Programming Flix

Essential Classes

Practical programming in Flix requires knowledge of three type classes: Eq, Order, and ToString.

The Eq Class

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

class 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.

The Order Class

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

class 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 class, 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
}

The ToString Class

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

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

Flix uses the ToString type class in string interpolations.

For example, the interpolated string

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

is actually syntactic sugar for the expression:

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

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