Mention `Val` and `Ref`

[soldasim]

I've mentioned both the Val trick for dispatch and Ref trick for broadcasts a few times, but from talking with the students during the project presentations I see, that noone remembers them. This is a shame as I consider these structures to be essential for efficient use of dispatch and broadcast.

I would mention them in the scripts and pay more attention to them.

[VaclavMacha]

Ref is mentioned several times in the scripts. For example, in the General Programming section. However, it definitely makes sense to highlight it more.

Val is probably never mentioned in scripts. It's a useful technique, but it can easily degrade code performance if used too often. To me, this is an advanced topic because you should discuss that using Val for dispatch is generally resolved at runtime, not at compile time. There are also other techniques that can be used to dispatch on value. For example, you can use parameter types in a similar way as used in Arrays for the dimension (Array{T, N}, where N is and integer). But both these things are quite complicated, at least in my opinion.

[soldasim]

True about the Ref. My bad for not putting enough emphasis on it I guess. :D

Regarding the Val trick; I wanted to show the students an alternative to large if-else blocks with many branches based on some runtime value, simulating switch statements from other languages, which I've seen in multiple projects already.

Using the Val trick provides the benefits of the dispatch (code readability & extendability) at the cost of performance. Unfortunately, in such cases there is nothing as fast as the huge ugly if-else blocks (correct me if I'm wrong). It's true that to discuss this extendability-performance trade-off, one would have to talk about the difference between runtime and compilation-time dispatch and that would be too confusing. Maybe it's better to not show Val at all, idk.

[VaclavMacha]

I hadn't thought about this use case, but using the Val trick as an alternative for large if-else blocks makes sense. An if-else block would probably be faster, but I would guess that the difference wouldn't be big. So I think that would be a really nice example for lectures, with a small note that it's probably better to avoid such a trick in a performance-critical part of the code :)