wisent: Start nonterminals
4.1.4 Start nonterminals
------------------------
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:
• The main grammar entry point
Of course!
• nonterminals passed to ‘EXPAND’/‘EXPANDFULL’
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 ‘EXPANDFULL’ expression. 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
The ‘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 the
‘reparse-symbol’ property of the tag returned by the ‘EXPANDFULL’
expression.
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.
*The ‘reparse-symbol’ property is set to the nonterminal symbol the
rule that explicitly uses ‘EXPANDTAG’ belongs to.*
For example:
rule:
rhs
(let* ((rhs $1)
name type comps prec action elt)
...
(EXPANDTAG
(TAG name 'rule :type type :value comps :prec prec :expr action)
))
;
Set the ‘reparse-symbol’ property of the expanded tag to ‘rule’. A
important consequence is that:
*Every nonterminal having any rule that calls ‘EXPANDTAG’ in a
semantic action, should be declared as a start symbol!*
Info Catalog
wisent: Mixed style
wisent: Grammar styles
wisent: Useful functions