Haskell：使用|

在浏览持久性源代码时，我在下面的文件中遇到了这个函数（我引用了一个标记，其相关代码与master branch中处于当前状态的代码相同，因为该标记的代码不太可能更改）。 在行

takeConstraint ps tableName defs（n:rest）| not（T.null n）&&isUpper（T.head n）=takeConstraint'

中，参数模式后面有一个管道（|）字符。

|

和

=

之间的表达式是否类似于模式中参数的约束？那么，我是否将这个

|

解释为数学中的同一符号，即“如此”

takeConstraint:：PersistSettings
->正文
->[FieldDef]
->[正文]
->（可能是FieldDef，可能是CompositeDef，可能是UniqueDef，可能是UnboundForeignDef）
takeConstraint ps tableName defs（n:rest）| not（T.null n）&&isUpper（T.head n）=takeConstraint'--是的，它的确切意思是“这样”。这些在Haskell中非常常见（通常优先于等效的
if
then
else
表达式）

case
对我来说也更合适。我同意，在防护装置有点误导之后，在两者之间塞满了整个功能块，我认为这是Haskell的一个全新功能。
takeConstraint :: PersistSettings
          -> Text
          -> [FieldDef]
          -> [Text]
          -> (Maybe FieldDef, Maybe CompositeDef, Maybe UniqueDef, Maybe UnboundForeignDef)
takeConstraint ps tableName defs (n:rest) | not (T.null n) && isUpper (T.head n) = takeConstraint' --- <<<<< This line
    where
      takeConstraint' 
            | n == "Unique"  = (Nothing, Nothing, Just $ takeUniq ps tableName defs rest, Nothing)
            | n == "Foreign" = (Nothing, Nothing, Nothing, Just $ takeForeign ps tableName defs rest)
            | n == "Primary" = (Nothing, Just $ takeComposite defs rest, Nothing, Nothing)
            | n == "Id"      = (Just $ takeId ps tableName (n:rest), Nothing, Nothing, Nothing)
            | otherwise      = (Nothing, Nothing, Just $ takeUniq ps "" defs (n:rest), Nothing) -- retain compatibility with original unique constraint
takeConstraint _ _ _ _ = (Nothing, Nothing, Nothing, Nothing)

f x
 | x > 2      = a
 | x < -4     = b
 | otherwise  = x

f x = if x > 2 then a
               else if x < -4 then b
                              else x

    takeConstraint' = case n of
        "Unique"  -> (Nothing, Nothing, Just $ takeUniq ps tableName defs rest, Nothing)
        "Foreign" -> (Nothing, Nothing, Nothing, Just $ takeForeign ps tableName defs rest)
        "Primary" -> (Nothing, Just $ takeComposite defs rest, Nothing, Nothing)
        "Id"      -> (Just $ takeId ps tableName (n:rest), Nothing, Nothing, Nothing)
        _         -> (Nothing, Nothing, Just $ takeUniq ps "" defs (n:rest), Nothing)