Quick impl of halting problem contradiction

halt.js

/*
Starting from http://my.opera.com/Vorlath/blog/2009/06/18/halting-problem-composability-and-compositionality
First, my apologies on reopening what is apparently not a recent blog post - the author might
long since have decided to no longer pursue this, but this was pointed out to me today, and
it looked like an interesting discussion to take part on. Whoops.

Consider any language able to turn a string into executable code: Javascript has an eval 
function able to take a string, and execute it. This can be used to turn a block of code into 
something which can be executed by simply calling a function.
*/

var haltCheckFunc = function(pgrm) {/*...*/};
// where ... in the implementation of a hypothetical check of another string of javascript, 
//here noted as pgrm. This is the input to which you refer - it can be asserted that this must 
//be valid javascript, and could be checked by wrapping it in function() { }, and evaling that 
//result. Note that this wrapping would mean that the only input is whatever is currently 
//available in the running js memory.

var contradiction = "if (haltCheckFunc(contradiction)) {while(true);} return;" 
// Declare a simple js program which will run itself through the halt check. No input is 
//directly required, except for other locally scoped variables

eval(contradiction);// If haltCheckFunc would return true on the string, then this eval will 
//not end, thus haltCheckFunc has lied to us

/*
This is trivial to do in JS, but just as doable in any other language where you can implement
 an interpreter. If I were to implement (for example) a JS interpreter in C, or Java, or any 
other Turing-complete language, I could build this same program, and though the haltCheckFunc 
could then be implemented in the underlying language, it would still be unable to predict how 
the function would complete.

Input matters, but allowing for a machine powerful enough to implement itself within that 
machine means you are too powerful to predict important things, as steps can be taken to 
demonstrate that input is a valid program, and yet you cannot specify certain properties about 
it. The input only matters in this example such that the haltCheck can parse the input, and 
the input can itself somehow be executed.
*/