Haskell 让/在哪里以及是否缩进

Question

我有一个函数：

isSimpleNumber :: Int -> Bool
isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x]
                       filterLength = length ( filter (\z -> z == 0) deriveList
                       ....

在filterLength之后我想检查多少filterLength，我尝试：

isSimpleNumber :: Int -> Bool
isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x]
                       filterLength = length ( filter (\z -> z == 0) deriveList
                       in if filterLength == 2
                          then true

我收到错误：

    parse error (possibly incorrect indentation)
Failed, modules loaded: none.

如何使用if和in正确放置缩进？

谢谢。

Answer 1

这将编译：

isSimpleNumber :: Int -> Bool
isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x]
                       filterLength = length ( filter (\z -> z == 0) deriveList )
                   in filterLength == 2

main = print $ isSimpleNumber 5

“deriveList”之后缺少结束符“)”。您也不需要 if-then-true 表达式。

Answer 2

if 始终需要 then 和 else 分支，因此您可能需要 if filterLength == 2 then true else false< /code>，相当于filterLength == 2。