Distinguish between empty trees and non-existent sub-trees

[samoht]

Change S.find_tree to return an optional value. This allows to not conflate
the meaning of `Empty: it is now only "an empty subtree". And we use None to
denote non-existent sub-trees.

[samoht]

/cc @talex5

[talex5]

Why is Empty not just Node Node.empty?

[samoht]

Because this you can create a tree (starting from an empty tree) without having to tie it to a store.

[samoht]

(e.g. Node.empty is the hash of the empty tree, so need to be stored in the store first)

[talex5]

Where is this? ir_node.ml defines:

let empty = of_list []

Seems like any other implementation of NODE must define an empty too. Maybe internally they distinguish between a hash and empty, but why expose it in the tree type?

[samoht]

I've pushed a commit to remove Empty, it's a bit more complex that I would like as it forces Tree.empty to become a function: it's because the all node values have an associated (lazy) hash, and this could be cached if you save the empty value once, delete your DB on disk and use the empty value again. Then you will have a "not found" when trying to look for that hash.

I think ideally we would cache the empty value in the Repo.t value, but currently it is a bit hard to do.

[talex5]

Ah, because when you save a tree the empty node changes from a map to a hash?

But couldn't export_map always export the empty map as P.Node.Val.empty to avoid that?

[samoht]

The empty () stuff is reverted and fixed in #435