lisp.hs

module Main where

import Control.Monad.Error
import Text.ParserCombinators.Parsec

-- Types

data Expr
  = Sym String
  | List [Expr]
  | Num Int
  | Str String
  | Bool Bool
  | Func Env [String] Expr
  -- | Prim ([Expr] -> Expr)
    deriving (Show, Eq)

data Err
  = NotInScope String
  | TypeMismatch String Expr
  | NumArgs Integer 
  | Parser ParseError
  | Default String
    deriving (Show)

instance Error Err

type ErrorOr = Either Err

-- Environment

type Env = [(String, Expr)]

nullEnv = [] :: Env
updateEnv env var exp = (var, exp) : env

-- Evaluation

eval :: Env -> Expr -> ErrorOr Expr
eval env (Sym x) = do
  let result = lookup x env
  case result of
    Just _ -> return $ Sym x
    Nothing -> throwError $ NotInScope x
eval env (List [(Sym "fn"), (List params), (List form)]) =
  return $ Func env (map (\(Sym x) -> x) params) (List form)
eval env (List [(Sym "if"), pred, consq, alt]) = do
  let result = eval env pred
  case result of
    Bool True -> eval env consq
    Bool False -> eval env alt
eval env (List [(Sym "quote"), xs]) = return xs
eval env (List (op:args)) = apply (eval env op) $ map (eval env) args
eval env x = x

apply :: Expr -> [Expr] -> ErrorOr Expr
apply _ = throwError $ Default "hello"

Main.hs:50:5:
Couldn't match expected type Either Err Expr' with actual typeExpr'
In the pattern: Bool True
In a case alternative: Bool True -> eval env consq
In a stmt of a 'do' block:
case result of {
Bool True -> eval env consq
Bool False -> eval env alt }

Main.hs:57:11:
No instance for (MonadError Err ((->) [Expr]))
arising from a use of throwError' Possible fix: add an instance declaration for (MonadError Err ((->) [Expr])) In the expression: throwError In the expression: throwError $ Default "hello" In an equation forapply': apply _ = throwError $ Default "hello"
Failed, modules loaded: none.