Please note currently Skila has only reference types (not that it is well thought over decision, I simply don’t have enough time to deal with value types).
The problem how to return data from the function efficiently while fitting in special “no result” value, haunts me for some time. I am aware of several approaches:
- magic values, like
-1 for indices (it is still used, see Go),
Nullable<T> type to mark there is no result (you can see this in Kotlin),
Option<T> type for wrapping the actual value (Swift, Scala approach),
- Try-Parse pattern (for sure it is used in C# libraries).
The first approach is dead simple and fast, yet it is disaster when it comes to serious programming — each time you have to check what magic value you can get. The second approach is much better but it does not scale very well — you can use it for a substring lookup, but it fails with dictionary getter or such functions as
first on collection. Yet, the performance is on par with magic values.
For me the only realistic answers are the last two — with
Option<T> type you can work with any function, you just add an extra layer of
Option<T> to the working type. The performance suffers but you always have fast counterpart functions
tryDoSomething. Of course some poor soul has to write all those pairs of functions now — because of that I was seriously considering supporting two types at the same time, lightweight
Nullable<T> and rich
Oh yes, there is another approach:
- communicate failure (like in Icon),
I have no idea what the performance is, but the Icon code is simply beautiful. It was about time to read a book about it — I barely even opened The Implementation of the Icon Programming Language by Ralph E. Griswold and Madge T. Griswold when I found this inspiring passage:
(…) the design of programming languages should not be overly inhibited by perceived implementation problems, since new implementation techniques often can be devised to solve such problems effectively and efficiently.
I am sold — I want beauty, and I want performance! Nothing less. I want stackable (nested)
Option<T> type, easy to use with speed of raw
null values. Until now I was focusing on optimizing nested
Option<T> type, but maybe I could somehow add a stack to
null… wait a second.
Let’s consider what happens on the first nesting level (think of
Option<T>) — we have either the actual value or a
null. On the second level (
Option<Option<T>>) — the actual value or a
null again. At both levels when we have the actual value we can recognize that we didn’t end up with no result because our reference is not equal to
null. It is the
null which has to be wrapped (because
null from the first level of nesting plays a role of true data on the second level), the real, actual value does not need any wrapping. It is some progress but we still have to do a little wrapping, right? No, we don’t — just turn the microscope on and take a look. In the first case the failure is indicated by
null¹, in the second case by another, different
Click, click, click — do you hear this sound?
Option<T> type. Who said it is a regular type in the first place? It does not have to be — our
Option<Option<T>> is a disjoint union of types. It is
Null² type, or
Null¹ type, or
T type —
Because we have to tell compiler what type we would like to get, it knows at what level it operates — i.e. how many layers it should pretend unwrapping to get the value. If it is any of the
null value cases, it will also know how long it should pretend it has real data — the show with single
null keyword is just for the user. Say the pointer is set to
null¹ and our current type is
Option<Option<String>> — do we have real value? Sure thing, it is not
null² and that’s all we have to care about.
Thanks to all those lies there is no wrapping values in the runtime, the speed is the same as working with plain old
null values. The only difference comparing to rich
Option<T> school is with down casting — we will be able to tell what type we hold in hand (
String for example), but we will not be able to deduce what union of types it comes from (
Null¹|String or maybe
Could it be this cookie is absolutely for free? Unfortunately — no. Union types does not work well with generic types (at least if you want to keep static control over types), but since we have here very specific case of union we can enrich type info with option counter. Whenever there is
Option<T> type used we have to take option counter from type
T and increment it.
This leaves me a syntax to think about and supporting three valued logic to consider — this could add an interesting twist to the language.