pub trait HasTruth
    fn isTruthy() -> Boolean

    def fn isFalsy() -> Boolean = !self.truthy
end

pub trait Functor[Self<T>]
    fn <U> map(f: fn (t: T) -> U) -> Self<U>
end

pub trait Lift[Self<T>]
    static fn lift(t: T) -> Self<T>
end

pub trait Applicative[Self<T>, U, V] : Functor[Self] + Lift[Self] where T is fn (u: U) -> V
    fn apply(us: Self<U>) -> Self<V>
end

pub trait Monad[Self<T>] : Functor[Self] + Lift[Self]
    fn <U> flatMap(f: fn (t: T) -> Self<U>) -> Self<U>
end

// Traits as higher-kinded types
// - Cannot be used as concrete types, casted, etc.
// - Cannot be constructed/instantiated
// - Imports are different:
//   ```
//   use std::core::Option
//   use std::trait::Monad[Option<*>] // bring in Monad for all Option<*>
//   ```
// - Once imported, fns are in scope
// - Can also be looked-up 'dictionary' style:
//   ```
//   use std::trait::HasTruth[Option<Boolean>]
//
//   fn main()
//     let maybeTruthy: Option<Boolean> = someOtherApiCall()
//     if HasTruth[maybeTruthy].isTruthy() then
//       println "It's truthy."
//     end
//   ```