有可能有一个数据类型只有另一个数据类型的一个构造函数吗？

Question

我有一个（相当复杂的）数据类型：

data SomeDataType = Constructor Blah Blah Blah | OtherConstructor Blah Yadda | YetAnotherConstructor Yadda Yadda Tittle | StillOneMoreConstructor Tittle Tattle Wish Wash

现在我发现自己需要另一个数据类型…有两个构造函数。一个与someDataType的yetanotherConstructor相同；另一个只存储一个double。我有什么选择？

data WantedDataType = ConstructorName1 Double | ConstructorName2 SomeDataType

虽然这会起作用，但它也允许类似ConstructorName2$StilloneMoreConstructorTittle tattle wish Wash这样的东西，这是没有意义的。

data WantedDataType = ConstructorName1 Double | ConstructorName2 Yadda Yadda Tittle

data WantedDataType = ConstructorName1 Double | YetAnotherConstructor Yadda Yadda Tittle

Answer 1

这使我认为YetanotherConstructor实际上“假定”是它自己的数据类型：

data YetAnotherData = YetAnotherConstructor Yadda Yadda Tittle
data SomeDataType   = Constructor Blah Blah Blah
                    | OtherConstructor Blah Yadda
                    | SomeYetAnotherConstructor {-!-}YetAnotherData
                    -- ! will make this EXACTLY isomorphic to the original 
                    -- but is likely unnecessary 
                    | StillOneMoreConstructor Tittle Tattle Wish Wash
data WantedDataType = ConstructorName1 Double
                    | ConstructorName2 {-!-}YetAnotherData

如果说someyeTanotherConstructor(yetanotherConstructor___)和ConstructorName2(yetanotherData___)让您很恼火，那么有一个扩展（不过我想您会认为它会让您回到起点）：

{-# LANGUAGE PatternSynonyms #-}

pattern SomeYetAnother :: Yadda -> Yadda -> Tittle -> SomeDataType
pattern SomeYetAnother x y z = SomeYetAnotherConstructor (YetAnotherConstructor x y z)
{-# COMPLETE Constructor, OtherConstructor, SomeYetAnother, StillOneMoreConstructor #-}

pattern WantedYetAnother :: Yadda -> Yadda -> Tittle -> WantedDataType
pattern WantedYetAnother x y z = ConstructorName2 (YetAnotherConstructor x y z)
{-# COMPLETE ConstructorName1, WantedYetAnother #-}

这将使someyetother和wantedyetother像数据构造器一样工作（完成覆盖检查(completpragmas)、模式匹配和构造）。当您不关心YetanotherData是每个类型自己的单元时，您可以使用它们对每个类型进行构造/匹配；如果您希望将YetanotherConstructor视为一个单元，则可以使用基础的SomeYetanotherConstructor和ConstructorName2构造函数。后者可用于例如。

someToWantedByYet :: SomeDataType -> Maybe WantedDataType
someToWantedByYet (SomeYetAnotherConstructor y) = Just $ ConstructorName2 y
someToWantedByYet _ = Nothing
wantedToSomeByYet :: WantedDataType -> Maybe SomeDataType
wantedToSomeByYet (ConstructorName2 y) = Just $ SomeYetAnotherConstructor y
wantedToSomeByYet _ = Nothing