scoreToLetter :: Integer -> Char
scoreToLetter n
  | n > 90 = 'A'
  | n > 80 = 'B'
  | n > 70 = 'C'
  | otherwise = 'D'


len :: [a] -> Integer
len [] = 0
len (_:xs) = 1 + len xs

duplicate :: [a] -> [a]
duplicate []     = []
duplicate (x:xs) = [x, x] ++ duplicate xs

firstn 0 _ = []
firstn n [] = []
firstn n (x:xs) = x : firstn (n-1) xs

ones :: [Integer]
ones = 1 : ones

zipOp :: (a -> b -> c) -> [a] -> [b] -> [c]
zipOp f (a:as) (b:bs) = (f a b) : (zipOp f as bs)
zipOp f [] _ = []
zipOp f _ [] = []

fibs :: [Integer]
fibs = 0:1: zipWith (+) (tail fibs) fibs



-- Interpreter of WAE
type Identifier = String
type Value = Integer

-------
type Env = [(Identifier,Value)]

eempty :: Env
eempty = []

eextend :: Identifier -> Value -> Env -> Env
eextend x v env = (x,v) : env

elookup :: Identifier -> Env -> Value
elookup x [] = error (x ++ " esta libre")
elookup x ((y,v):ys)
  | x == y    = v
  | otherwise = elookup x ys
--------

data Expr = Num Integer
          | Add Expr Expr
          | Sub Expr Expr
          | Id Identifier
          | With Identifier Expr Expr

eval :: Expr -> Env -> Integer
eval (Num n) env      = n
eval (Add e1 e2) env  = eval e1 env + eval e2 env
eval (Sub e1 e2) env  = eval e1 env - eval e2 env
eval (Id x) env       = elookup x env
eval (With x e b) env = eval b (eextend x (eval e env) env)

myex :: Expr
myex = (With "x" (Num 5) (Add (Id "x") (Num 8)))

main :: IO ()
main = return ()