skosch
To my knowledge, put_in, Map.update etc. all have the one limitation of not automatically creating intermediate keys when needed (for example, put_in(%{a: %{}}, [:a, :b, :c], 42) will fail). But – despite all the risks – this would be useful to have: for instance, I often use reduce to create a nested, aggregate representation of a list of complex objects (say, counting occurrences by categories and sub-categories), which is a one-liner when you don’t have to worry about intermediate keys (think mkdir -p).
The implementation is simple enough (see deep_update example below, which I’m already using all over my projects). I have a nagging feeling this must already be part of Elixir’s standard library and I just don’t know about it? If not, is there a particular reason it isn’t?
%spec deep_update(map(), [any()], any(), any() -> any()) :: map()
def deep_update(map, [leaf_key], default, update_func) do
updated_leaf = if is_map(map) and Map.has_key?(map, leaf_key) do
update_func.(map[leaf_key])
else
default
end
Map.put(map, leaf_key, updated_leaf)
end
def deep_update(map, [key | key_tail], default, update_func) do
if is_map(map) and Map.has_key?(map, key) do
new_branch = deep_update(map[key], key_tail, default, update_func)
Map.put(map, key, new_branch)
else
new_branch = deep_update(%{}, key_tail, default, update_func)
Map.put(map, key, new_branch)
end
end
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peerreynders
Not entirely sure if this addresses what you are trying to accomplish but have you had a look at
Kernel.put_in/3,Kernel.get_and_update_in/3and the like - and Access behavior?skosch
put_inandget_and_update_inhave the same limitation I described, and I don’t know of any Access functions that do what I’m looking for (do correct me if they exist).peerreynders
So if I understand you correctly you want
to result in
One problem I see is that you are assuming that
:bshould refer to a Map value i.e. that the nested structure is a homogenous Map. The Accessdata[key]syntax is right out of the box also supported by Keyword lists - and any other structure that cares to implement the Access behaviour so there is some ambiguity right there - just because there is a key doesn’t automatically mean it should be a Map.skosch
Yeah, that would explain why Access behaviour doesn’t cover this. I obviously only use this for plain maps-of-maps. Still, it could be part of the
Mapmodule?peerreynders
Nobody is stopping you from creating helpers that you use because that is a simplifying assumption that you want to make in your own code and your own structures.
But in general terms looking at just
[:a, :b, :c]the types of the values referenced by:aandb:can’t be known unless they already exist - with:cthe type is known because you are giving it the value. Map simply manages the keys and the values - it doesn’t want to make any assumptions about the type of values it manages so that it can be generic.skosch
Fair point, thanks @peerreynders
michalmuskala
You can achieve this today using
Access.key/2:If you use it often, you could wrap it in a function to do this automatically for you.
skosch
Ahhh, very cool. I’d seen
Access.keybefore but never made the connection that I could use it like that. Thanks Michał!tjdam
This is really great, does anyone one know why it’s in Kernel and not Map, though?
al2o3cr
put_incan manipulate anything thatAccesssupports, not just maps. For instance: