Generics

Generic Constraints

Constraints let a generic function require the members it needs.

generic constraint A generic constraint such as `T extends HasLength` limits which values the generic function accepts.

Generic Constraints

word
constraints.ts
Replay: real traced execution (multi-file project)
interface HasLength {
    length: number;
}

function lengthLabel<T extends HasLength>(value: T): string {
    return `length=${value.length}`;
}

const word: string = "generic";
const label: string = lengthLabel<string>(word);

console.log(label);

interface HasLength {
    length: number;
}

function lengthLabel<T extends HasLength>(value: T): string {
    return `length=${value.length}`;
}

const word: string = "type";
const label: string = lengthLabel<string>(word);

console.log(label);

interface HasLength {
    length: number;
}

function lengthLabel<T extends HasLength>(value: T): string {
    return `length=${value.length}`;
}

const word: string = "constraint";
const label: string = lengthLabel<string>(word);

console.log(label);

  1. word ← generic

    6    return `length=${value.length}`;7}89const word→ generic: string = "generic";  //@word="type", "constraint"10const label: string = lengthLabel<string>(wordgeneric);

  2. function lengthLabel<T extends HasLength>(value: T): string

    2    length: number;3}45function lengthLabel<T extends HasLength>(valuegeneric: T): string {6    return `length=${value.length7}`;7}

  3. label ← length=7

    9const word: string = "generic";  //@word="type", "constraint"10const label→ length=7: string = lengthLabel<string>(wordgeneric);1112console.log(labellength=7);
    outputlength=7

  1. word ← type

    6    return `length=${value.length}`;7}89const word→ type: string = "type";10const label: string = lengthLabel<string>(wordtype);

  2. function lengthLabel<T extends HasLength>(value: T): string

    2    length: number;3}45function lengthLabel<T extends HasLength>(valuetype: T): string {6    return `length=${value.length4}`;7}

  3. label ← length=4

    9const word: string = "type";10const label→ length=4: string = lengthLabel<string>(wordtype);1112console.log(labellength=4);
    outputlength=4

  1. word ← constraint

    6    return `length=${value.length}`;7}89const word→ constraint: string = "constraint";10const label: string = lengthLabel<string>(wordconstraint);

  2. function lengthLabel<T extends HasLength>(value: T): string

    2    length: number;3}45function lengthLabel<T extends HasLength>(valueconstraint: T): string {6    return `length=${value.length10}`;7}

  3. label ← length=10

    9const word: string = "constraint";10const label→ length=10: string = lengthLabel<string>(wordconstraint);1112console.log(labellength=10);
    outputlength=10

Follow the Length

  1. word starts as generic.
  2. lengthLabel<string>(word) receives that string.
  3. The function reads value.length.
  4. generic has 7 letters.
  5. The program prints length=7. | word | value.length | printed output | | --- | ---: | --- | | generic | 7 | length=7 | | type | 4 | length=4 | | constraint | 10 | length=10 |

Exercise: constraints.ts

Reproduce length=7, then use word type and constraint to predict length=4 and length=10.