A typeclass (or in Rust, a trait) is something that describes a set of behaviours that must be implemented by a type (in OOP languages, abstract classes are bro

A typeclass (or in Rust, a trait) is something that describes a set of behaviours that must be implemented by a type (in OOP languages, abstract classes are broadly equivalent). For example, the typeclass `Eq` describes how values of a type might be compared for equality. class Eq = => eq : Self -> Self -> Bool The typeclass requires that types conforming to it provide a function, `eq`, that takes two instances of itself and returns a bool (indicating whether the instances are equal). And member of this typeclass might look like: for A, B member (A, B) of Eq where A < Eq, B < Eq, = => eq = fn (a0, b0), (b0, b1) => a0 == a1 && b0 == b1 You can read this as "for any two types, A and B, a tuple of both can also be compared for equality provided both A and B can themselves be compared for equality" Typeclasses become useful because they allow you to constrain generic type parameters. For example, we can now write a function that determines whether a list contains a matching value: fn contains A : A -> [A] -> Bool where A < Eq, = | _, [] => False \ k, [x .. xs] => x == k || xs->contains(k) All of this is checked at compile-time by the compiler, ensuring that the function can only be called with types that can be compared for equality.