[Rd] `dendrapply` Enhancements

[Ivan Krylov]

Hello Aidan,

Sorry for dropping this for a while.

В Thu, 2 Mar 2023 21:03:59 +0000
"Lakshman, Aidan H" <AHL27 using pitt.edu> пишет:

> //after
> curnode = eval(lang3(R_Bracket2Symbol, parent->node, DEND_IND), env);

lang3() always constructs a new language object. If you do end up using
eval(), it may make sense to move lang3() out of the loop and reuse the
existing object by referring to the DEND_IND variable using its symbol,
like it's done in the lapply() implementation.

> The problem is, it seems like the returned value from `eval` is not
> protected, whereas the value from VECTOR_ELT is (if the source list
> is protected). My understanding is that this happens because
> VECTOR_ELT just copies the pointer, whereas eval(…) calls R code,
> which returns a copy of the object.

That's right, `[[.dendrogram` returns a new object which is not
protected, unlike the raw elements of node that you're keeping a
protected pointer to.

> This ends up being problematic, since it isn’t feasible to protect
> all the nodes until we’re done with them.

I see. It's not easy in R to unprotect arbitrary previously-allocated
objects in a safe way. Have you considered "precious multi-sets"
<https://blog.r-project.org/2018/12/10/unprotecting-by-value/index.html>?
They have R_ReleaseFromMSet(), making it possible to free arbitrary
objects from the set, and they automatically unprotect everything they
contain on destruction.

> It’s also possible to use eval(…) to get the node, apply the function
> to it, save its class in the linked list, and then save the object
> using VECTOR_ELT. This way we get the benefits of `[[` dispatch,
> class preservation, and constant stack space. However, this ends up
> hurting performance significantly (about 4x slower than the current
> new version, making it around half the speed of the version in stats).

That's a clever solution! Can you profile the code to see if there are
visible sources of slowdown? Maybe this can be salvaged.

-- 
Best regards,
Ivan