Elixir: find by value prefix in nested JSON - json

Question

I'm trying to find URLs in a nested JSON response and map them. My function so far looks like this:
def list(env, id) do
Service.get_document(env, id)
|> Poison.decode!
|> Enum.find(fn {_key, val} -> String.starts_with?(val, 'https') end)
end
The JSON looks roughly like this:
"stacks": [
{
"boxes": [
{
"content": "https://ddd.cloudfront.net/photos/uploaded_images/000/001/610/original/1449447147677.jpg?1505956120",
"box": "photo"
}
]
}
],
"logo": "https://ddd.cloudfront.net/users/cmyk_banners/000/000/002/original/banner_CMYK.jpg?1397201875"
So URLs can have any key, and be at any level.
With that code I get this error:
no function clause matching in String.starts_with?/2
Anyone got a better way to find in JSON responses?

Answer 1

You'll have to use recursive function for this, which handles three types of data:
For map, it recurses over all its values.
For list, it recurses over all its elements.
For string, it selects strings that start with "https"
Here's a simple implementation which accepts a term and a string to check with starts_with?:
defmodule A do
def recursive_starts_with(thing, start, acc \\ [])
def recursive_starts_with(binary, start, acc) when is_binary(binary) do
if String.starts_with?(binary, start) do
[binary | acc]
else
acc
end
end
def recursive_starts_with(map, start, acc) when is_map(map) do
Enum.reduce(map, acc, fn {_, v}, acc -> A.recursive_starts_with(v, start, acc) end)
end
def recursive_starts_with(list, start, acc) when is_list(list) do
Enum.reduce(list, acc, fn v, acc -> A.recursive_starts_with(v, start, acc) end)
end
end
data = %{
"stacks" => [
%{
"boxes" => [
%{
"content" => "https://ddd.cloudfront.net/photos/uploaded_images/000/001/610/original/1449447147677.jpg?1505956120",
"box" => "photo"
}
]
}
],
"logo" => "https://ddd.cloudfront.net/users/cmyk_banners/000/000/002/original/banner_CMYK.jpg?1397201875"
}
data |> A.recursive_starts_with("https") |> IO.inspect
Output:
["https://ddd.cloudfront.net/photos/uploaded_images/000/001/610/original/1449447147677.jpg?1505956120",
"https://ddd.cloudfront.net/users/cmyk_banners/000/000/002/original/banner_CMYK.jpg?1397201875"]