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WebExpo 2022: Railway Oriented Typescript
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/* | |
============================= | |
Railway Oriented Typescript | |
============================= | |
by @robinpokorny | |
*/ | |
/* === 1. Union basics === */ | |
const a: string | number = 42; | |
const b: "hey" | "ahoy" = "hey"; | |
const c: "hey" | "ahoy" = "heyyyy"; // Error | |
const code: 200 | 400 | 500 = 200; | |
const f = ( | |
a: { name: string } | { age: number } | |
) => { | |
a.name; // Property 'name' does not exist on type '{ name: string; } | { age: number; }'. | |
if ("name" in a) { | |
a.name; | |
} | |
}; | |
type Person = { tag: "person"; name: string }; | |
type Pet = { tag: "pet"; age: number }; | |
const g = (a: Person | Pet) => { | |
if (a.tag === "person") { | |
a.name; | |
a.age; // Error | |
} | |
}; | |
/* === 2. Success, Failure, Result === */ | |
type Success<T> = { | |
tag: "success"; | |
value: T; | |
}; | |
type Failure<E> = { | |
tag: "failure"; | |
value: E; | |
}; | |
type Result<T, E> = Success<T> | Failure<E>; | |
/* === 3. Use Result === */ | |
const validate = ( | |
input: unknown | |
): Result<number, "NOT_AN_INTEGER"> => { | |
if (Number.isInteger(input)) { | |
return { | |
tag: "success", | |
value: input as number, | |
}; | |
} else { | |
return { | |
tag: "failure", | |
value: "NOT_AN_INTEGER", | |
}; | |
} | |
}; | |
const updateDB = ( | |
input: number | |
): Result<Date, "DB_UNAVAILABLE"> => { | |
DB.write(input); | |
if (DB.success) { | |
return { | |
tag: "success", | |
value: new Date(), | |
}; | |
} else { | |
return { | |
tag: "failure", | |
value: "DB_UNAVAILABLE" as const, | |
}; | |
} | |
}; | |
const log = (value: unknown) => | |
console.log("[RP]:", value); | |
/* === 4. Chain and Tee === */ | |
const chain = | |
<T, S, E>(f: (a: T) => Result<S, E>) => | |
<F>(result: Result<T, F>): Result<S, E | F> => { | |
if (result.tag === "failure") { | |
return result; | |
} | |
return f(result.value); | |
}; | |
// <F>(result: Result<number, F>) => Result<Date, "DB_UNAVAILABLE" | F> | |
const chainedUpdateDB = chain(updateDB); | |
const tee = | |
<T, E>(f: (a: unknown) => void) => | |
(result: Result<T, E>): Result<T, E> => { | |
if (result.tag === "success") { | |
f(result.value); | |
} | |
return result; | |
}; | |
// (result: Result<unknown, unknown>) => Result<unknown, unknown> | |
const teedLog = tee(log); | |
const action = (input: unknown) => { | |
// pipe(a, b, c) == c(b(a)) | |
// Result<Date, "DB_UNAVAILABLE" | "NOT_AN_INTEGER"> | |
const response = pipe( | |
input, | |
validate, | |
chain(updateDB), | |
tee(log) | |
); | |
if (response.tag === "failure") { | |
switch (response.value) { | |
case "DB_UNAVAILABLE": | |
return "…"; | |
case "NOT_AN_INTEGER": | |
return "…"; | |
case "NOT_AN_ARRAY": | |
return "…"; | |
} | |
if (response) { | |
// Unreachable code detected | |
} | |
} | |
}; | |
// A: `string`-`number` mismatch | |
// B: Forgot `chain` | |
// UTILS | |
const DB = { | |
success: false, | |
write: (x: unknown) => {}, | |
}; | |
export const input = 3; | |
import { pipe } from "fp-ts/function"; | |
[a, b, c, code, action, f, g, chainedUpdateDB, teedLog] |
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