构造函数为高阶 function

Question

I'm facing the problem that I want to map from a Stream 4 elements to the contructor of the record Data:

record Data(String firstname, String middlename, String lastname, String zip) {}

Function4<String, String, String, String, Data> constructor = Data::new;
List<Data> data = Stream.of("chris", "p", "bacon", "black")
                .map(constructor)
                .collect(Collectors.toList())

I found a matching higher-order function ( Function4 from vavr) to describe this constructor with 4 parameter at least. But I cant figure out how to use it in this case.

Side note: what I try to avoid is building a Collection of 4 elements and have to pass them "manually".:

    List<Data> datas = rowData.stream()
                .map(row -> new Data(row.get(0), row.get(1), row.get(2), row.get(3))).toList();

I think this can't be done in Java because at some point in the stream I have to construct a type with parameter arity 4.

Answer 1

Your idea is close to Applicative Functor which is already implemented in any monadic library, vavr also implemented it with a limitation: you can have a maximum of 8 arities. To solve your problem you have to follow this step:

• Lift each your param to Monadic world.
• Use Applicative Functor to do the rest

        // Validation is a monad, you lift your param here
        Validation<String,String> firstname = Validation.valid("chris");
        Validation<String,String> middlename = Validation.valid("p");
        Validation<String,String> lastname = Validation.valid("lastname");
        Validation<String,String> zip = Validation.valid("zip");

        // Here is the limitation of vavr, only Validation implement applicative functor so you can only Validation class
        // to combine four field to Data object. Beside you have to handle the error -> all the error is collected in 
        // the left side Seq<String>
        Validation<Seq<String>, Data> result = Validation.combine(firstname, middlename, lastname, zip).ap(Data::new);