初试Haskell

昨天初次尝试用Haskell编了一个小的程序，用来检测一个列表有没有重复的元素。习惯了命令式编程语言后，尝试这种宣告式编程（Declarative programming） 稍微有点不习惯。

module Main where

multiply :: [a] -> [b] -> [(a, b)]
multiply [] []         = []
multiply _ []          = []
multiply [] _          = []
multiply [x] [y]       = [(x, y)]
multiply [x] (y:ys)    = (x, y) : (multiply [x] ys)
multiply (x:xs) [y]    = (x, y) : (multiply xs [y])
multiply (x:xs) (y:ys) = (x, y) : ((multiply xs [y]) ++ (multiply [x] ys) ++ (multiply xs ys))

equalPair :: Eq a => (a, a) -> Bool
equalPair (a, b) = fst (a, b) == snd (a, b)

makePair :: [a] -> [(a, a)]
makePair []     = []
makePair [x]    = []
makePair [x, y] = [(x, y)]
makePair (x:xs)   = (multiply [x] xs) ++ (makePair xs)

main = all ( (==) False )
           ( map equalPair (makePair [1,2,3,4]) )

这个程序思路很直接，就是先两两配对，然后检查这些对中有无相等的。但这或许不是最快的一个实现。