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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

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.

brundolf 5y ago
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.
- hikarudo 5y ago
  Good point. A generic type is a great example of a type constructor.
zabzonk 5y ago
> 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.
- hikarudo 5y ago
  You are correct, my mistake.
- jkhdigital 5y ago
  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.

brundolf 5y ago

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.

hikarudo 5y ago
Good point. A generic type is a great example of a type constructor.

zabzonk 5y ago

> 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.

hikarudo 5y ago
You are correct, my mistake.
jkhdigital 5y ago
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.