Monad example in JavaScript

monad-example.js

var cube = function ( x ) { return [x * x * x, "Cube was called"]; };

var square = function ( x ) { return [x * x, "Square was called"]; };

var compose = function ( f, g ) { return function ( a ) { return f( g( a ) ); }; };

var Debuggable = function ( f ) {
  return function ( debugInfo ) {
    var x  = debugInfo[0],
        s  = debugInfo[1],
        fx = f( x ),
        y  = fx[0],
        t  = fx[1];
    return [ y, s + t ];
  }
}

var DebuggableUnit = function ( x ) { return [ x, '' ]; }

var liftedComposition = compose( Debuggable( cube ), Debuggable( square ) );

liftedComposition( DebuggableUnit( 3 ) );