What is the way to chain methods with different signatures describing a business process?

[Andras-Csanyi]

Hi all,

I am new in the functional programming world, so please be gentle... :D And I believe my Java knowledge is a bit rusty too.

I have the following process I'd like to describe with chained methods returning Either.

The process receives a context object containing a WhateverInput and correlationId and processes these in the following steps:

• validation of the WhateverInput
• mapping the WhateverInput to WhateverEntity
• saving the WhateverEntity in a storage
• mapping the returned WhateverEntity (having Id at this point) to WhateverResult

The correlationId is there to mark which process the operation belongs to if the process fails.

What I tried is map and flatMap, but both of them expects a function where the input is the Either's Right and an Either as a return value. However my method signatures look like below due to that I don't want to put the correlation id into a field.

public class VavrPractice {

    public static void handle(Context context) {

        var result = Either.right(context)
            .map(validateInput(context.getWhateverEntityInput(), context.getCorrelationId())) // shows error
    }

    private static Either<ErrorResult, WhateverEntityInput> validateInput(WhateverEntityInput whateverEntityInput, UUID correlationId) {}

    private static Either<ErrorResult, WhateverEntity> mapToWhateverEntity(WhateverEntityInput whateverEntityInput, UUID correlationId) {}

    private static Either<ErrorResult, WhateverEntity> saveToDatabase(WhateverEntityInput whateverEntityInput, UUID correlationId) {}

    private static Either<ErrorResult, WhateverEntityResult> mapToWhateverEntityResult(WhateverEntity whateverEntity, UUID correlationId) {}
}

How to do this in a nice, best/good practices way? What I can think of is creating a Context object adding to it both the WhateverInput and correlationId and this Context will be the Right side of the Either and moving this object between methods. But, it feels a bit clunky, heavy.

I study functional programming in both Java and C# in the same time and the example above can be done in C# using Bind and I am looking for an equivalent for that or the solution how the Bind, what it seems like map or flatMap, can be done.

Thanks the answers in advance.

[Andras-Csanyi]

I posted this to Stackoverlow.

[EarthCitizen]

@Andras-Csanyi Where you are using .map(), you need to use .flatMap() (bind). .map() is for only transforming what is inside the Either.Right.

Also, .map() and .flatMap() expect a function as a parameter, so you need to do this:

.flatMap(__ -> validateInput(context.getWhateverEntityInput(), context.getCorrelationId()))

I am using __ here because even though .flatMap() will provide context to its function parameter, this is not needed in this case because context is already in scope from the handle() method.

[EarthCitizen]

If you don't want to create a bunch of classes that represent transitional stages between your methods, you could simply use a Tuple, and it can grow and shrink as needed as each stage of your process is completed.