Lenses that seem to work in Flow

lenses.js

// @flow strict-local

// Lenses
// ------------------------------------------------------------------------------------------------

export type LensInternal<S, T, A, B> = {
    getVal: (S) => A,
    setVal: (B, S) => T,
};

export type Lens<S, T, A, B> = {
    (): LensInternal<S, T, A, B>,
    compose: <C, D>(Lens<A, B, C, D>) => Lens<S, T, C, D>,
};

export const lens = <S, T, A, B>(getVal: (S) => A, setVal: (B, S) => T): Lens<S, T, A, B> => {
    const lensFunc = () => ({ getVal, setVal });
    lensFunc.compose = <C, D>(l: Lens<A, B, C, D>): Lens<S, T, C, D> => {
        const { getVal: g2, setVal: s2 } = l();
        return lens(
            (x: S) => g2(getVal(x)),
            (val: D, obj: S): T => setVal(s2(val, getVal(obj)), obj),
        );
    };
    return lensFunc;
};

export const view = <S, T, A, B>(l: Lens<S, T, A, B>, obj: S): A => l().getVal(obj);

export const set = <S, T, A, B>(l: Lens<S, T, A, B>, val: B, obj: S): T => l().setVal(val, obj);

export const lensCompose = <S, T, A, B, C, D>(
    l1: Lens<S, T, A, B>,
    l2: Lens<A, B, C, D>,
): Lens<S, T, C, D> => l1.compose(l2);

type L<A, B> = Lens<A, A, B, B>;

export const propLens = <S, K: string, A, B>(
    k: K,
): Lens<{ ...S, +[K]: A }, { ...S, +[K]: B }, A, B> =>
    lens(
        (s) => s[k],
        (val, obj: S) => ({ ...obj, [k]: val }),
    );

export const idxLens = <T>(idx: number): L<$ReadOnlyArray<T>, T> =>
    lens(
        (arr) => arr[idx],
        (val, arr) => arr.slice(0, idx).concat([val]).concat(arr.slice(idx)),
    );