When run-time traces are enabled (see Tracing Your Parser), the parser reports its actions, such as reductions. When a symbol involved in an action is reported, only its kind is displayed, as the parser cannot know how semantic values should be formatted.
The %printer directive defines code that is called when a symbol is
reported. Its syntax is the same as %destructor (see Freeing Discarded Symbols).
Invoke the braced code whenever the parser displays one of the symbols. Within code,
yyoutputdenotes the output stream (aFILE*in C, and anstd::ostream&in C++),$$(or$<tag>$) designates the semantic value associated with the symbol, and@$its location. The additional parser parameters are also available (see The Parser Functionyyparse).The symbols are defined as for
%destructor(see Freeing Discarded Symbols.): they can be per-type (e.g., ‘<ival>’), per-symbol (e.g., ‘exp’, ‘NUM’, ‘"float"’), typed per-default (i.e., ‘<*>’, or untyped per-default (i.e., ‘<>’).
For example:
%union { char *string; }
%token <string> STRING1
%token <string> STRING2
%type <string> string1
%type <string> string2
%union { char character; }
%token <character> CHR
%type <character> chr
%token TAGLESS
%printer { fprintf (yyoutput, "'%c'", $$); } <character>
%printer { fprintf (yyoutput, "&%p", $$); } <*>
%printer { fprintf (yyoutput, "\"%s\"", $$); } STRING1 string1
%printer { fprintf (yyoutput, "<>"); } <>
guarantees that, when the parser print any symbol that has a semantic type
tag other than <character>, it display the address of the semantic
value by default. However, when the parser displays a STRING1 or a
string1, it formats it as a string in double quotes. It performs
only the second %printer in this case, so it prints only once.
Finally, the parser print ‘<>’ for any symbol, such as TAGLESS,
that has no semantic type tag. See also