Parametric Polymorphism

We can define type variables as part of the definition of named types and then use them in the type definition in order to create type aliases with type parameters. For example, suppose in our foods example we wanted to specify that `Other could carry a value other than a string,

type FoodKind<'a> = [
  `Vegetable,
  `Fruit,
  `Meat,
  `Grain,
  `Other('a)
];
let v: FoodKind<`Cookie> = `Other(`Cookie);
v

if we paste this program into a file and run it we get,

$ graphix test.gx
`Other(`Cookie)

We can even place constraints on the type that a type variable can take. For example,

type Point3<'a: Number> = {x: 'a, y: 'a, z: 'a};
let f = |p: Point3<'a>, x: 'a| {p with x: p.x + x};
f({x: 0., y: 1., z: 3.14}, 1.)

Running this program we get,

$ graphix test.gx
{x: 0, y: 1, z: 3.14}

However, consider,

type Point3<'a: Number> = {x: 'a, y: 'a, z: 'a};
let v: Point3<'a> = {x: "foo", y: "bar", z: "baz"};
v

Running this, we can see that 'a is indeed constrained, since we get

$ graphix test.gx
Error: in file "test.gx"

Caused by:
    0: at: line: 2, column: 21, in: { x: "foo", y: "bar", z: "baz" }
    1: type mismatch Point3<'a: [Int, Real]> does not contain {x: string, y: string, z: string}

Indicating that we can't construct a Point3 with the type parameter of string, because the constraint is violated.