hw2: init

hw2/hs/WhileInterp.hs

@@ -0,0 +1,80 @@
+{-
+  Name: <Your name here>
+  Class: CS 252
+  Assigment: HW2
+  Date: <Date assignment is due>
+  Description: <Describe the program and what it does>
+-}
+
+
+module WhileInterp (
+  Expression(..),
+  Binop(..),
+  Value(..),
+  testProgram,
+  run
+) where
+
+import Data.Map (Map)
+import qualified Data.Map as Map
+
+-- We represent variables as strings.
+type Variable = String
+
+-- The store is an associative map from variables to values.
+-- (The store roughly corresponds with the heap in a language like Java).
+type Store = Map Variable Value
+
+data Expression =
+    Var Variable                            -- x
+  | Val Value                               -- v
+  | Assign Variable Expression              -- x := e
+  | Sequence Expression Expression          -- e1; e2
+  | Op Binop Expression Expression
+  | If Expression Expression Expression     -- if e1 then e2 else e3
+  | While Expression Expression             -- while (e1) e2
+  deriving (Show)
+
+data Binop =
+    Plus     -- +  :: Int  -> Int  -> Int
+  | Minus    -- -  :: Int  -> Int  -> Int
+  | Times    -- *  :: Int  -> Int  -> Int
+  | Divide   -- /  :: Int  -> Int  -> Int
+  | Gt       -- >  :: Int -> Int -> Bool
+  | Ge       -- >= :: Int -> Int -> Bool
+  | Lt       -- <  :: Int -> Int -> Bool
+  | Le       -- <= :: Int -> Int -> Bool
+  deriving (Show)
+
+data Value =
+    IntVal Int
+  | BoolVal Bool
+  deriving (Show)
+
+
+-- This function will be useful for defining binary operations.
+-- The first case is done for you.
+-- Be sure to explicitly check for a divide by 0 and throw an error.
+applyOp :: Binop -> Value -> Value -> Value
+applyOp Plus (IntVal i) (IntVal j) = IntVal $ i + j
+applyOp _ _ _ = error "TBD"
+
+
+-- Implement this function according to the specified semantics
+evaluate :: Expression -> Store -> (Value, Store)
+evaluate (Op o e1 e2) s =
+  let (v1,s1) = evaluate e1 s
+      (v2,s') = evaluate e2 s1
+  in (applyOp o v1 v2, s')
+evaluate _ _ = error "TBD"
+
+
+-- Evaluates a program with an initially empty state
+run :: Expression -> (Value, Store)
+run prog = evaluate prog Map.empty
+
+-- The same as run, but only returns the Store
+testProgram :: Expression -> Store
+testProgram prog = snd $ run prog
+
+