快速檢查發電機 - 自定義類型

Question

我試圖生成任意大小的元素爲我的自定義數據類型的任意元素：

newtype ZippList a = ZPL ([a], [a]) 
    deriving (Show) 

這是我得到：

instance Arbitrary a => Arbitrary (ZippList a) where 
arbitrary = sized zipplist where 
    zipplist n = do 
     firstLength <- choose(0,n) 
     secondLength <- n - firstLength 
     firstList <- [arbitrary :: Gen a | [1..firstLength]] 
     secondList <- [arbitrary :: Gen a | [1..secondLength]] 
     return $ ZPL (firstList, secondList) 

但是它並不能編譯。該編譯失敗了生成a的兩個列表。我怎樣才能生成一個任意的？

該死的，我有點忘了實際產生的值與_ <- ..對不起，一個微不足道的問題，我是編碼的時間很晚。

Answer 1

這對我的作品

instance Arbitrary a => Arbitrary (ZippList a) where 
    arbitrary = sized zipplist where 
    zipplist n = do 
     firstLength <- choose (0, n) 
     let secondLength = n - firstLength 
     firstList <- sequence [arbitrary | _ <- [1..firstLength]] 
     secondList <- sequence [arbitrary | _ <- [1..secondLength]] 
     return $ ZPL (firstList, secondList) 

注意secondLength的定義不使用單子，所以你應該使用let