Work in progress : macros for b4a.js ... These are actually more like mini-compilers. Basically the idea is that when you see a word like `.if` , it's going to define `.then` and `.else` on the fly... This way if you encounter an `.else` that is not inside an `.if`, it will be a syntax error. The assembly code should wind up looking a lot more l…

b4-macros.js

  // when asm( ) encounters a word that starts with punctuation,
  // it will not try to compile a call to that word, but will instead
  // look up the word's definition in the "macro" vocabulary, and 
  // execute the macro immediately.
  //
  // The macros are written in javascript.

  // semicolon. in forth this would issue a 'return' instruction,
  // but in b4, it's used as punctuation. In the default context,
  // it doesn't do anything, but it can be used to break long
  // strings of words down into easier to understand chunks.
  def( ';', function( ) {
  });

  // REPEAT ... UNTIL ... ; loop, as in pascal or oberon.
  def( '.repeat', function {
    // repeat-loop ::= REPEAT <code> UNTIL <condition> ";"
    // so define what UNTIL means in this context.
    pushdef( '.until', function( ) {
      // ... and inside UNTIL, redefine ";"
      pushdef( ';', function( ) {
        // when we encounter the ';' emit code to conditionally jump back to the REPEAT.
        // That is, "UNTIL x;" compiles down to "IF x NOT THEN JUMP: <top-of-loop> END"
        // so by the time we're here, we will have already compiled
        // 'IF x', and we can continue with:
        asm( "not .then" );
          // REPEAT will have put the <top-of-loop> label on the stack.
          // ".emit" will take it off the stack and compile it:
          asm( "JUMP: .emit" );
        // close the IF statement, so it can clean up after itself:
        asm( ".end" );
        // and then step out of the REPEAT context:
        popdefs( ";", ".until" );
      });
      // with the plan for ";" in place, UNTIL now compiles the IF
      asm( ".if" );
    });
    // and with the plan for "UNTIL" in place, REPEAT now records <top-of-loop>.
    asm( ".here" );
  });