Transducer expansion

transducerExpansion.js

function mapping(f) {
    return function(rf) {
        return function(acc, x) {
            return rf(acc, f(x))
        }
    }
}

function filtering(f) {
    return function(rf) {
        return function(acc, x) {
            if(f(x))
                return rf(acc, x)
            else
                return acc
        }
    }
}

function comp(f, g) {
    return function(x) {
        let x2 = g(x)
        let x3 = f(x2)
        return x3
    }
}

let xf = comp(
  mapping(inc), 
  filtering(isEven)
)

// =>

let xf = function(rf) {
    let rf2 = filtering(isEven)(rf)
    let rf3 = mapping(inc)(rf2)
    return rf3
}

// =>

let xf = function(rf) {
    let rf2 = function(acc, x) {
        if(isEven(x))
            return rf(acc, x)
        else
            return acc
    }
    let rf3 = function(acc, x) {
        return rf2(acc, inc(x))
    }
    return rf3
}

// =>

let xf = function(rf) {
    function(acc, x) {
        acc = acc
        x = inc(x)

        if(isEven(x))
            return rf(acc, x)
        else
            return acc
    }
}