main.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) =
  case lookup x env 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]) =
  case (eval env pred) 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 xs = return $ Str "hello"

Main.hs:48:5:
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 the expression:
case (eval env pred) of {
Bool True -> eval env consq
Bool False -> eval env alt }

Main.hs:55:12:
Couldn't match expected type ErrorOr Expr' with actual typeExpr'
Expected type: [Expr] -> ErrorOr Expr
Actual type: [Expr] -> Expr
In the expression: return $ Str "hello"
In an equation for `apply': apply xs = return $ Str "hello"
Failed, modules loaded: none.