REBOL [
	author: 'chl
	date: 2003-01-13
]

eval-expr: func [expr /local e r] [
	if number? expr/1 [
		return expr/1	
	]
	switch expr/1 [
		"+" [r: 0 foreach e next expr [r: r + e] r]
		"/" [r: 0 foreach e next expr [r: r + (1 / e)] 1 / r]
	]
]

op-chars: 	charset "+/"
num-chars: 	charset "1234567890"
op:		[
			copy o
			op-chars 
			(
				either number? pick (last stack) 1 [
					insert (last stack) o
				] [
					if not (o = pick (last stack) 1) [
						make error! "wrong op"
					]
				]
			)
		]
number:		[copy n some num-chars (append (last stack) to-integer n)]
arg:		[number | "(" expr ")"]
expr: 		[
			(append/only stack copy []) 
			arg any [op arg]
			(
				append (pick stack (-1 + length? stack)) (eval-expr last stack)
				remove back tail stack
			)
		]

eval: func [expr-str] [
	stack: reduce [copy []]
	parse expr-str expr
	stack/1/1
]