[ruby-core:68928] [Ruby trunk - Feature #10982] Clarify location of NoMethod error

Issue #10982 has been updated by Yuki Nishijima.


It wasn't so hard to implement this in pure Ruby, the last example doesn't work, though.

```ruby
# -*- coding: utf-8 -*-
class NoMethodError
  REPL_LABELS = {
    "irb_binding" => -> { Readline::HISTORY.to_a.last },
    "__pry__"     => -> { ::Pry.history.to_a.last }
  }

  def to_s
    msg = super
    msg << "\n\n".freeze
    msg << "  #{code}\n"
    msg << "  #{marker}\n"
  rescue
    super
  end

  def code
    # Memoize since the #backtrace_locations method creates a lot of objects.
    @code ||= begin
      loc = backtrace_locations.first

      if REPL_LABELS.include?(loc.label)
        REPL_LABELS[loc.label].call
      elsif File.exist?(loc.absolute_path)
        File.open(loc.absolute_path) do |file|
          file.detect { file.lineno == loc.lineno }
        end
      end
    end.to_s.strip
  end

  def marker
    code.strip.gsub(/(.*)(#{name}).*/) do
      (" ".freeze * $1.size.pred) + '^'.freeze + ("‾".freeze * $2.size)
    end if !code.empty?
  end
end

# works
e = 1.foo rescue $!
puts e
# => undefined method `foo' for 1:Fixnum
#
#  e = 1.foo rescue $!
#       ^‾‾‾

# works.
e = 1.send(:bar) rescue $!
puts e
# => undefined method `bar' for 1:Fixnum
#
#  e = 1.send(:bar) rescue $!
#             ^‾‾‾

# works
@foo = ->{ true }
e = (@foo.call && @bar.call) rescue $!
puts e
# => undefined method `call' for nil:NilClass
#
#  e = (@foo.call && @bar.call) rescue $!
#                        ^‾‾‾‾

# doesn't work
e = (@something.call && @else.call) rescue $!
puts e
# => undefined method `call' for nil:NilClass
#
#  e = (@something.call && @else.call) rescue $!
#                               ^‾‾‾‾
```

----------------------------------------
Feature #10982: Clarify location of NoMethod error
https://bugs.ruby-lang.org/issues/10982#change-52194

* Author: Richard Schneeman
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
In Ruby, the error `NoMethodError` on happens frequently, especially when it occurs on `nil`. This error can be confusing to beginners since, many of them think there is a problem with the method instead of the receiver. This error can be confusing to advanced developers when there are multiple method calls in the on the same line. In this example it is unclear if `foo` or `bar` returned `nil`:

```
foo.call && bar.call
NoMethodError: undefined method `call' for nil:NilClass
```

I would like to make this a better error message and to indicate where the exception occurs.

```
@foo.call && @bar.call
                 ^----
NoMethodError: The method `call` is undefined on receiver nil:NilClass
```

Now it is more clear that the `@bar` is currently `nil` and that is the source of the error. I believe exposing this information will help developers of all abilities. 




-- 
https://bugs.ruby-lang.org/