what is the difference between thunk, futures, and promises?

Question

There are wiki articles about them: (http://en.wikipedia.org/wiki/Futures_and_promises, http://en.wikipedia.org/wiki/Thunk_(delayed_computation)). But I am not sure what are the exact differences between the three as a programming language concept? Are futures and promises only applicable in concurrent programming?

Answer 1

An example of each, using javascript since everybody can read it.

Please don't use this code in production, use a real library, there are plenty of good ones.

var a, b, c, async_obj;  // assume exist  // CommonJS - for reference purposes  try {    async_obj.asyncMethod(a, b, c, function (error1, result1) {      if (error1) {       console.error(error1);     } else {       console.log(result1);     }    });  } catch (ex1) {   console.error(ex1); }  // Thunk - "a parameterless closure created to prevent the evaluation of an expression until forced at a later time"  function makeThunkForAsyncMethod (cb) {   return function () {     async_obj.asyncMethod(a, b, c, cb);   } }  var my_thunk = makeThunkForAsyncMethod(function (error1, result1) {   if (error1) {     console.error(error1);   } else {     console.log(result1);   } });  setTimeout(function () {   try {     my_thunk();   } catch (ex1) {     console.error(ex1);   } }, 5e3);   // Promise - "a writable, single assignment container which sets the value of the future"  function makePromiseForAsyncMethod () {   var     future_then_cb,     future_catch_cb,     future   ;    future = {     then: function (cb) {       future_then_cb = cb;     },     catch: function (cb) {       future_catch_cb = cb;     };   };    try {     async_obj.asyncMethod(a, b, c, function (error1, result1) {       if (error1) {         if (future_catch_cb) {           future_catch_cb(error1)         }       } else {         if (future_then_cb) {           future_then_cb(result1);         }       }     });   } catch (ex1) {     setTimeout(function () {       if (future_catch_cb) {         future_catch_cb(ex1);       }     });   }    return future; }  // Future - "a read-only placeholder view of a variable"  var my_future = makePromiseForAsyncMethod();  my_future   .then(function (result) {     console.log(result);   })   .catch(function (error) {     console.error(error);   }) ; 

A Promise chain would be like the above contrived example, but it would work on collections and be more robust.