PrettyBacktrace でデバッグを楽にする

Backtrace を表示する際に、ローカル変数名や値も表示してくれるようにしてくれる Gem

使ってみる

README のサンプルをそのまま使う

sample.rb

def recursive n
  str = "Hi #{n}!!  " * 128
  if n > 0
    recursive n - 1
  else
    raise "bottom of recursive"
  end
end

recursive 3

これを実行するといつものエラー表示

pretty_backtrace.rb:10:in `recursive': bottom of recursive (RuntimeError)
	from pretty_backtrace.rb:8:in `recursive'
	from pretty_backtrace.rb:8:in `recursive'
	from pretty_backtrace.rb:8:in `recursive'
	from pretty_backtrace.rb:14:in `<main>'

PrettyBacktrace を有効にすると

sample.rb

require 'pretty_backtrace'
PrettyBacktrace.enable

def recursive n
  str = "Hi #{n}!!  " * 128
  if n > 0
    recursive n - 1
  else
    raise "bottom of recursive"
  end
end

recursive 3

こうなる

pretty_backtrace.rb:10:in `recursive' (n = 0, str = "Hi 0!!  Hi 0!!  Hi 0...): bottom of recursive (RuntimeError)
	from pretty_backtrace.rb:8:in `recursive' (n = 1, str = "Hi 1!!  Hi 1!!  Hi 1...)
	from pretty_backtrace.rb:8:in `recursive' (n = 2, str = "Hi 2!!  Hi 2!!  Hi 2...)
	from pretty_backtrace.rb:8:in `recursive' (n = 3, str = "Hi 3!!  Hi 3!!  Hi 3...)
	from pretty_backtrace.rb:14:in `<main>'

おー n と str の状態がよくわかる！
さらに multi line モードにすると

sample.rb

require 'pretty_backtrace'
PrettyBacktrace.enable
PrettyBacktrace.multi_line = true

...

Pretty！

pretty_backtrace.rb:10:in `recursive'
          [FILE]
             8|    recursive n - 1
             9|  else
        ->  10|    raise "bottom of recursive"
            11|  end
            12|end

          [LOCAL VARIABLES]
            n = 0
            str = "Hi 0!!  Hi 0!!  Hi 0!!  Hi 0!!  Hi 0!!  Hi 0!!  Hi 0!!  Hi 0...
: bottom of recursive (RuntimeError)
	from pretty_backtrace.rb:8:in `recursive'
          [FILE]
             6|  str = "Hi #{n}!!  " * 128
             7|  if n > 0
        ->   8|    recursive n - 1
             9|  else
            10|    raise "bottom of recursive"

          [LOCAL VARIABLES]
            n = 1
            str = "Hi 1!!  Hi 1!!  Hi 1!!  Hi 1!!  Hi 1!!  Hi 1!!  Hi 1!!  Hi 1...

	from pretty_backtrace.rb:8:in `recursive'
          [FILE]
             6|  str = "Hi #{n}!!  " * 128
             7|  if n > 0
        ->   8|    recursive n - 1
             9|  else
            10|    raise "bottom of recursive"

          [LOCAL VARIABLES]
            n = 2
            str = "Hi 2!!  Hi 2!!  Hi 2!!  Hi 2!!  Hi 2!!  Hi 2!!  Hi 2!!  Hi 2...

	from pretty_backtrace.rb:8:in `recursive'
          [FILE]
             6|  str = "Hi #{n}!!  " * 128
             7|  if n > 0
        ->   8|    recursive n - 1
             9|  else
            10|    raise "bottom of recursive"

          [LOCAL VARIABLES]
            n = 3
            str = "Hi 3!!  Hi 3!!  Hi 3!!  Hi 3!!  Hi 3!!  Hi 3!!  Hi 3!!  Hi 3...

	from pretty_backtrace.rb:14:in `<main>'
          [FILE]
            12|end
            13|
        ->  14|recursive 3

たった 3 行足すだけで、とっても便利！
いろいろな場面で重宝しそうですね。