解析错误并且没有关于错误的线索

Question

我不知道下面的代码有什么问题。 这段代码的目的是给我一个列表中字符索引的列表。

示例：elemIndices 3 [1,2,3,4,3,2,3,4,5]表示[2,4,6]

elemIndices' :: Eq a => a -> [a] -> [Int]
elemIndices' x [] = []
elemIndices' x l = reverse (listaind x l)

listaind :: Eq a => a -> [a] ->[Int]
listaind x [] = []
listaind x (y:ys) = if x == y then (length ys) : listaind x ys

Answer 1

您的if遗漏了相应的else。

Answer 2

我会这样做，一旦你对递归感到满意，试着把它做成折叠

elemIndices :: Eq a => a -> [a] -> [Int]
elemIndices = elemIndices' 0

elemIndices' :: Int -> a -> [a] -> [Int]
elemIndices' _ _ [] = []
elemIndices' idx s (x:xs) =
    if s == x
    then idx : elemIndices' (idx + 1) s xs
    else elemIndices' (idx + 1) s xs