Virtasant Compose Type Safety

type-safety-compose.ts

// Function composition is a way to combine several simpler function into a more complicated one.

const readFromTextInput = (): string => {
  // some pseudocode
  return "abcde"
}

const addSignature = (input: string): string => {
  return input + 'Hello AI.';
}

const fitInSMS = (input: string, limit: number = 160): boolean => {
  return input.length <= limit
}

// Given these three simple functions, we can compose the following chain.
// const canSMS = fitInSMS(addSignature(readFromTextInput()))

// Alternatively, we can use compose(...) function
// const canSMS = compose(readFromTextInput, addSignature, fitInSMS)

// 1. The task is to write type-safe compose function
function compose<A, B, C>(
  f1: (c: C) => string,
  f2: (b: B) => string,
  f3: (a: A) => boolean,
): boolean

function compose(...funcs: any[]) {
  if (funcs.length === 0) {
    return <T>(arg: T) => arg
  }

  if (funcs.length === 1) {
    return funcs[0]
  }

  return funcs.reduce((a, b) => (...args: any) => a(b(...args)))
}


// 1.1. Success case
// TS Transpiler should understand the return type of compose function
// const canSMS: boolean = compose(readFromTextInput, addSignature, fitInSMS)
const canSMS: boolean = compose(readFromTextInput, addSignature, fitInSMS)

// 1.2. Fail case
// TS Transpiler should validate types of parameters passed from one function to another, e.g.

const readFromNumberInput = (): number => {
  // pseudocode
  return 13
}

// This scenario should fail, because readFromNumberInput returns a number, however addSignature accepts string
const canFail: boolean = compose(readFromNumberInput, addSignature, fitInSMS)