simplify.go

// Expr is something that can be simplified.
type Expr interface {
	simplify() Expr
}

// binary operator on int.
type operator struct {
	op func(a int, b int) int
	a Expr
	b Expr
}

func valueOf(e Expr) (int, error) {
	switch t := e.(type) {

	}
}

func (o *operator) simplify() Expr {
	switch t := a.(type) {

	}
}

func add(a int, b int) int {
	return a + b
}

func mult(a int, b int) int {
	return a * b
}

func Add(a Expr, b Expr) Expr {
	return &operator{op: add, a: a, b: b}
}

func Mult(a Expr, b Expr) Expr {
	return &operator{op: mult, a: a, b: b}
}

Add(
	Mult(
		Add(
			Int(1), 
			Mult(
				Int(0), 
				Var("x")
			)
		),
		Int(3)
	),
	Int(12)
)