Add `lengthable?`

[primo-ppcg]

I've proposed this previously, and determined that it wasn't necessary, but it seems that it is.

JANET_TFLAG_LENGTHABLE is used within the capi to efficiently determine if a given Janet is a bytes, indexed, or dictionary - in other words, a vaild target for length. Many functions in boot.janet can be implemented more efficiently if the argument type is known to have a fixed length, primarily by creating an array of the correct length, rather than iteratively pushing to an empty array. I think it would be a good idea to expose lengthable? through the corelib.

I don't know that lengthable? is the best name, but it is certainly accurate and unambiguous.

What I'm trying to avoid with all of this is maintaining a user library which re-implements core functions more efficiently. Contributing faster implementations seems like a better idea to me.

[sogaiu]

Exposing doesn't seem bad also from a symmetry perspective with bytes?, indexed?, and dictionary?. Though there doesn't seem to be a callable? (may be there isn't yet a need?).

I tried but failed to think of a better name. If going forward with this, I am in favor of lengthable?. I think it's better than sizable?, has-length?, or i-can-has-length?.

[pepe]

The name is OK, but I am not a native speaker, so :-D.

I would like to see callable? here: https://git.sr.ht/~pepe/gp/tree/master/item/gp/data/navigation.janet#L26 :-).

And 100% for the last sentence of the original post.

[sogaiu]

Regarding callable?, it looks like the definition might end up not being exactly equivalent to the type being :function or :cfunction?

#define JANET_TFLAG_CALLABLE (JANET_TFLAG_FUNCTION | JANET_TFLAG_CFUNCTION | \
        JANET_TFLAG_LENGTHABLE | JANET_TFLAG_ABSTRACT)

Does that work for:

                 callable (or (= :function typ)
                              (= :cfunction typ))]]

?

[pepe]

Oh my bad. Thank you @sogaiu.

[pepe]

But it could be more correct in that place. I will think about it.