{"id":370,"date":"2013-03-01T11:09:04","date_gmt":"2013-03-01T11:09:04","guid":{"rendered":"http:\/\/nicholshayes.co.uk\/blog\/?p=370"},"modified":"2013-10-31T15:56:11","modified_gmt":"2013-10-31T15:56:11","slug":"using-__line__-in-dynamic-methods","status":"publish","type":"post","link":"http:\/\/nicholshayes.co.uk\/blog\/?p=370","title":{"rendered":"Using __LINE__ in dynamic methods"},"content":{"rendered":"

I’ve been digging into active_support to see how config_accessor<\/em><\/a> was working. This is the code I found:<\/p>\n

      def<\/span> config_accessor(<\/span>*<\/span>names)<\/span>
\n        options = names.extract_options<\/span>!
\n
\n        names.each<\/span> do<\/span> |<\/span>name|<\/span>
\n          reader, line = "def #{name}; config.#{name}; end"<\/span>, __LINE__<\/span>
\n          writer, line = "def #{name}=(value); config.#{name} = value; end"<\/span>, __LINE__<\/span>
\n
\n          singleton_class.class_eval<\/span> reader, __FILE__<\/span>, line
\n          singleton_class.class_eval<\/span> writer, __FILE__<\/span>, line
\n          class_eval reader, __FILE__<\/span>, line unless<\/span> options[<\/span>:instance_reader<\/span>]<\/span> == false<\/span>
\n          class_eval writer, __FILE__<\/span>, line unless<\/span> options[<\/span>:instance_writer<\/span>]<\/span> == false<\/span>
\n        end<\/span>
\n      end<\/span><\/div><\/div>\n

What it is doing is fairly straight forward. It is creating setters and getters for attributes of a config<\/em> object. The setter definition being held in the variable writer<\/em> and the getter in reader<\/em>.<\/p>\n

The thing that really piqued my interest, was the construction of reader<\/em>, writer<\/em> and line<\/em>. In particular, what line<\/em> was doing.<\/p>\n

After a little reading around I learnt that line<\/em> is used to ensure the exception backtrace points to the place where the method is being defined. So for reader<\/em>, the back trace will point to the place where the method was defined, and not to where class_eval<\/em> was called.<\/p>\n

To help me get my head around this, I wrote this:<\/p>\n

module<\/span> LinePlay
\n
\n  def<\/span> self<\/span>.raise_error_here<\/span>
\n    method, line = "def self.raise_here; raise; end"<\/span>, __LINE__<\/span>
\n    make_into_method(<\/span>method, line)<\/span>
\n  end<\/span>
\n
\n  def<\/span> self<\/span>.raise_error_there<\/span>
\n    method, line = "def self.raise_there; raise; end"<\/span>, __LINE__<\/span>
\n    make_into_method(<\/span>method)<\/span>
\n  end<\/span>
\n
\n  def<\/span> self<\/span>.make_into_method<\/span>(<\/span>method, line = nil<\/span>)<\/span>
\n    if<\/span> line
\n      module_eval method, __FILE__<\/span>, line
\n    else<\/span>
\n      module_eval method
\n    end<\/span>
\n  end<\/span>
\n
\n  def<\/span> self<\/span>.rescue_error<\/span>(<\/span>method)<\/span>
\n    begin<\/span>
\n      send(<\/span>method)<\/span>
\n    rescue<\/span> =><\/span> e
\n      puts<\/span> "\\n<\/span>Output for #{method}"<\/span>
\n      puts<\/span> e.backtrace<\/span>
\n    end<\/span>
\n  end<\/span>
\n
\n  raise_error_here
\n  raise_error_there
\n
\nend<\/span>
\n
\nLinePlay.rescue_error<\/span>(<\/span>:raise_here<\/span>)<\/span>
\nLinePlay.rescue_error<\/span>(<\/span>:raise_there<\/span>)<\/span><\/div><\/div>\n

This dynamically creates two methods via module_eval<\/em>: LinePlay.raise_here<\/em>, and LinePlay.raise_there<\/em>. I called module_eval<\/em> within make_into_method<\/em>, to separate the definition method from the eval method. It then calls the two dynamically defined methods, rescues the exceptions and outputs the backtraces.<\/p>\n

This is the output:<\/p>\n

Output for raise_here
\nline_play.rb:4:in `raise_here'
\nline_play.rb:23:in `rescue_error'
\nline_play.rb:35:in `<main>'
\n
\nOutput for raise_there
\n(eval):1:in `raise_there'
\nline_play.rb:23:in `rescue_error'
\nline_play.rb:36:in `<main>'<\/div><\/div>\n

For raise_here<\/em>, the line number where that method is defined, is passed to module_eval<\/em>. If you look at the resulting backtrace, this shows line 4 as the source of the error. That is, the line where the method causing the error, was defined.<\/p>\n

For raise_there<\/em>, __FILE__ and line number are not passed to module_eval<\/em>. In this case the backtrace points to where class_eval was called and not where it was defined. <\/p>\n

So using this technique generates much more useful error reporting. A neat trick I think.<\/p>\n

However, have you spotted the error in the original code? The variable line<\/em> for reader<\/em> is overwritten on the next line when writer<\/em> is assigned. That means backtraces for both reader<\/em> and writer<\/em> will point at the writer<\/em> definition. This error has since been corrected. See this commit<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve been digging into active_support to see how config_accessor was working. This is the code I found:       def config_accessor(*names)         options = names.extract_options!         names.each do |name|         … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[3],"tags":[],"_links":{"self":[{"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/370"}],"collection":[{"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=370"}],"version-history":[{"count":16,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/370\/revisions"}],"predecessor-version":[{"id":457,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/370\/revisions\/457"}],"wp:attachment":[{"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=370"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=370"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/nicholshayes.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=370"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}