How can I simulate macros in JavaScript?

Question

I know that JavaScript doesn't support macros (Lisp-style ones) but I was wondering if anyone had a solution to maybe simulate macros? I Googled it, and one of the solutions suggested using eval(), but as he said, would be quite costly.

They don't really have to be very fancy. I just want to do simple stuff with them. And it shouldn't make debugging significantly harder :)

Answer 1

Heh, that is a clever solution. I guess I could roll some Emacs lisp code to do this. Not a bad suggestion, but it only covers one part of the equation: entering the stuff. Maintaining it code "generated" is at least as important.

Answer 2

Javascript is interpreted. Eval isn't any more costly that anything else in Javascript.

Answer 3

As much as it might be considered overhead by some, the best approach is to use functions, shorthand and ternary conditions as a replacement for macros. Most of the JavaScript libraries do that these days. For example:

1.)

var a, b;
if ( !a ) {
    a = b;
}

// shorthand approach:

var a, b;
a = a || b;

2.)

var a, b, c;
if ( true ) {
    a = b;
} else {
    a = c;
}

// shorthand approach:

var a, b, c;
a = ( true ) ? b : c;

3.)

var a, b, c, d;
b = a;
c = b;
d = c;

// shorthand approach:

var a, b, c, d;
d = c = b = a;

Several methods in jQuery are all essentially macros, although you have to define them as functions first, since the browser does not automatically interpret them. For example, some of the more popular of these are each(), next() and filter().

Answer 4

You could use parenscript. That'll give you macros for Javascript.

Answer 5

I've written a gameboy emulator in javascript and I simulate macros for cpu emulation this way:

macro code (the function returns a string with the macro code):

function CPU_CP_A(R,C) { // this function simulates the CP instruction, 
  return ''+             // sets CPU flags and stores in CCC the number
  'FZ=(RA=='+R+');'+     // of cpu cycles needed
  'FN=1;'+
  'FC=RA<'+R+';'+
  'FH=(RA&0x0F)<('+R+'&0x0F);'+
  'ICC='+C+';';
}

Using the "macro", so the code is generated "on the fly" and we don't need to make function calls to it or write lots of repeated code for each istruction...

OP[0xB8]=new Function(CPU_CP_A('RB',4)); // CP B
OP[0xB9]=new Function(CPU_CP_A('RC',4)); // CP C
OP[0xBA]=new Function(CPU_CP_A('RD',4)); // CP D
OP[0xBB]=new Function(CPU_CP_A('RE',4)); // CP E
OP[0xBC]=new Function('T1=HL>>8;'+CPU_CP_A('T1',4)); // CP H
OP[0xBD]=new Function('T1=HL&0xFF;'+CPU_CP_A('T1',4)); // CP L
OP[0xBE]=new Function('T1=MEM[HL];'+CPU_CP_A('T1',8)); // CP (HL)
OP[0xBF]=new Function(CPU_CP_A('RA',4)); // CP A

Now we can execute emulated code like this:

OP[MEM[PC]](); // MEM is an array of bytes and PC the program counter

Hope it helps...

Answer 6

what about using iMacros for Firefox?

Answer 7

function unless(condition,body) {
    return 'if(! '+condition.toSource()+'() ) {' + body.toSource()+'(); }';
}


eval(unless( function() {
    return false;
  }, function() {
    alert("OK");
}));