yet-another-monad-example.go

package main

import (
	"errors"
	"fmt"
)

type Number float64

type Operator func(Number) Number

func Adder(i Number) Operator {
	return func(n Number) Number {
		return n + i
	}
}

func Subtractor(i Number) Operator {
	return func(n Number) Number {
		return n - i
	}
}

func Multiplier(i Number) Operator {
	return func(n Number) Number {
		return n * i
	}
}

func Divider(i Number) Operator {
	return func(n Number) Number {
		return n / i // FIXME: divide by zero
	}
}

// f(g(n))
func Compose(f, g Operator) Operator {
	return func(n Number) Number {
		x := g(n)
		y := f(x)
		return y
	}
}

type Result struct {
	Val Number
	Err error
}

type LiftingOperator func(Number) Result

func LiftingAdder(i Number) LiftingOperator {
	return func(n Number) Result {
		return Result{n + i, nil}
	}
}

func LiftingSubtractor(i Number) LiftingOperator {
	return func(n Number) Result {
		return Result{n - i, nil}
	}
}

func LiftingMultiplier(i Number) LiftingOperator {
	return func(n Number) Result {
		return Result{n * i, nil}
	}
}

func LiftingDivider(i Number) LiftingOperator {
	return func(n Number) Result {
		if i == 0 {
			return Result{0.0, errors.New("divide by zero")}
		}
		return Result{n / i, nil}
	}
}

func ComposeLifting(f, g LiftingOperator) LiftingOperator {
	return func(n Number) Result {
		x := g(n)
		if x.Err != nil {
			return Result{0.0, fmt.Errorf("call inner function: %w", x.Err)}
		}
		y := f(x.Val)
		if y.Err != nil {
			return Result{0.0, fmt.Errorf("call outer function: %w", y.Err)}
		}
		return Result{y.Val, nil}
	}
}

func main() {
	var n Number = 1
	addTwo := Adder(2)
	subOne := Subtractor(1)
	mulThree := Multiplier(3)
	divFour := Divider(4)

	// ((((1 + 2) * 3) - 1) / 4) = 2.0
	fmt.Println(divFour(subOne(mulThree(addTwo(n)))))

	f := Compose(mulThree, addTwo)
	g := Compose(subOne, f)
	h := Compose(divFour, g)
	fmt.Println(h(n))

	//divZero := Divider(0)
	//f = Compose(mulThree, addTwo)
	//g = Compose(subOne, f)
	//h = Compose(divZero, g) // NOTE: this would break everything
	//fmt.Println(h(n))

	addLiftingTwo := LiftingAdder(2)
	subLiftingOne := LiftingSubtractor(1)
	mulLiftingThree := LiftingMultiplier(3)
	divLiftingFour := LiftingDivider(4)

	fl := ComposeLifting(mulLiftingThree, addLiftingTwo)
	gl := ComposeLifting(subLiftingOne, fl)
	hl := ComposeLifting(divLiftingFour, gl)
	fmt.Println(hl(n))
}