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鉴于类型类

class Dictionary w where
  insert :: String -> String -> w -> w
  remove :: String -> w -> w
  lookUp :: String -> w ->  String

我写不出来

instance Dictionary [(String,String)] where
  insert key value dic = (key,value) : remove key dic
  remove key dic = filter (\entry -> (fst entry) /= key) dic
  lookUp key [] = "not found"
  lookUp key ((k,v):xs) | k == key = v
                        | otherwise = lookUp key xs 

因为

Illegal instance declaration …

作为练习,我在Haskell中实现了一个'cons'操作,它从任意类型的两个值形成一对.实现所需的数据类型非常简单:

data Nil = Nil deriving (Eq)
data Pair a b = Cons a b deriving (Eq)

car (Cons x _) = x
cdr (Cons _ y) = y

caar = car . car
cdar = cdr . car
cadr = car . cdr
cddr = cdr . cdr

*Main> cddr (Cons 55 (Cons (1,2,3,4) "hello, world!"))
"hello, world!"
*Main> 

但是受到这个主题的启发,我想让结果对打印出来像Scheme列表一样 - 包括臭名昭着的"不正确的列表"(1 2 3 .4).我的实施(见下文)适用于Char的:

*Main> Cons 'a' (Cons 'b' (Cons 'c' Nil))
('a' 'b' 'c')
*Main> …

crypto-api包中有一个Crypto.Random API,用于指定"伪随机数生成器"的含义.

我使用System.Random的RandomGen类的实例,即StdGen实现了这个API:

instance CryptoRandomGen StdGen where
  newGen bs = Right $ mkStdGen $ shift e1 24 + shift e2 16 + shift e3 8 + e4
    where (e1 : e2 : e3 : e4 : _) = Prelude.map fromIntegral $ unpack bs
  genSeedLength = Tagged 4
  genBytes n g = Right $ genBytesHelper n empty g
    where genBytesHelper 0 partial gen = (partial, gen)
          genBytesHelper n partial gen = genBytesHelper (n-1) (partial `snoc` nextitem) newgen
            where (nextitem, newgen) = …