[BUG] The field storing the number of items in a List or Dict should be named "length", not "size"

[nmsmith]

Bug description

The field List.size should probably be renamed to List.length, because:

• That's what Python's len() function returns (it returns the # of elements in any collection)
• "length" is used in everyday English when talking about lists

A similar reasoning applies to Dict.size. The expression len(my_dict) obtains the number of elements, so we should probably be referring to the number of elements as the "length" of the dictionary.

Also, the term "size" is more often used to refer to memory consumption, e.g. C++'s sizeof operator returns the number of bytes that an object consumes. It would be good to avoid this conflation.

Steps to reproduce

N/A

System information

No response

[lattner]

+1, this makes sense to me. @JoeLoser wdyt?

[JoeLoser]

+1, this makes sense to me. @JoeLoser wdyt?

+1 to renaming the field names here for consistency (using length instead of size). We should also signal that they are meant to be private as well by prefixing these with an _. It would be bad for users to do something like

var xs = List[String]()
xs.size = 10 # oops

# When we run the destructor for `xs` here, we'd try to free uninitialized memory

Our team can do all this internally and fix up all the uses.

[nmsmith]

Also, if we're renaming the word "size", it might also make sense to rename the Sized trait (that includes the __len__ method) to something different. Notably, in Rust, the Sized trait means something completely different. Unfortunately there isn't really an English adjective that means "has a length", but the name Countable seems very accurate.

(I'll open a different issue for this if desired.)

[JoeLoser]

We definitely had similar thoughts when we introduced the Sized trait since the "able" suffix naming doesn't quite work with the (currently) named Sized, e.g. Lengthable isn't great. Mind opening a separate GH issue for that and we can discuss with the community, please?

[nmsmith]

That said, given Mojo already invented the adjectives Intable and Stringable, maybe Lengthable would be fine. 🤷‍♀️

[soraros]

Let's use _len: it's short, less specific (can be applied to types where a literal "length" doesn't make sense, but still maintains parity with len), and somewhat "ugly", indicating its internal nature.

the name Countable seems very accurate.

+1. Or HasLen.

[nmsmith]

For discussing the Sized trait, I opened #3514.