Extends Mini-Parser from Getting Started to be able to use funcalls wherever a simple integer would do

funcall.rb

require 'pp'
require 'parslet'
require 'parslet/convenience'

class MiniP < Parslet::Parser
  # Single character rules
  rule(:lparen)     { str('(') >> space? }
  rule(:rparen)     { str(')') >> space? }
  rule(:comma)      { str(',') >> space? }

  rule(:space)      { match('\s').repeat(1) }
  rule(:space?)     { space.maybe }

  # Things
  rule(:integer)    { match('[0-9]').repeat(1).as(:int) >> space? }
  rule(:identifier) { match['a-z'].repeat(1) }
  rule(:operator)   { match('[+]') >> space? }
  
  # New
  rule(:term)       { integer | funcall }
  
  # Grammar parts
  rule(:sum)        { term.as(:left) >> operator.as(:op) >> expression.as(:right) }
  rule(:arglist)    { expression >> (comma >> expression).repeat }
  rule(:funcall)    { identifier.as(:funcall) >> lparen >> arglist.as(:arglist) >> rparen }
  
  rule(:expression) { sum | term }
  root :expression
end

pp MiniP.new.parse_with_debug('add(add(1, 2) + add(3, 4), add(5, 6))')