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Typescript IO Monad Implementation
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abstract class Monad<T> { | |
protected value: T; | |
public abstract map(f: Function): Monad<T>; | |
public abstract flatMap(f: Function): Monad<T>; | |
} | |
type Effect<T> = () => T; | |
class IO<T> extends Monad<Effect<T>> { | |
constructor(val: Effect<T>) { | |
super(); | |
this.value = val; | |
} | |
static of<T>(val: Effect<T>): IO<T> { | |
return new IO(val); | |
} | |
map<B>(f: (val: T) => T | B): IO<T | B> { | |
return new IO(() => f(this.eval())); | |
} | |
asyncMap<B>(f: (val: T) => Promise<T | B>): IO<Promise<T | B>> { | |
return new IO(async () => f(await this.eval())); | |
} | |
flatMap<B>(f: (val: T) => IO<T | B>): IO<T | B> { | |
return new IO(() => f(this.eval()).eval()); | |
} | |
asyncFlatMap<B>(f: (val: T) => IO<Promise<T | B>>): IO<Promise<T | B>> { | |
return new IO(async () => f(await this.eval()).eval()); | |
} | |
eval(): T { | |
return this.value(); | |
} | |
toString(): string { | |
return `IO ${typeof this.value}`; | |
} | |
} | |
// ----- | |
// utils | |
// ----- | |
const log = (...args: any[]) => console.log(`=> ${args.join(' ')}`); | |
const tapLog = (x: any): any => { | |
log(x); | |
return x; | |
}; | |
const randBetween = (...args: Array<any>): any => { | |
const rand = Math.random(); | |
const step = 1 / args.length; | |
return args.find((a, index) => { | |
if (rand >= index * step && rand < (index + 1) * step) return a; | |
}); | |
}; | |
// ----- | |
// examples | |
// ----- | |
// ----- | |
// sync usage | |
// ----- | |
const readdirSyncIO = (...args: any[]) => IO.of(() => fs.readdirSync(...args)); | |
const readFileSyncIO = (...args: any[]) => | |
IO.of(() => fs.readFileSync(...args)); | |
const writeFileSyncIO = (...args: any[]) => | |
IO.of(() => fs.writeFileSync(...args)); | |
const countAndWriteFilesWithExt = (ext: string): IO<string> => | |
readdirSyncIO('./') | |
.map( | |
files => files.filter((filename: any) => filename.endsWith(ext)).length, | |
) | |
.flatMap(count => writeFileSyncIO(`count.txt`, count.toString(), 'utf8')) | |
.flatMap(() => readFileSyncIO(`count.txt`, 'utf8')); | |
log(countAndWriteFilesWithExt('.json').eval()); | |
// ----- | |
// async usage with error handling | |
// ----- | |
const unsafeFetchDataIO = () => | |
IO.of( | |
randBetween( | |
() => tapLog('1 worked') && Promise.resolve({ status: 200, data: 10 }), | |
() => | |
tapLog('1 failed') && | |
Promise.reject({ status: 500, error: 'Internal server error.' }), | |
), | |
); // same chances to fail or succeed ; | |
const unsafeFetchMoreDataIO = () => | |
IO.of( | |
randBetween( | |
() => | |
tapLog('2 worked') && | |
Promise.resolve({ status: 200, data: 'Am some API data' }), | |
() => | |
tapLog('2 failed') && | |
Promise.reject({ status: 500, error: 'Internal server error.' }), | |
), | |
); // same chances to fail or succeed ;; | |
const asyncTasksMess = unsafeFetchDataIO() | |
.asyncMap(res => ({ ...res, data: res.data * 2 })) | |
.asyncFlatMap((data: any) => | |
writeFileSyncIO(`data.txt`, JSON.stringify(data), 'utf8'), | |
) | |
.asyncFlatMap(() => readFileSyncIO(`data.txt`, 'utf8')) | |
.asyncMap(log) // => { status: 200, data: 20 } | |
.asyncFlatMap(() => unsafeFetchMoreDataIO()) | |
.asyncFlatMap((data: any) => | |
writeFileSyncIO(`data.txt`, JSON.stringify(data), 'utf8'), | |
) | |
.asyncFlatMap(() => readFileSyncIO(`data.txt`, 'utf8')); | |
asyncTasksMess | |
.eval() | |
.then(log) // => { status: 200, data: "Am some API data" } | |
.catch(e => log(e.error)); // => Internal server error. |
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