The specific structure does not match the generic format

type Identity = <T>(input: T) => T

const identity: Identity = (input: number) => input;

When using generics like this, it results in a compiler error:

Type '(input: number) => number' is not compatible with type 'Identity'.
  Parameters 'input' and 'a' have incompatible types.
    Type 'T' cannot be assigned to type 'number'.

To resolve this, we can define the generic type as follows: 

type Identity<T> = (input: T) => T
But sometimes, we may not want to explicitly declare the type every time. Is there a way for it to be inferred or flow through automatically?

typescript

Answer №1

It appears that the issue you're encountering stems from the contrasting nature of these two type definitions.

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

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

When defining a function of type Func1<A>, the type must be specified at the time of definition.

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

func1(10); // This works
func1("x"); // This fails - we have already determined it to be of type `number`

On the other hand, any function of type Func2 shouldn't specify its type until it is invoked.

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

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

You can explore this concept further with this playground link demonstration.

The error occurred because you attempted to define the function's type at the declaration stage rather than during invocation.

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