当值是Just

Question

我想在值为Just时设置对象的属性，而在值为Nothing时不设置属性。但是，如果值为Nothing，则返回的对象将变为Nothing。

let person = {name: 'Franz'}
const address = getAddress(response) // getAddress returns Maybe

// person will be Nothing if address is Nothing 
person = S.map(R.assoc('address', R.__, person))(address)

Answer 1

您的Person类型似乎是这样的：

Person = { name :: String, (address :: Address)? }

如果可能的话，我建议避免使用null，undefined和可选的记录字段，因为它们都是bug的来源。我建议这样：

//    person :: Person
const person = {name: 'Franz', address: S.Nothing};

//    address :: Maybe Address
const address = getAddress (response);

//    person$ :: Person
const person$ = R.assoc ('address') (address) (person);

在这种情况下，如果由于某种原因必须有一个可选的address字段，则可以使用S.maybe：

//    person :: Person
const person = {name: 'Franz'}

//    address :: Maybe Address
const address = getAddress (response);

//    person$ :: Person
const person$ = S.maybe (person)
                        (address => R.assoc ('address') (address) (person))
                        (address);

结果将为{name: 'Franz'}或类似{name: 'Franz', address: 'Frankfurter Allee 42'}的值。但是，正如我所提到的，{name: 'Franz', address: S.Nothing}和{name: 'Franz', address: S.Just ('Frankfurter Allee 42')}是这些“人”的首选表示形式。