Does Pharo have immutable data structures such as maps and sets in its standard library?

Question

I might have just missed them, but I can't seem to find any mention of immutable data structures in Pharo. Coming from functional languages, I've found the immutable map and set useful on various occasions. Even though Pharo has a particular bias towards using mutation, I'd be surprised if nobody got around to implementing them yet.

Answer 1

The code at http://source.lukas-renggli.ch/container/ implements a modern container and iterator library; with mutable and immutable lists; unmodifiable views; and sorted, ordered and unordered set and map data structures. It also supports efficient lazy iteration over all containers using common filtering, mapping, flattening, partitioning, ... operations.

I am not claiming the library has a perfect design or is more performant than the standard collection library, but it is certainly a good starting point for further exploration.

Answer 2

It is completely possible that someone implemented something like that. And maybe there will be immutable collections as a part of the main library in the future. However, for now, there is nothing like that and it is for a very simple reason: what for? When I started to learn Pharo I was fascinated by the null-propagation idea of Objective-C (if you have null, and you send a message to a null you get null back, etc...) So the first thing that I did was to implement null propagation in Pharo. It was fun, it was educational, and it was completely useless. It was useless because no one uses Pharo in that way, it was a wrong approach for that context. I strongly encourage you to make your own immutable collections in Pharo.

But while you do this, think about what should be immutable and why. Is it about shrinking or growing a collection? Arrays are like that — they are fixed size. Is it about not being able to add/remove/swap elements? But what if you get an element and modify it? Finally, consider this example:

array := #('a' 'b' 'c').

array first become: 'd'.

array = #('d' 'b' 'c')

I don't use any setters and still, I can end up with a different array in the end.

Pharo community cares about transparency and good design. It is known that you shouldn't modify contents of collections directly, you shouldn't interact with the internal state of the objects from outside, etc… On the other hand, no one will punch you in the face if you want to do that. I mean, what if you prototype? what if you hack? what if there is literally no other way? You are always able to choose, the question is how can we help people to learn about better choices.

P.S. My answer may sound like immutability is not important. It's not the case. There were even prototypes of read-only objects that can be used to ensure a certain degree of security. It's not that simple to come up with a single concept that will work for everything though