Hi, I was looking at the `Either` source code and ...

# arrow

[niltsiar]

Hi, I was looking at the Either source code and out of curiosity I was wondering why functions like map, fold , tap ... are member functions while flatMap, getOrElse and some other are extension functions.

[stojan]

flatMap is an extension because of how generics work in Kotlin, so you can have a different error type after flatMap ... or maybe ti was variance related.... I don't remember exactly

[raulraja]

Hi @niltsiar, to expand on @stojan point:
Either is defined as covariant out in both its type arguments but if we were to define flatMap inside Either:

public inline fun <B, C> flatMap(f: (B) -> Either<A, C>): Either<A, C>

in the lambda f A appears in contravariant in position over the lambda return type. If we move it to an extension then A is no longer coming from the instance but from the environment in invariant position which is acceptable. We could have included flatMap inside the Either data type if we had changed Either’s type arguments to be invariant but this would cascade poorly for inference. for example:

1.right() // inferred to Either<Nothing, Int>

would not unify properly with other expressions that would yield Either<String, Nothing> as compatible sub types.

[niltsiar]

Thanks for sharing the information 👍 👏