在typescript中的type参数中使用的never关键字

Question

I am trying to understand the following code:

export const getArraySemigroup = <A = never>(): Semigroup<Array<A>> => {
  return {
    concat: (x, y) => concat(x, y)
  }
}

My understanding of never is that it is something that will never be resolved, is it being used as a default paramter her?

I've never (pun intended) seen a type parameter with an equals sign before.

Answer 1

<A = never> is generic type default value . By default, generic type default value is empty object, <{}> .

As stated in documentation ,

The never type represents the type of values that never occur. For instance, never is the return type for a function expression or an arrow function expression that always throws an exception or one that never returns; Variables also acquire the type never when narrowed by any type guards that can never be true.

This piece of code basically forces a developer to always use generic function with explicit type argument.