Lisp インタープリタ勉強で学んだことを試した。

Myistp.hs

module Mylisp where

import Data.HashMap.Strict  as HM hiding (map)
import Data.Maybe
import Data.List as L hiding (insert)

data Value = Func ([Value] -> Value) |
            List [Value] |
            Symbol String |
            Number Int

instance Show Value where
  show (Func x) = "#fn"
  show (List xs) = "(" ++ intercalate " " (map show xs) ++ ")"
  show (Symbol x) = x
  show (Number x) = show x

type Env = HashMap String Value

-- |
-- >>> putStrLn . show $ List [Symbol "+", Number 1, Number 2, Number 3]
-- (+ 1 2 3)
-- >>> evaluate global $ List [Symbol "+", Number 1, Number 2, Number 3]
-- 6
-- >>> eval $ List [Symbol "+", Number 1, Number 2, Number 3]
-- 6
evaluate :: Env -> Value -> Value
evaluate _ (Number x) = Number x
evaluate env (Symbol x) = fromMaybe nil $ HM.lookup x env
evaluate _ (List []) = nil
evaluate env (List (x:xs)) = case x of
  Symbol x' -> case fromMaybe nil $ HM.lookup x' env of
    x''@(Func _) -> apply env x'' xs
    _ -> nil
  _ -> nil

eval = evaluate global

apply :: Env -> Value -> [Value] -> Value
apply env (Func x) = x . map (evaluate env)

global = insert "+" plus $
         HM.empty
  where
    number (Number x) = x
    car (x:_) = x
    cdr (_:x) = x
    list xs = List xs

plus = Func plus'
  where
    number (Number x) = x
    plus' = Number . sum . map number

nil = List []
n1 = Number 1
n2 = Number 2
n3 = Number 3
sPlus = Symbol "+"
-- >>> eval $ List [sPlus, n1, n2, n3]
-- 6