test2.rb

require "parslet"

class MiniP < Parslet::Parser
  rule(:wsp)    { match("\\s").repeat(1) }
  rule(:wsp?)   { wsp.maybe }

  rule(:t_int)  { match("[0-9]").repeat(1).as(:int) }

  # this is causing Stack Level Too Deep
  rule(:sum)    { expr.as(:left) >> wsp? >> str("+") >> wsp? >> expr.as(:right) }

  # but this is fine
  rule(:arg_list) { expr >> wsp? >> (str(",") >> wsp? >> expr).repeat }
  rule(:add_call) { str("add") >> wsp? >> str("(") >> wsp? >> arg_list.as(:args) >> wsp? >> str(")") }

  rule(:expr)      { sum.as(:sum) | add_call.as(:add_call) | t_int }

  root(:expr)
end

class MiniT < Parslet::Transform
  rule(:int => simple(:int)) { |dict| Integer(dict[:int]) }
  rule(:args => sequence(:args)) { |dict| dict[:args] }
  rule(:sum => { :left => simple(:left), :right => simple(:right) }) { |dict| dict[:left] + dict[:right] }
  rule(:add_call => subtree(:args)) { |dict| dict[:args].reduce(&:+) }
end

expr = "add(add(4, 5), add(1, 1 + add(0, 1)))"
p MiniP.new.parse(expr)
p MiniT.new.apply(MiniP.new.parse(expr))