英文:
Non-exhaustive patterns in function buildTree' Haskell
问题
我使用返回元组的函数。但当我尝试运行该函数时，它抛出一个异常：函数中存在非穷尽模式。
buildTree' :: String -> Tree -> (String, Tree)
buildTree' (x:xs) currenttree
| null (x:xs) = ("empty", currenttree)
| isDigit x && (take 1 xs == ['+'] || take 1 xs == ['-']) = buildTree' xs (Node [x] Empty Empty)
| isDigit x = buildTree' newstring1 (snd (buildrecursion (getminiexpr (x:xs)) Empty))
| elem x "+-" = buildTree' newstring (buildTree2 currenttree newtree [x])
where newtree = snd (buildrecursion (getminiexpr xs) Empty)
newstring = drop (length (getminiexpr xs)) xs
newstring1 = drop (length (getminiexpr (x:xs))) (x:xs)
getminiexpr :: String -> String
getminiexpr input = takeWhile (\y -> y /= '+' && y /= '-') input
英文:
I use function that returns tuples. But when I try to run the function it gives me an exception: Non-exhaustive patterns in function.
buildTree' :: String -> Tree -> (String,Tree)
buildTree' (x:xs) currenttree
|null (x:xs) = ("empty", currenttree)
|isDigit x && ( take 1 xs == ['+'] || take 1 xs == ['-'])= buildTree' xs (Node [x] Empty Empty)
|isDigit x = buildTree' newstring1 (snd (buildrecursion (getminiexpr(x:xs)) Empty))
|elem x "+-" = buildTree' newstring (buildTree2 currenttree newtree [x])
where newtree = (snd (buildrecursion (getminiexpr xs) Empty))
newstring = drop (length(getminiexpr xs)) xs
newstring1 = drop (length(getminiexpr (x:xs))) (x:xs)
getminiexpr :: String -> String
getminiexpr input = takeWhile ( \y -> y /= '+' && y /= '-') input
答案1
得分: 3
(x:xs) 不是一个任意的列表/字符串，它是一个非空的列表/字符串，其头部是 x，尾部是 xs。因此
buildTree' (x:xs) currenttree
...
只处理 非空 列表。使用 -Wall 编译会警告缺少空列表的情况。因此，你需要：
buildTree' [] currenttree = ...
buildTree' (x:xs) currenttree
...
根据你的代码，我们可以移除 null 守卫：
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x:xs) currenttree
| isDigit x && (take 1 xs == ['+'] || take 1 xs == ['-']) = buildTree' xs (Node [x] Empty Empty)
...
类似地，take 检查需要不仅仅是列表的头部 x。你可以改写为：
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x1:x2:xs) currenttree
| isDigit x1 && (x2 == '+' || x2 == '-') = buildTree' (x2:xs) (Node [x1] Empty Empty)
...
甚至可以这样写：
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x1:x2:xs) currenttree
| isDigit x1 && (x2 `elem` "+-") = buildTree' (x2:xs) (Node [x1] Empty Empty)
...
我无法确定你的逻辑是否正确。
英文:
(x:xs) is not an arbitrary list/string, it is a non empty list/string whose head is x and whose tail is xs. Hence
buildTree' (x:xs) currenttree
...
only handles non empty lists. Compiling with -Wall would warn that the empty list case is missing. So, you need:
buildTree' [] currenttree = ...
buildTree' (x:xs) currenttree
...
Adapting your code, we can remove the null guard:
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x:xs) currenttree
| isDigit x && ( take 1 xs == ['+'] || take 1 xs == ['-'])= buildTree' xs (Node [x] Empty Empty)
...
Similarly, the take checks need more than the head x of the list. You can write, instead:
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x1:x2:xs) currenttree
| isDigit x1 && ( x2 == '+' || x2 == '-') = buildTree' (x2:xs) (Node [x1] Empty Empty)
...
or even
buildTree' [] currenttree = ("empty", currenttree)
buildTree' (x1:x2:xs) currenttree
| isDigit x1 && (x2 `elem` "+-") = buildTree' (x2:xs) (Node [x1] Empty Empty)
...
I have no idea about whether your logic is correct.
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