the given nonterminal.
For example, the following will parse ‘PAREN_BLOCK’ data using the ‘formal_parameters’ rules:formal_parameter_list : PAREN_BLOCK (EXPANDFULL $1 formal_parameters) ;
The semantic value of ‘formal_parameters’ becomes the value of the
EXPANDFULLexpression. It is a list of Semantic tags spliced in the tags tree.
Because the automaton must know that ‘formal_parameters’ is a start symbol, you must declare it like this:%start formal_parameters
EXPANDFULL macro has a side effect it is important to know,
related to the incremental re-parse mechanism of Semantic: the
nonterminal symbol parameter passed to
EXPANDFULL also becomes
reparse-symbol property of the tag returned by the
When buffer's data mapped by a tag is modified, Semantic
schedules an incremental re-parse of that data, using the tag's
reparse-symbol property as start nonterminal.
The rules associated to such start symbols must be carefully reviewed to ensure that the incremental parser will work!
Things are a little bit different when the grammar is written in Bison style.
reparse-symbol property is set to the nonterminal
symbol the rule that explicitly uses
EXPANDTAG belongs to.
rule: rhs (let* ((rhs $1) name type comps prec action elt) ... (EXPANDTAG (TAG name 'rule :type type :value comps :prec prec :expr action) )) ;
reparse-symbol property of the expanded tag to
‘rule’. A important consequence is that:
Every nonterminal having any rule that calls
in a semantic action, should be declared as a start symbol!