Crystal-lang: Recursive JSON or Hash

Question

I'm trying to create a JSON or a Hash that can have N depth. Example: X people with unique names might have Y kids and those kids might have Z kids (and goes on till N generations). I want to create a Hash (or JSON) that would look like this:

{
  "John" => {
              "Lara" => { 
                          "Niko" => "Doe"
                        },
              "Kobe" => "Doe"
            },
  "Jess" => {
              "Alex" => "Patrik"
            }
}

I tried working with recursive aliases but couldn't achieve that.

alias Person = Hash(String, Person) | Hash(String, String)

The input could come from arrays of String like

["John|Lara|Niko", "John|Kobe", "Jess|Alex"]
["Doe", "Patrik"]

(I can deal with the loops. My issue is adding them to the Hash as their size is unknown.)

I came across this discussion https://forum.crystal-lang.org/t/how-do-i-create-a-nested-hash-type/885 but unfortunately I can't achieve what I want and also keep the Hash's (or JSON's) methods (which are needed).

Answer 1

I couldn't quite make out how you arrived at your example result from your example input, so I'm going to use a different setup: Let's assume we have a simple configuration file format where keys are structured and grouped through a dotted sequence and all values are always strings.

app.name = test
app.mail.enable = true
app.mail.host = mail.local
server.host = localhost
server.port = 3000
log_level = debug

We can parse it to a recursive Hash like so:

alias ParsedConfig = Hash(String, ParsedConfig)|String

config = Hash(String, ParsedConfig).new

# CONFIG being our input from above
CONFIG.each_line do |entry|
  keys, value = entry.split(" = ")
  keys = keys.split(".")
  current = config
  keys[0..-2].each do |key|
    if current.has_key?(key)
      item = current[key]
      if item.is_a?(Hash)
        current = item
      else
        raise "Malformed config"
      end
    else
      item = Hash(String, ParsedConfig).new
      current[key] = item
      current = item
    end
  end

  current[keys.last] = value
end

pp! config

The output will be:

config # => {"app" =>
  {"name" => "test", "mail" => {"enable" => "true", "host" => "mail.local"}},
 "server" => {"host" => "localhost", "port" => "3000"},
 "log_level" => "debug"}

Alternatively we can parse it to a recursive struct:

record ConfigGroup, entries = Hash(String, ConfigGroup|String).new

config = ConfigGroup.new

# CONFIG being our input from above
CONFIG.each_line do |entry|
  keys, value = entry.split(" = ")
  keys = keys.split(".")
  current = config
  keys[0..-2].each do |key|
    if current.entries.has_key?(key)
      item = current.entries[key]
      if item.is_a?(ConfigGroup)
        current = item
      else
        raise "Malformed config"
      end
    else
      item = ConfigGroup.new
      current.entries[key] = item
      current = item
    end
  end

  current.entries[keys.last] = value
end

pp! config

The output then will be:

config # => ConfigGroup(
 @entries=
  {"app" =>
    ConfigGroup(
     @entries=
      {"name" => "test",
       "mail" =>
        ConfigGroup(@entries={"enable" => "true", "host" => "mail.local"})}),
   "server" => ConfigGroup(@entries={"host" => "localhost", "port" => "3000"}),
   "log_level" => "debug"})

Recursive structs currently are a bit less buggy, offer a nice place for custom methods on your parsed domain objects and generally have a more certain future than recursive aliases, which are sometimes a bit buggy.

Full example on carc.in: https://carc.in/#/r/9mxr