Call by name?

Main.hs

module Main where
import Data.Maybe (fromJust)

data Expr
  = Var String
  | Lam String Expr
  | App Expr Expr
  | Lit Int
  | Prim PrimOp Expr Expr
  | Let (String, Expr) Expr
  deriving (Show)

data Value
  = VInt Int
  | VClosure String Expr Env
  deriving (Show)

data PrimOp = Add | Mul deriving (Show)

type Env = [(String, Expr)]

eval :: Env -> Expr -> Value
eval env term = case term of
  Var n -> eval env $ fromJust $ lookup n env
  Lam n a -> VClosure n a env
  App a b ->
    let VClosure n c env' = eval env a in
    eval ((n, b) : env') c

  Lit n -> VInt n
  Prim p a b -> (evalPrim p) (eval env a) (eval env b)
  Let (n, e) e' -> eval ((n, e) : env) e'

evalPrim :: PrimOp -> Value -> Value -> Value
evalPrim Add (VInt a) (VInt b) = VInt (a + b)
evalPrim Mul (VInt a) (VInt b) = VInt (a + b)

emptyEnv :: Env
emptyEnv = []