故障基本情况哈斯克尔

Question

我的功能：

type Block = [Maybe Int] 

checkBlock' :: Block -> Block 
checkBlock' (r:rs) 
    | r == [] = [] 
    | isJust r == True = r:checkBlock' rs 
    | isNothing r == True = checkBlock' rs 

checkBlock :: Block -> Block 
checkBlock (r:rs) 
    | r == [] = [] 
    | isNothing r == True = r:checkBlock rs 
    | isJust r == True = checkBlock rs 

我想第1功能检查就INTS和空话的列表，只有返回刚才值。而第二个函数只返回Nothings。

他们搜集精而不basecase其中r == [] = []，但它我得到的错误：

Sudoku.hs:104:12: 
    Couldn't match expected type `Maybe Int' with actual type `[t0]' 
    In the second argument of `(==)', namely `[]' 
    In the expression: r == [] 
    In a stmt of a pattern guard for 
        an equation for checkBlock': 
     r == [] 
Failed, modules loaded: none. 

我可以把那里，而不是[]，使这个编译？我没有想法。

Answer 1

你应该使用模式匹配在这里，而不是后卫：

checkBlock :: Block -> Block 
checkBlock [] = [] 
checkBlock (r:rs) 
    | isNothing r == True = r:checkBlock rs 
    | isJust r == True = checkBlock rs 

，或最好：

checkBlock :: Block -> Block 
checkBlock [] = [] 
checkBlock (Nothing : rs) = Nothing : checkBlock rs 
checkBlock (Just _ : rs) = checkBlock rs 

Answer 2

你不能空单比较r，你只能比较完整列表。

作为替代方案，你可以写

checkBlock :: Block -> Block 
checkBlock rs 
    | rs == [] = [] 
    | isNothing (head rs) == True = (head rs):checkBlock (tail rs) 
    | isJust (head rs) == True = checkBlock (tail rs) 

其次，警卫已经使用Bool，所以== True是不必要的。顺便说一下，尝试使用null rs istaed的rs == []

checkBlock :: Block -> Block 
checkBlock rs 
    | null rs    = [] 
    | isNothing (head rs) = (head rs):checkBlock (tail rs) 
    | isJust (head rs)  = checkBlock (tail rs) 

但是，这看起来有点丑，让我们使用模式匹配：

checkBlock :: Block -> Block 
checkBlock [] = [] 
checkBlock (r:rs) 
    | isNothing r = r:checkBlock rs 
    | isJust r  = checkBlock rs