[ufo]

This is a neat trick that pops up from time to time. Instead of representing an algebraic data type as a bunch of cases that you can test with if-statements at runtime, you represent it as a pattern-matching function that receives a code block to run for each possibility.

https://en.wikipedia.org/wiki/Smalltalk#Control_structures

http://c2.com/cgi/wiki?VisitorPattern

https://en.wikipedia.org/wiki/Mogensen%E2%80%93Scott_encodin...

One neat example of this correspondence is continuation-passing style. For example, in Javascript:

    function foo(){ return value; }
    function fooCps(cb){ cb(value); }

    function bar(){
       if(x){
          return {tag:'OK', value:value};
       }else{
          return {tag:'ERROR', errmsg:"message"};
       }
    }
    function barCps(okCb, errCb){
       if(x){
          okCb(value);
       }else{
          errCb("message");
       }
    }