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December 19, 2017 09:28
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An attempt at understanding and implementing a naive model for algebraic effects in JS.
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const Effect = (type, val) => ({ type, val }); | |
const run = gen => { | |
let r; | |
while (!(r = gen.next()).done) {} | |
return r.value; | |
}; | |
function* handle(fn, map = {}) { | |
const gen = fn(); | |
let r; | |
let matchVal = undefined; | |
while (!(r = gen.next(matchVal)).done) { | |
if (r.value) { | |
const matchFn = map[r.value.type]; | |
if (matchFn) { | |
matchVal = matchFn(r.value.val); | |
} else { | |
yield r.value; | |
} | |
} | |
} | |
return r.value; | |
} | |
// Pseudo state | |
let state = 1; | |
// The "app"; | |
const runner = handle( | |
function*() { | |
const number = yield* handle( | |
function*() { | |
const current = yield Effect("State", 1); | |
yield Effect("Print", "Hello, world!"); | |
return current; | |
}, | |
// "Pattern matching" | |
{ | |
State: val => { | |
state += val; | |
return state; | |
} | |
} | |
); | |
return number + 1; | |
}, | |
{ | |
Print: val => console.log(`Print: ${val}`) | |
} | |
); | |
const output = run(runner); | |
console.log(output); | |
/* | |
OUTPUT | |
-> Print: Hello, world! | |
-> 3 | |
*/ |
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