In Ruby, What structures can a `rescue` statement be nested in

Question

In ruby to catch an error one uses the rescue statement. generally this statement occurs between begin and end. One can also use a rescue statement as part of a block (do ... end) or a method (def ... end). My question is what other structures (loop, while, if, ...) if any will rescue nest within?

Abduction

do ... end blocks can't be rescued from without an explicit begin ... end. – Yearlong

Since ruby 2.5 do ... end blocks can be rescued without an explicit begin ... end. – Eustache

Answer 1

You can only use rescue in two cases:

• Within a begin ... end block

  begin
    raise
  rescue 
    nil
  end
  

• As a statement modifier

  i = raise rescue nil
  

Function, module, and class bodies (thanks Jörg) are implicit begin...end blocks, so you can rescue within any function without an explicit begin/end.

    def foo
      raise
    rescue
      nil
    end

The block form takes an optional list of parameters, specifying which exceptions (and descendants) to rescue:

    begin
      eval string
    rescue SyntaxError, NameError => boom
      print "String doesn't compile: " + boom
    rescue StandardError => bang
      print "Error running script: " + bang
    end

If called inline as a statement modifier, or without argument within a begin/end block, rescue will catch StandardError and its descendants.

Here's the 1.9 documentation on rescue.

Wing

module and class bodies are implicit begin blocks, too. – Stuartstub

@Jörg W Mittag: as are do ... end blocks and def ... end method definitions. IS there anything else that is an implicit begin? while, case, or if for example? – Abduction

@john - do...end isn't an implicit begin...end. – Wing

I suggest adding the comment above about do...end to the answer... that is really what I was looking for. – Hophead

Although not very well documented, as of ruby 2.5 rescue works in regular do/end blocks (not in-line blocks {...} though). commit – Meldameldoh

Answer 2

As said in recent comment, response has changed since Ruby 2.5.

do ... end blocks are now implicit begin ... end blocks; like module, class and method bodies.

In-line blocks {...} still can't.

Eustache

my favorite is each blocks – Ardisardisj