CODEWITHSUNDEEP
typescript
bbz
bbz

Reputation: 123

Concrete type is not assignable to generic type

type Id = <A>(a: A) => A

const id: Id = (a: number) => a;

If I use generics in this way the compiler will return an error saying

Type '(a: number) => number' is not assignable to type 'Id'.
  Types of parameters 'a' and 'a' are incompatible.
    Type 'A' is not assignable to type 'number'.

I know it can be solved with type Id<A> = (a: A) => A But what if I can't declare A at every point. Is there a way to just have it flow through, or be inferred?

Upvotes: 1

Views: 198

Answers (1)

Shaun Luttin
Shaun Luttin

Reputation: 141512

I think what you're experiencing is the difference between these two type definitions.

type Func1<A> = (a: A) =>  A;

type Func2 = <A>(a: A) =>  A;

Any function of type Func1<A> must specify its type at the time it is defined.

const func1: Func1<number> = (a: number): number => a;

func1(10); // works
func1("x"); // fails - we already decided it is of type `number`

Anything of type Func2 must not specify its type until it is called.

const func2: Func2 = <X>(a: X): X => a;

func2(10); // works
func2("x"); // works

Here is a playground link that illustrates.

The problem you were experiencing happened because you tried to specify the type of the function when you defined it instead of when you called it.

Upvotes: 2

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