Evaluator.hs

module Main where
import qualified Data.Map as Map

type Env = Map.Map String Expression

data ArithmeticExpression = Add Expression Expression
                          | Multiply Expression Expression
                          deriving Show

data Expression = Constant Integer
                | Variable String
                | ArithmeticExpression ArithmeticExpression
                | Let [(String, Expression)] Expression
                | Lambda String Expression
                | Call String Expression
                | Closure Env String Expression
                deriving Show

evaluateArithmeticExpression :: Env -> ArithmeticExpression -> Either String Expression

evaluateArithmeticExpression env (Add aExpr bExpr) = do
  aValue <- evaluate env aExpr
  bValue <- evaluate env bExpr
  case (aValue, bValue) of
    (Constant aInt, Constant bInt) -> Right $ Constant (aInt + bInt)
    _ -> Left "Add applied to non-integer"

evaluateArithmeticExpression env (Multiply aExpr bExpr) = do
  aValue <- evaluate env aExpr
  bValue <- evaluate env bExpr
  case (aValue, bValue) of
    (Constant aInt, Constant bInt) -> Right $ Constant (aInt * bInt)
    _ -> Left "Multiply applied to non-integer"

evaluateClosure :: Env -> String -> Expression -> Expression -> Either String Expression

evaluateClosure capturedEnv param body argValue = do
  evaluate (Map.insert param argValue capturedEnv) body

evaluate :: Env -> Expression -> Either String Expression

evaluate env constant@(Constant _) = Right constant

evaluate env (Variable name) =
  case Map.lookup name env of
    Just expr -> Right expr
    _ -> Left $ "No such variable " ++ name ++ " in Variable expression"

evaluate env (ArithmeticExpression e) =
  evaluateArithmeticExpression env e

evaluate env (Let bindings body) =
  let
    addBindingsToEnv :: Env -> [(String, Expression)] -> Either String Env
    addBindingsToEnv acc [] = Right acc
    addBindingsToEnv acc (x : xs) = do
      xValue <- evaluate env (snd x)
      addBindingsToEnv (Map.insert (fst x) xValue acc) xs
  in
    do
      letEnv <- addBindingsToEnv env bindings
      evaluate letEnv body

evaluate env (Lambda param body) = Right $ Closure env param body

evaluate env (Call name arg) =
  case Map.lookup name env of
    Just (Closure capturedEnv param body) -> do
      argValue <- evaluate env arg
      evaluateClosure capturedEnv param body argValue
    Just x -> Left $ "Variable " ++ name ++ " is not closure in Call expression"
    _ -> Left $ "No such variable " ++ name ++ " in Call expression"

main :: IO ()
main = do
  let result = evaluate
                (Map.fromList [("x", Constant 5)])
                (Let [
                    ("f", (Lambda "p" (ArithmeticExpression (Add (Variable "p") (Constant 10))))),
                    ("y", (Constant 20))
                  ]
                  (Let [("temp", (Call "f" (Variable "x")))]
                    (ArithmeticExpression (Multiply (Variable "temp") (Variable "y")))))
  putStrLn . show $ result

LICENSE

The MIT License (MIT)

Copyright (c) 2014 Richard Cook

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.