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examples.js
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examples.js
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LJSON = require("./ljson.js")
// TODO: add simpler examples
// Example #1: trusting an user-defined array library.
// Suppose you want your users to define an array library for you, but you
// don't trust them. As a good programmer, you know that using `foldr` and
// `cons` as primitives, one is able to encode any array-manipulating
// algorithm. So, you enable those primitives and nothing else.
// A creative user, then, writes this function that, given the promised
// primitives, returns 3 new functions: `foldl`, `reverse` and `concat`.
function userLib(foldr,cons){
function id(x){
return x;
};
function foldl(snoc,nil,array){
function cons(head,tail){
return function(result){
return tail(snoc(result,head));
};
};
return foldr(cons, id, array)(nil);
};
function reverse(array){
return foldl(function(head,tail){return cons(tail,head)}, [], array);
};
function concat(a,b){
return foldr(cons,foldr(cons,[],b),a);
};
return {
foldl : foldl,
reverse : reverse,
concat : concat};
};
// That user stringifies his library and sends it to you via sockets:
var userLibStr = LJSON.stringify(userLib);
// For the curious, this is the stringification of `userLibStr`:
// (function(v0, v1) {
// return {
// foldl: (function(v2, v3, v4) {
// return v0((function(v5, v6) {
// return (function(v7) {
// return v6(v2(v7, v5))
// })
// }), (function(v8) {
// return v8
// }), v4)(v3)
// }),
// reverse: (function(v9) {
// return v0((function(v10, v11) {
// return (function(v12) {
// return v11(v1(v10, v12))
// })
// }), (function(v13) {
// return v13
// }), v9)([])
// }),
// concat: (function(v13, v15) {
// return v0(v1, v0(v1, [], v15), v14)
// })
// }
// })
// You recover his original code by parsing that string:
var userLib = LJSON.unsafeParse(userLibStr);
// Now, to get the actual lib, you need to give it the promised primitives:
function foldr(cons, nil, array){
var result = nil;
for (var i=array.length-1; i>=0; --i)
result = cons(array[i],result);
return result;
};
function cons(head, tail){
return [head].concat(tail);
};
var lib = userLib(foldr, cons);
// You can then use his code safely, knowing he won't steal your password or
// mine bitcoins, because you gave him no power other than array manipulation.
// On this particular case, you can also be sure his programs will halt.
console.log(lib.reverse([1,2,3]));
console.log(lib.concat([1,2,3],[4,5,6]));