Essential Traits

Practical programming in Flix requires knowledge of at least three traits: Eq, Order, and ToString.

The Eq Trait

The Eq trait captures when two values of a specific type are equal:

trait Eq[a] {

    ///
    /// Returns `true` if and only if `x` is equal to `y`.
    ///
    pub def eq(x: a, y: a): Bool

    // ... additional members omitted ...
}

To implement Eq, we only have to implement the eq function. When we implement eq we automatically get an implementation of Eq.neq.

The Order Trait

The Order trait captures when one value is smaller or equal to another value of the same type:

trait Order[a] with Eq[a] {

    ///
    /// Returns `Comparison.LessThan` if `x` < `y`, 
    /// `Equal` if `x` == `y` or 
    /// `Comparison.GreaterThan` if `x` > `y`.
    ///
    pub def compare(x: a, y: a): Comparison

    // ... additional members omitted ...
}

To implement the Order trait, we must implement the compare function which returns value of type Comparison. The Comparison data type is defined as:

enum Comparison {
    case LessThan
    case EqualTo
    case GreaterThan
}

When we implement compare, we automatically get implementations of Order.less, Order.lessThan, Order.greater, Order.greaterEqual, Order.max, and Order.min.

The ToString Trait

The ToString trait is used to obtain a string representation of a specific value:

trait ToString[a] {
    ///
    /// Returns a string representation of the given `x`.
    ///
    pub def toString(x: a): String
}

Flix uses the ToString trait in string interpolations.

For example, the interpolated string

"Good morning ${name}, it is ${hour} o'clock."

is actually syntactic sugar for the expression:

"Good morning " + ToString.toString(name) + ", it is " 
                + ToString.toString(hour) + " o'clock."

In the following subsection, we discuss how to automatically derive implementations of the Eq, Order, and ToString traits.