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Phantom Types

Phantom types are type parameters of a custom type that are not used in any of the value constructors of that type.

That's a little abstract, so here is an example:

pub type Length(unit) {
  Length(amount: Float)
}

In this example the unit type parameter is not used in the Length value constructor, so unit is a phantom type.

This unused type parameter may seem useless, but it can be used to add further restrictions on how Length values can be used.

For example, we could have a function double, which multiplies the length. It works with lengths of any unit, so the type parameter is a generic type variable.

// This function accepts all Length values
pub fn double(length: Length(unit)) -> Length(unit) {
  Length(length.amount *. 2.0)
}

We could also have a function add_inch, which only works if the length is in inches.

// A unit type for inches. It is never constructed so we don't
// define any constructors for it.
pub type Inches

pub fn add_inch(length: Length(Inches)) -> Length(Inches) {
  Length(length.amount +. 1.0)
}

The add_inch function will not accept lengths of any other unit parameter, the phantom type has been used to ensure only the correct unit is used.

A function can also be written to ensure that two length values are of the same unit, by using the same type variable for both.

pub fn add(a: Length(unit), b: Length(unit)) -> Length(unit) {
  Length(a.amount +. b.amount)
}

let two_meters: Length(Meters) = Length(2.0)
let two_inches: Length(Inches) = Length(2.0)

add(two_meters, two_meters)
// -> Length(4.0): Length(Meters)

add(two_meters, two_inches)
// Type error! The unit type parameters do not match.

Phantom types can work well with opaque types. If other modules cannot construct Length values then we can ensure they are not constructed with an invalid unit type, and that only the functions defined above can be used with them.