Question

我收到以下错误，我假设所有的递归案例，缺少什么？

Prelude> product [] =1
Prelude> product (x:xs) =x * product xs
Prelude> product [1,2,3]
*** Exception: <interactive>:48:1-30: Non-exhaustive patterns in function product

Answer 1

GHCi分别处理每一行，因此您定义了一个函数

Query : select count(*) from tests3;

Vertex failed, vertexName=Map 1, vertexId=vertex_1513582536692_0022_1_00
, diagnostics=[Vertex vertex_1513582536692_0022_1_00 [Map 1] killed/failed due to:ROOT_INPUT_INIT_FAILURE, Vertex Input: tests3 initializer fail
ed, vertex=vertex_1513582536692_0022_1_00 [Map 1], com.amazon.ws.emr.hadoop.fs.shaded.com.amazonaws.services.s3.model.AmazonS3Exception: Bad Req
uest (Service: Amazon S3; Status Code: 400; Error Code: 400 Bad Request; Request ID: DEBA2E241B9DE8C3),

然后通过定义新函数

来遮蔽

mkdir("\\\\.\\C:\\path\\to\\con");

product [] = 1

要解决此问题，您可以将product和product (x:xs) = x * product xs用于多行广告块：

:{

或者，这就是我的建议，将你的函数定义放在一个文件中并加载到GHCi中。

Answer 2

我强烈建议您使用-Wall打开警告。在这里，这样做也会暗示每个定义都是单独考虑的事实。

> :set -Wall
> product [] = 1

<interactive>:2:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘product’: Patterns not matched: (_:_)
> product (x:xs) = x * product xs

<interactive>:3:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘product’: Patterns not matched: []

确实，请注意关于[]的最后警告投诉如何不匹配，表明忽略了第一个定义product []。

haskell非详尽模式缺失

2 个答案: