.irb_exception_cause_2_3_4.rb

##
# Monkey-patch IRB in Ruby 2.3.4 to address the following Ruby issue:
#
#   * https://bugs.ruby-lang.org/issues/14685
#     IRB doesn't print exception cause
##
# After loading this file, one should observe in an IRB session:
#
#   * Printed exceptions contain the `#cause`
#     In fact, this should work for a chain of `#cause` relationships    
##
# This file is a copied and modified version of the method `IRB::Irb#eval_input`
# from Ruby version 2.3.4.
#
# It would be great to turn this into a PR to Ruby. To do so, one would first
# need to:
#   a. Make an updated version based on the latest trunk Ruby
#   b. Add tests
#
# To make a later PR easier, the modified lines are called out with comments
# below.
#
# Author: "Marc Siegel" <msiegel@panoramaed.com>
##
require "irb"

module IRB
  class Irb
    # Monkey-patch this large method by copying it and modifying.
    def eval_input
      @scanner.set_prompt do
        |ltype, indent, continue, line_no|
        if ltype
          f = @context.prompt_s
        elsif continue
          f = @context.prompt_c
        elsif indent > 0
          f = @context.prompt_n
        else
          f = @context.prompt_i
        end
        f = "" unless f
        if @context.prompting?
          @context.io.prompt = p = prompt(f, ltype, indent, line_no)
        else
          @context.io.prompt = p = ""
        end
        if @context.auto_indent_mode
          unless ltype
            ind = prompt(@context.prompt_i, ltype, indent, line_no)[/.*\z/].size +
              indent * 2 - p.size
            ind += 2 if continue
            @context.io.prompt = p + " " * ind if ind > 0
          end
        end
      end

      @scanner.set_input(@context.io) do
        signal_status(:IN_INPUT) do
          if l = @context.io.gets
            print l if @context.verbose?
          else
            if @context.ignore_eof? and @context.io.readable_after_eof?
              l = "\n"
              if @context.verbose?
                printf "Use \"exit\" to leave %s\n", @context.ap_name
              end
            else
              print "\n"
            end
          end
          l
        end
      end

      @scanner.each_top_level_statement do |line, line_no|
        signal_status(:IN_EVAL) do
          begin
            line.untaint
            @context.evaluate(line, line_no)
            output_value if @context.echo?
            exc = nil
          rescue Interrupt => exc
          rescue SystemExit, SignalException
            raise
          rescue Exception => exc
          end
          ## BEGIN Monkey-patch 1
          #- if exc
          is_cause = false
          while exc
            #- print exc.class, ": ", exc, "\n"
            print (is_cause ? "Caused by: " : ""), exc.class, ": ", exc, "\n"
            ## END Monkey-patch 1

            if exc.backtrace && exc.backtrace[0] =~ /irb(2)?(\/.*|-.*|\.rb)?:/ && exc.class.to_s !~ /^IRB/ &&
              !(SyntaxError === exc)
              irb_bug = true
            else
              irb_bug = false
            end

            messages = []
            lasts = []
            levels = 0

            if exc.backtrace
              for m in exc.backtrace
                m = @context.workspace.filter_backtrace(m) unless irb_bug
                if m
                  if messages.size < @context.back_trace_limit
                    messages.push "\tfrom "+m
                  else
                    lasts.push "\tfrom "+m
                    if lasts.size > @context.back_trace_limit
                      lasts.shift
                      levels += 1
                    end
                  end
                end
              end
            end
            print messages.join("\n"), "\n"
            unless lasts.empty?
              printf "... %d levels...\n", levels if levels > 0
              print lasts.join("\n"), "\n"
            end
            print "Maybe IRB bug!\n" if irb_bug

            ## BEGIN Monkey-patch 2
            exc = exc.cause
            is_cause = true
            ## END Monkey-patch 2
          end
          if $SAFE > 2
            abort "Error: irb does not work for $SAFE level higher than 2"
          end
        end
      end
    end
  end
end