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Comment by hikarudo | Hacker News Reader

A type constructor is something that takes a type and gives you a new type.

For example, in C, if you have any type Foo, the pointer operator * will give you a new type Foo*.

Another example: in Haskell, Maybe is a type constructor, since it takes a type parameter a (Maybe a). So 'Maybe' is a type constructor, but 'Maybe Int' is a type.

So a type constructor is like a function, but it operates on types, not on values.

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So this is a perfect example of how things could be better-explained to the average developer via analogy:

"You can think of a type constructor like a generic type, particularly the way those work in languages like TypeScript"

Edit: The C example was added after my initial comment, and is better than nothing, though I'd tend to say the pointer example is a bit of a weird one to go with. Of course intuitiveness is relative and I'm not saying an example has to mention TypeScript or whatever to be valid. But I do think, like, C++ templates would be a somewhat clearer comparison to make for that audience.

Good point. A generic type is a great example of a type constructor.

> the pointer operator *

Do you mean declarator?, As in:

     Foo * f;    // f is a pointer to a Foo

The operator dereferences something - it certainly does not create a new type. And neither does the above - the concept of pointers to addressable types is innate to the C and C++ type systems - if you create a type Foo, you get pointers to Foo for free.

You are correct, my mistake.

The fact that pointers to Foo are "free" doesn't invalidate the notion that the * is, in some sense, a semantic extension of whatever Foo is and thus a generic type. I know that this is not literally true in the "theory" of the C programming language and type system, but conceptually it matches up pretty well.

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