Designing function returns on generic programming

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## What does this function return?

Is it coupled in time? Is it synchronous?

YES

    How many results are there?

    NONE

        Unit

    ONE

        Can it fail?

        YES

            Can it fail with a known value like Exception or String?

            YES

                Try<Value> or Either<String, Value>

            NO

                Optional<Value>

        NO

            Value

    A COUPLE BUT I KNOW THE AMOUNT AND THE TYPES

        Can it fail?

        YES

            Can one of them fail and the rest succeed?

            YES

                Can it fail with a known value like Exception or String?

                YES

                    Tuple<Try<Value1>, Value2, Value3> or Tuple<Either<String, Value1>, Value2, Value3>

                NO

                    Tuple<Optional<Value1>, Value2, Value3>

            NO

                Can it fail with a known value like Exception or String?

                YES

                    Try<Tuple<Value1, Value2, Value3>> or Either<String, Tuple<Value1, Value2, Value3>>

                NO

                    Optional<Tuple<Value1, Value2, Value3>>

        NO

            Tuple<Value1, Value2, Value3>


    MANY

        Can it fail?

        YES

            Can one of them fail and the rest succeed?

            YES

                Can it fail with a known value like Exception or String?

                YES

                    Iterable<Try<Value>> or Iterable<Either<String, Value>>

                NO

                    Iterable<Optional<Value>>

            NO

                Can it fail with a known value like Exception or String?

                YES

                    Try<Iterable<Value>> or Either<String, Iterable<Value>>

                NO

                    Optional<Iterable<Value>>

NO

    How many results are there?

    NONE

        Can it fail?

        YES

            Can it fail with a known value like Exception or String?

            YES

                Single<Optional<Exception>> -- absence of exception is success

            NO

                Single<Optional<Unit>> -- absence is failure

        NO

            Completable

    ONE

        Can it fail?

        YES

            Can it fail with a known value like Exception or String?

            YES

                Single<Try<Value>> or Single<Either<String, Value>>

            NO

                Single<Optional<Value>>

        NO

            Single<Value>

    MANY

        Can it fail?

        YES

            Can one of them fail and the rest succeed?

            YES

                Can it fail with a known value like Exception or String?

                YES

                    Observable<Try<Value>> or Observable<Either<String, Value>>

                NO

                    Observable<Optional<Value>>

            NO

                Observable<Value> -- use onErrorResumeNext()

        NO

            Observable<Value>


## How does that Value look like?

How many possible value types or states can it contain? Failure cases like Exceptions or error Strings can also be rolled into the possible states

NONE

    Unfeasable in Java or Void in Kotlin 

ONE

    Value

TWO

    Either<Value1, Value2>

THREE TO NINE

    Union3<Value1, Value2, Value3>
    Union6<Value1, Value2, Value3, Value4, Value5, Value6>
    Union9<Value1, Value2, Value3, Value4, Value5, Value6, Value7, Value8, Value9>

MORE THAN NINE

    Look into Kotlin's sealed unions


## What does Value contain?

Is it?

NOTHING

    Unit

PRIMITIVE OR DATA CLASS

    Int, String, Boolean, User, DogDto...

FUNCTION

    Function<InputValue, OutputValue>

OBSERVABLE, SINGLE, OR COMPLETABLE

    You should be using a nested flatMap instead

ANOTHER CLASS

    You're outside the paradigm so here be dragons


## Silly Types Time

    A sequence of 3 integers that can completely fail without a reason

        Optional<Iterable<Integer>>

    A sequence of 3 groups of names to users that can fail individually with a reason. A reason can be either an Exception or a string

        Iterable<Either<Either<Exception, String>, Tuple<String, User>> -- probably wrong

        Iterable<Union3<Exception, String, Tuple<String, User>>>

    A grouping of a map of string to user, a map of id to list of strings, and a list of ids. Users can be one of admin, luser, or guest. An id can be either a number or a string

        Tuple<Map<String, Union3<Admin, Luser, Guest>, Map<Either<NumberId, StringId>, List<String>>, List<Either<NumberId, StringId>>>

    An asynchronous sequence of N elements, some of which are a pair of an id to a user, a map of user ids to users, or a sequence of groupings of ids to their users. They can individually fail but only some give an Exception

        Observable<Union4<Optional<Exception>, Pair<Either<NumberId, StringId>, Union3<Admin, Luser, Guest>>, Map<Either<NumberId, StringId>, Union3<Admin, Luser, Guest>>, Iterable<Pair<Either<NumberId, StringId>, Union3<Admin, Luser, Guest>>>>> -- this needs type aliases

        typealias Id = Either<NumberId, StringId>

        typealias User = Union3<Admin, Luser, Guest>

        Observable<Union4<Optional<Exception>, Pair<Id, User>, Map<Id, User>, Iterable<Pair<Id, User>>>> -- can do better

        typealias UserInfo = Pair<Id, User>

        Observable<Union4<Optional<Exception>, UserInfo, Map<Id, User>, Iterable<UserInfo>>> -- hmmm

        Observable<Union5<Unit, Exception, UserInfo, Map<Id, User>, Iterable<UserInfo>>>