4.1 User Code Blocks

User code blocks begin following a "<<" token and end with a ">>" token found at the end of a line. All text lines in the code block are copied verbatim into the output file.

4.1.1 Standalone Code Blocks

C example:

<<
#include <stdio.h>
>>

D example:

<<
import std.stdio;
>>

Standalone code blocks are emitted early in the output file as top-level code outside the context of any function. Standalone code blocks are a good place to include/import any other necessary supporting code modules. They can also define helper functions that can be reused by lexer or parser user code blocks. They are emitted in the order they are defined in the grammar file.

For a C target, the word "header" may immediately follow the "<<" token to cause Propane to emit the code block in the generated header file rather than the generated implementation file. This allows including another header that may be necessary to define any types needed by a ptype directive, for example:

<<header
#include "mytypes.h"
>>

4.1.2 Lexer pattern code blocks

Example:

ptype ulong;

token integer /\\d+/ <<
  ulong v;
  foreach (c; match)
  {
    v *= 10;
    v += (c - '0');
  }
  $$ = v;
>>

Lexer code blocks appear following a token or pattern expression. User code in a lexer code block will be executed when the lexer matches the given pattern. Assignment to the $$ symbol will associate a parser value with the lexed token. This parser value can then be used later in a parser rule.

4.1.3 Parser rule code blocks

Example:

E1 -> E1 plus E2 << $$ = $1 + $3; >>

Parser rule code blocks appear following a rule expression. User code in a parser rule code block will be executed when the parser reduces the given rule. Assignment to the $$ symbol will associate a parser value with the reduced rule. Parser values for the rules or tokens in the rule pattern can be accessed positionally with tokens $1, $2, $3, etc...

Parser rule code blocks are not available in AST generation mode. In AST generation mode, a full parse tree is automatically constructed in memory for user code to traverse after parsing is complete.