Self type and pinned spiral

It is amazing how different ideas can all of the sudden click and create one coherent blueprint for a cool feature. Yes, I know, I should be doing generator — but this is exactly where the problem began.

Here is how it went — I was implementing rich `Iterable` interface and stumbled on `concat` method. On the first glance it is obvious — it takes another `Iterable` and also returns `Iterable`, no problem. But think about using it — when you have some arbitrary `Iterable` it works as expected, but if you have for example `String`… Do you really expect to concatenate two strings and get some `Iterable` in result?

OK, so let’s make a correction ¹ — the outcome should be `Self`. It will work perfectly with `String`, we can also define `prepend` and `append`. But… wait a second, `Set` is `Iterable` as well, right? How can you prepend anything to a set and get a set back? If you don’t see a problem — you can add something to set, but not concatenate sets, or append anything to it, because except for one special case, set is not a sequence (there is no order in a set).

I found a solution, it waits right now for implementation (I needed a break from coding, thus this post) — `Set` is not an `Iterable`, it can be treated as one (thanks to implicit conversion) — subtle difference, but making things consistent. You can call `append` on `String` and get back a `String` and at the same time you can `append` on `Set` and get an `Iterable`, because it will be silently converted to this type on call.

Implicit conversions were easy — they are just a parameterless methods overloaded on result type. In Skila with “have to read the result of the function” feature it simply fitted right in.

One thing was still missing — how can you ensure that regular method returns `Self` type? You can track whether the returned value is really pure `Self`, but sometimes you don’t work with `Self` type in the first place. The only alternative would be relying on the fact that every descendant type will override such method and produce its own type, making effectively given method a factory of `Self`. And this concept was already present in Skila — `pinned` regular methods waited long for such cause, but now it is done. So either you write one general method with `Self` or you will guarantee with `pinned` every type will have its own implementation.


¹ If I am not mistaken, Scala solves similar kind of problems with what-can-build-what types. This part seems too convoluted for me so I even didn’t dig deeper.

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