Product Type

A product type is essentially an “and” type, a combination of types. Like a struct:

struct Foo {
  bar: bool,
  baz: i32
}

Foo is a bool AND i32 - both types are required to make Foo. In type theory, this is called a “product” type (for reasons I’ll explain later).

Sum Type

A sum type, on the other hand, is more like an “or” type, represented in C++ as a discriminated union:

union Foo {
  bool bar;
  int baz;
}

foo.bar = false;
foo.baz = 10