Closure Like Protocol

A.js

export const f1 = a => a.x

f1.arity = 1

export const f2 = c => a => a.x + c

f2.arity = 2

export const f3 = d => c => a => a.x + c + d

f3.arity = 3

B.js

export const f1 = b => b.y

f1.arity = 1

export const f2 = c => b => c + b.y

f2.arity = 2

export const f3 = d => c => b => d + c + b.y

f3.arity = 3

index.js

// export const f1 = 'C'
import * as A from './A.js'
import * as B from './B.js'

const a = {
    '@@type': 'A',
    x: '.A.'
}

const b = {
    '@@type': 'B',
    y: '.B.'
}

const a2 = {
    ...a,
    x: '.A2.'
}

const protocol = mods => {
    const err = m => {
        throw new Error(m)
    }
    const fSet = Object.entries(mods).reduce(
        (fSet, [tn, ns]) => ({
            ...fSet,
            ...Object.entries(ns).reduce(
                (rSet, [fn, f]) => ({
                    ...rSet,
                    [fn]:
                        fSet[fn] && fSet[fn] !== f.arity
                            ? err('arity')
                            : f.arity
                }),
                {}
            )
        }),
        {}
    )
    const f = fn => {
        !fSet[fn] && err('not implemented')
        switch (fSet[fn]) {
        case 1:
            return v => mods[v['@@type']][fn](v)
        case 2:
            return v => v2 => mods[v2['@@type']][fn](v)(v2)
        case 3:
            return v => v2 => v3 => mods[v3['@@type']][fn](v)(v2)(v3)
        case 4:
            return v => v2 => v3 => v4 =>
                mods[v4['@@type']][fn](v)(v2)(v3)(v4)
        case 5:
            return v => v2 => v3 => v4 => v5 =>
                mods[v5['@@type']][fn](v)(v2)(v3)(v4)(v5)
        case 6:
            return v => v2 => v3 => v4 => v5 => v6 =>
                mods[v6['@@type']][fn](v)(v2)(v3)(v4)(v5)(v6)
        }
    }
    return Object.keys(fSet).reduce(
        (out, fn) => ({
            ...out,
            [fn]: f(fn)
        }),
        {}
    )
}

const _ = protocol({ A, B })

console.log(_.f1(a), _.f1(b), _.f1(a2))

console.log(_.f2('X')(a), _.f2('Y')(b), _.f2('Z')(a2))

console.log(_.f3('X2')('X')(a), _.f3('Y2')('Y')(b), _.f3('Z2')('Z')(a2))