arithmetic_eval.moon

import P, S, V, R, C, Ct, match from require "lpeg"

Space = S' \n\t'^0
Num = C(P'-'^-1 * R'09'^1 ) * Space
Term = C(S'+-') * Space
Fact = C(S'*/%') * Space
Open = '(' * Space
Close = ')' * Space

G = P{
	"Exp"
	Exp: Ct(V'Term' * (Term * V'Term')^0)
	Term: Ct(V'Fact' * (Fact * V'Fact')^0)
	Fact: Num + Open * V'Exp' * Close
}

G = Space * G * -1

eval = (x) ->
	local ret_num
	if type(x) == "string"
		-- tonumber x
		ret_num = tonumber x
	else
		op1 = eval x[1]

		for i = 2, #x, 2
			op = x[i]
			op2 = eval x[i + 1]

			switch op
				when '+' op1 += op2
				when '-' op1 -= op2
				when '*' op1 *= op2
				when '/' op1 /= op2

		ret_num = op1

	ret_num

eval_exp = (s) ->
	t = match(G, s)
	if not t
		error "Syntax error", 2

	eval t

repl = ->
	while true
		io.write "> "
		l = io.read!

		if not l
			break

		print eval_exp l

repl!