Ruby: Parsing a string representation of nested arrays into an Array?

Question

Let\'s say I had the string

\"[1,2,[3,4,[5,6]],7]\"

How would I parse that into the array

[1,2,[3,4,[5,6]],7]

Answer 1

"Obviously" the best solution is to write your own parser. [ If you like writing parsers, have never done it before and want to learn something new, or want control over the exact grammar ]

require 'parslet'

class Parser < Parslet::Parser
  rule(:space)       { str(' ') }
  rule(:space?)      { space.repeat(0) }
  rule(:openbrace_)  { str('[').as(:op) >> space? }
  rule(:closebrace_) { str(']').as(:cl) >> space? }
  rule(:comma_)      { str(',') >> space?  }
  rule(:integer)     { match['0-9'].repeat(1).as(:int) }
  rule(:value)       { (array | integer) >> space? }
  rule(:list)        { value >> ( comma_ >> value ).repeat(0) }
  rule(:array)       { (openbrace_ >> list.maybe.as(:list) >> closebrace_ )}
  rule(:nest)        { space? >> array.maybe }
  root(:nest)
end

class Arr
  def initialize(args)
    @val = args
  end
  def val
    @val.map{|v| v.is_a?(Arr) ? v.val : v}
  end
end


class MyTransform < Parslet::Transform
  rule(:int => simple(:x))      { Integer(x) }
  rule(:op => '[', :cl => ']')  { Arr.new([]) }
  rule(:op => '[', :list => simple(:x), :cl => ']')   {  Arr.new([x]) }
  rule(:op => '[', :list => sequence(:x), :cl => ']')   { Arr.new(x) }
end

def parse(s)
  MyTransform.new.apply(Parser.new.parse(s)).val
end

parse " [   1  ,   2  ,  [  3  ,  4  ,  [  5   ,  6  , [ ]]   ]  ,  7  ]  "

Parslet transforms will match a single value as "simple" but if that value returns an array, you soon get arrays of arrays, then you have to start using subtree. returning objects however are fine as they represent a single value when transforming the layer above... so sequence will match fine.

Couple the trouble with returning bare arrays, with the problem that Array([x]) and Array(x) give you the same thing... and you get very confusing results.

To avoid this I made a helper class called Arr which represents an array of items. I could then dictate what I pass into it. Then I can get the parser to keep all the brackets even if you have the example that @MateuszFryc called out :) (thanks @MateuszFryc)