Int expressions.md

eval.hs

type Name = String
data Expr =  Var Name
            | Lit Int
            | Expr :+: Expr
            | Expr :*: Expr
            deriving (Eq, Show)

eval :: Env -> Expr -> Int
type Env = [(Name, Int)]
eval e (Var s) = lookUp e s
eval e (Lit n) = n
eval e (x :+: y) = eval e x + eval e y
eval e (x :*: y) = eval e x * eval e y

lookUp :: Eq a => [(a,b)] -> a -> b
lookUp xys x = the [ y | (x', y) <- xys, x == x']
    where the [x] = x

main =  do
    print $ eval [("P", 3), ("Q", 4)] ((Var "P" :+: Var "Q") :*: Var "Q") -- 28

Int expressions.md

Int expressions

Define the “evaluate (eval)” function for the following data types of Int expressions.

Type Name = String
data Expr =  Var Name | Lit Int  --constant
					  | Expr :+: Expr
		              | Expr :*: Expr
	                  deriving (Eq, Show)