CODEWITHSUNDEEP
elixirphoenix-framework
George Morris
George Morris

Reputation: 484

Elixir what is the best way to push from anonymous function into an array?

I am wondering, what is the best way to obtain results by the given algorithm? The current solution doesn't help because results are nil. How can I make the variable results accessible in iterate_json without passing it as an argument?

def lookup(json) do
    results = []
    iterate_json(json, fn(x) -> 
        if x["title"] =~ "sunt" do
            results = [x|results]
        end
    end)
    results
end

def iterate_json([head|tail],f) do
    f.(head)
    iterate_json(tail,f)
end

def iterate_json([],f), do: nil

Upvotes: 0

Views: 327

Answers (1)

Aleksei Matiushkin
Aleksei Matiushkin

Reputation: 121000

There are few different issues with this code. The main is one cannot access the parent context from within a closure (even in Ruby that won’t change the results variable:

results = nil
fn(x) -> results = 42 end.()
results
#⇒ nil

warning: variable "results" is unused

Note variables defined inside case, cond, fn, if and similar do not leak. If you want to conditionally override an existing variable "results", you will have to explicitly return the variable. For example:

The above is what iex spits out when one tries to execute this code.

How can I make the variable results accessible in iterate_json without passing it as an argument?

It’s accessible in iterate_json. The value that was set upfront is captured and used within a closure. What you cannot do, is you cannot change it from inside a closure.

What you are trying to achieve, is actually a Enum.reduce/3 operation:

def lookup(json) do
  Enum.reduce(json, [], fn x, acc ->
    if x["title"] =~ "sunt", do: [x|acc], else: acc
  end)
end

Upvotes: 1

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