possum.hs

import Prelude hiding (toInteger)
import Data.List

data Value = StringValue  String
           | IntegerValue Integer
           | NameValue    String
           | FuncValue    [String] AST
           | NilValue
           deriving Show

type AST = [Value]

toString :: Value -> String
toString (StringValue  s) = s
toString (IntegerValue i) = show i
toString (NameValue    s) = '\'' : s
toString (FuncValue  a b) = "(" ++ intercalate ", " a ++ ") -> ..."
toString  NilValue        = "nil"

toInteger :: Value -> Integer
toInteger (StringValue  s) = read s
toInteger (IntegerValue i) = i
toInteger _                = 0

consume :: AST -> IO (Value,AST)

consume (x@(StringValue  _):xs) = return (x,xs)
consume (x@(IntegerValue _):xs) = return (x,xs)

consume (NameValue "print":xs) = do (s,xs') <- consume xs
                                    putStrLn $ toString s
                                    return (NilValue, xs')

consume (NameValue "plus":xs) = do (a,xs')  <- consume xs
                                   (b,xs'') <- consume xs'
                                   return (IntegerValue $ toInteger a + toInteger b, xs'')

consume [] = error "not enough arguments"
consume _  = undefined

eval :: AST -> IO Value

eval ast = do (x,xs') <- consume ast
              if null xs'
                 then return x
                 else eval xs'

main = eval [NameValue "print"
            ,NameValue "plus"
            ,NameValue "plus"
            ,IntegerValue 1
            ,IntegerValue 2
            ,IntegerValue 3] >>= print