7stud
Updating a map's values that are lists
What is the Elixir equivalent to Ruby’s Hash.new where you can specify a block that returns an empty list when you access a non-existent key (or Perl’s autovivification feature)?
The idea is that when you access a key in a Map that doesn’t exist you get back an empty list, which you can then append values to. And, if the key does exist, then you get back a non-empty list, which you can also append values to.
Also, why is it that the editor for this forum has a textbox that says “choose optional tags for this topic”, yet when I submit my question, I get an alert that says “You must choose at least 1 tag”, yet none of the tags are relevant to my question?
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First Post!
OvermindDL1
In functional languages that kind of access is ‘inverted’, you specify the default arguments on the calls instead of the object, thus:
Which of course you can wrap up as new calls in your own custom module.
Or do something generic like make a
DefaultMapmodule that delegates to the normalMapmodule but sets an override default key in the map or something.Lots of options.
A relevant tag would just be ‘elixir’ or so, but adding tags is a good way to be able to search for topics in the future.
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sribe
Elixir is not Perl. The phrase “if you treat a value returned by a key as a list” makes no sense in the Elixir context–you’re basically saying “if you expect of type XXX and you get nil, then automagically create an XXX and substitute it”, and that is exactly the kind of implicit magic that Elixir tries to avoid, by design.
AstonJ
We generally don’t use ‘elixir’ as a tag since pretty much everything is related to Elixir on this forum
(and so I’ve made it a staff only tag)
I’ve added maps and lists as tags as they seem most relevant
sribe
Don’t confuse syntactic sugar and implicit magic–the examples you post are all completely explicit.
Last Post!
Ryzey
The definition for Map.update is
def update(map, key, initial, fun) when is_function(fun, 1)wherefunis a 1 arity function that receives the existing value.I don’t think it would be a breaking change to add this definition:
def update(map, key, initial, fun) when is_function(fun, 2)wherefunis a 2 arity function that receives the existing value and theinitialvalue. Byinitialvalue, I mean the value that the user supplied as the 3rd argument.The second definition is what I expected the functionality of update to be and what I would prefer (and I think this is what @7stud and @OvermindDL1 would prefer). Importantly, there would be no impact for anyone who did not want to use the
initialvalue infun. They could simply continue to provide a 1 arity function asfun.As @overmindDL1 points out, there are options to work around this limitation, but I agree with @7stud that this is an oversight.