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#!/usr/bin/env runghc
-- run <script> <RPN expression with space-separated tokens>
import Data.Maybe (isJust,fromJust,)
import System.Environment (getArgs,)
import Test.Tasty.Options (safeRead,)
rpnEval :: [Float] -> [(String,Float->Float->Float)] -> [String] -> [Float]
rpnEval stack dict input
|null input =stack
|isJust nxt =rpnEval ((fromJust nxt):stack) dict (tail input)
|isJust op =rpnEval (((fromJust op) s f):rest) dict (tail input)
|otherwise =error "parsing error" where
nxt = safeRead (head input)::Maybe Float
op = lookup (head input) dict
(f:s:rest) = stack
main = do
input <- getArgs
let dict = [ ("+",(+))
, ("-",(-))
, ("*",(*))
, ("/",(/))]
print $ rpnEval [] dict input
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