cevado
nm - a tool to work with deeply nested data structures
I’ve just publish something I’ve been experimenting with:
Since the discussion about local accumulators I’ve joined some discussions on ways to deal with it properly.
I’ve always argued that this is difficult and yes, it should be difficult, you’re trying to change a deeply nested tree of data and you’re trying to apply mutability adhoc to a immutable data structure. Thinking on ways to handle that more easily I’ve always come to the conclusion that the “simple”(not the best and not the fastest) is just to unnest that tree into a more “linear” structure. That’s where the starting point for nm.
So nm is a erlang library because there is no need to anything special to elixir. It has some limitations, it only consider “nesteable data structures” as lists and maps, so keywordlists, tuples and other data types will not be “unnested”.
This is very early work, i’ve just come with the most simple and basic api that can be useful and somewhat easy to use. I’ve been thinking to improve that api with something close to the Access behavior, but first I gonna improve documentation and provide some examples(I’m planning to do an example in elixir and erlang for doing the problem that Valim shows in the local accumulators repository).
I’d like to receive some feedback and opinions on this type of library if it makes sense and all that. If you could just apply it to your most nasty deeply nested structures and see if it’s useful, i’d be thankful. 
First Post!
cevado
someone drew my attention that on hex there is no link to the repository.
Also I made this example using the library for the problem statement that Valim did in the local accumators thread.
defmodule Traversal do
def traverse(data) do
{:ok, t} = :nm.unnest(data)
{:ok, root} = :nm.get_value(t, :root)
Enum.reduce(root, {t, 1, 1}, &root_traversal/2)
:nm.nest(t)
end
defp root_traversal({:"$nm_ref", id}, {t, c1, c2}) do
{:ok, current} = :nm.get_value(t, id)
{:"$nm_ref", list_id} = current["lessons"]
{:ok, list} = :nm.get_value(t, list_id)
updated = Map.put(current, "position", c1)
:nm.fast_update(t, id, updated)
c2 = if current["reset_lesson_position"], do: 1, else: c2
{_, c2} = Enum.reduce(list, {t, c2}, &lessons_traversal/2)
{t, c1 + 1, c2}
end
defp lessons_traversal({:"$nm_ref", id}, {t, c}) do
{:ok, current} = :nm.get_value(t, id)
updated = Map.put(current, "position", c)
:nm.fast_update(t, id, updated)
{t, c + 1}
end
end
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