Eval by need (with named varriables)

Main.hs

module Main where

import Data.Either.Extra (maybeToEither)

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

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

data PrimOp = Add | Mul deriving (Show)

type Env = [(String, Value)]

data EvalError
  = UndefinedVar String
  | Unaplicable Value
  | IncorrectArgs PrimOp Value Value
  deriving (Show)

eval :: Env -> Expr -> Either EvalError Value
eval env term = case term of
  Var n -> maybeToEither (UndefinedVar n) $ lookup n env
  Lam n a -> pure $ VClosure n a env
  App a b -> do
    aRes <- eval env a
    case aRes of
      VClosure n c env' -> do
        bRes <- eval env b
        eval ((n, bRes) : env') c
      x ->
        Left $ Unaplicable x
  Lit n ->
    pure $ VInt n
  Prim p a b -> do
    resA <- eval env a
    resB <- eval env b
    evalPrim p resA resB

evalPrim :: PrimOp -> Value -> Value -> Either EvalError Value
evalPrim Add (VInt a) (VInt b) = pure $ VInt (a + b)
evalPrim Mul (VInt a) (VInt b) = pure $ VInt (a + b)
evalPrim op  x1       x2       = Left $ IncorrectArgs op x1 x2

emptyEnv :: Env
emptyEnv = []