@markandrus/effect-derive/lib/data/Free.js

Derive Covariant (Functor), Foldable, and Traversable instances, as well as base functors, for algebraic data types (ADTs)

export const mapComposition = (F) => {
    return function map(self, f) {
        switch (self.type) {
            case 'Pure':
                return { type: 'Pure', a: f(self.a) };
            case 'Free':
                return { type: 'Free', fa: F.map(self.fa, _ => map(_, f)) };
        }
    };
};