Haskell : Reverse Polish Notation

rpn.hs

{-
Reverse Polish Notation
>example
rpn "10 4 3 + 2 * - "
-4.0

rpn "30 4 +"
34.0

rpn "4 3 + 10 *"
70.0

-}
import System.IO          -- putStrLn
import System.Environment -- getArgs
import Data.Char          -- isDigit
 
main = do
     args <- getArgs
     dispatch args     
 
dispatch :: [String] -> IO ()
dispatch [expression] = putStrLn $ show $ rpn expression
dispatch _          = help 
 
{-
Command format is ...
-}
help :: IO ()
help = do
       	 putStr $ "The number of arguments is one.\n" ++ 
       	 	  "argument ::= <expression>\n" ++ 
		  "<expression> :: = {0..9| |+|-|*|/|} \n" ++ 
	 	  "*** Example *** \n" ++
		  "prn \"10 4 3 + 2 * -\" \n" ++
	 	  "-4.0" ++ 
		  "\n"
 
{-
This program's body definition is ...
-}
rpn :: String -> Double
rpn expression = parse [] $ words expression
 
 
{- *** Description *** 
 The list for the stack should has only one value when input string is empty.
-}
type Stack = [Double]
 
parse :: Stack -> [String] -> Double
parse [num] []       =  num -- finished parse
parse [] []          = error "[???????] Syntax error!!\n" -- maybe not be happned.
parse (num:nums) []  = error "[Operand] Syntax error!!\n"
--parse (num:nums) []  = num 
parse stack (x:xs)   = parse (operand x stack) xs
 
-- (L) left side value  : older data on stack
-- (R) right side value : newer data on stack

-- "+-*/"
operand :: String -> Stack -> Stack
operand "+" (right:left:stack) = ( left + right ) :stack
operand "+" _ 		       = error "Operand[+] Syntax Error!!"

operand "-" (right:left:stack) = ( left - right ) :stack
operand "-" _ 		       = error "Operand[-] Syntax Error!!"

operand "*" (right:left:stack) = ( left * right ) :stack
operand "*" _ 		       = error "Operand[*] Syntax Error!!"

operand "/" (right:left:stack) = ( left / right ) :stack
operand "/" _                  = error "Operand[/] Syntax Error!!"

operand numString  stack = read numString : stack