[Feature Request] Rename the Sized trait (perhaps to Countable)

[nmsmith]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Mojo currently has a trait named Sized. This denotes a type that has the __len__ dunder method. I am proposing that we rename this trait.

What is your motivation for this change?

The name "Sized" seems problematic for a few reasons:

• As discussed in [BUG] The field storing the number of items in a List or Dict should be named "length", not "size" #3511, Python has standardized on the word "length" for referring to the number of elements in a collection. We should probably avoid conflating "length" with the term "size" which is commonly used to refer to the memory consumption of a data type, e.g. the sizeof operator in C++.
• Rust already has a Sized trait, and it denotes a type whose memory consumption is known at compile-time. Rust devs would be quite confused when encountering "Sized" in Mojo.

Unfortunately there isn't really an English adjective that means "has a length", or more accurately "its number of elements can be queried". The most accurate English adjective is probably Countable, so that would be my top recommendation. (len() probably should have been called count(), but that ship sailed decades ago.)

Here's a list of plausible trait names:

• Countable
• HasLength
• Lengthable
• Len (this is probably the least surprising trait name)

More generally though, I'm wondering if Mojo actually needs a trait specifically for the __len__ dunder method. A function that takes a parameter T bounded by Sized seems pretty useless, because you can't actually do anything useful with those instances of T. Perhaps the trait hierarchy should be structured a bit differently? Should the minimal trait be something like Container or Iterable?

The answer is probably "we need it because that's what len() needs". Which is a bit circular, but I suppose we might have to live with that.

Any other details?

No response

[martinvuyk]

+1 on renaming, though I'd rather go for HasLength or HasLen than the others since it aligns more with the discussion in issue #2924

[nmsmith]

Hmm... what if the trait was itself named __len__? And we could have an __int__ trait etc. One for every dunder method.