[ruby-core:87273] [Ruby trunk Feature#12906] do/end blocks work with ensure/rescue/else

Issue #12906 has been updated by perlun (Per Lundberg).


Old issue, but still perhaps the right place to mention this: the new syntax ONLY works in do/end, not in {} blocks (as mentioned above.)

It also does not work in "block-like" places like a for loop. So this is not valid syntax:

```
for i in 1..100
  puts 'hello'
rescue # Syntax error; not valid code in existing Ruby versions like 2.5.0 and 2.5.1.
end
```

Suggested workaround: Use a `(1..100).each do |i|` construct instead. Then you can use a `rescue` or `ensure` within the block in Ruby 2.5 and newer.

(This is actually an arguably valid use case for "fine grained error handling", in cases where the loop can fail in `n` of the `100` cases and you want the failing ones to be ignored. I agree that too-finegrained error handling can otherwise become an antipattern.)


----------------------------------------
Feature #12906: do/end blocks work with ensure/rescue/else
https://bugs.ruby-lang.org/issues/12906#change-72263

* Author: josh.cheek (Josh Cheek)
* Status: Closed
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 2.5
----------------------------------------
When you want to rescue in a block, you must do this:

~~~ruby
lambda do
  begin
    raise 'err'
  rescue
    $! # => #<RuntimeError: err>
  end
end.call
~~~

I've wished on numerous occasions that I could omit the begin/end and not need the extra wrapper:

~~~ruby
lambda do
  raise 'err'
rescue
  $! # => #<RuntimeError: err>
end.call
~~~

This would be consistent with how classes and methods work:

~~~ruby
class C
  raise 'err'
rescue
  $! # => #<RuntimeError: err>
end

send def m
  raise 'err'
rescue
  $! # => #<RuntimeError: err>
end
~~~


It's not really clear to me how to submit this since it may require some discussion, but this is the diff:

~~~diff
diff --git a/parse.y b/parse.y
index 54ccc52..223e5d3 100644
--- a/parse.y
+++ b/parse.y
@@ -3757,7 +3757,7 @@ brace_body	: {$<vars>$ = dyna_push();}
 
 do_body 	: {$<vars>$ = dyna_push();}
 		  {$<val>$ = cmdarg_stack >> 1; CMDARG_SET(0);}
-		  opt_block_param compstmt
+		  opt_block_param bodystmt
 		    {
 			$$ = new_do_body($3, $4);
 			dyna_pop($<vars>1);
~~~


I added tests for ensure to rubyspec, but there wasn't an obvious place to talk about rescue/else in this context (the spec for rescue only uses it in a begin/end block) It's probably fine as the spec for ensure does hit rescue, too, and they ultimately delegate to the same pieces. Not totally clear, though. I can do more with that if you need.

~~~diff
diff --git a/language/ensure_spec.rb b/language/ensure_spec.rb
index 13575fc..b14b0b5 100644
--- a/language/ensure_spec.rb
+++ b/language/ensure_spec.rb
@@ -124,3 +124,74 @@ describe "An ensure block inside a method" do
     @obj.explicit_return_in_method_with_ensure.should == :ensure
   end
 end
+
+describe "An ensure block inside a do block" do
+  before :each do
+    ScratchPad.record []
+  end
+
+  it "is executed when an exception is raised in it's corresponding do block" do
+    begin
+      lambda do
+        ScratchPad << :begin
+        raise "An exception occured!"
+      ensure
+        ScratchPad << :ensure
+      end.should raise_error(RuntimeError)
+
+      ScratchPad.recorded.should == [:begin, :ensure]
+    end
+  end
+
+  it "is executed when an exception is raised and rescued in it's corresponding do block" do
+    begin
+      lambda do
+        ScratchPad << :begin
+        raise "An exception occured!"
+      rescue
+        ScratchPad << :rescue
+      ensure
+        ScratchPad << :ensure
+      end.call
+
+      ScratchPad.recorded.should == [:begin, :rescue, :ensure]
+    end
+  end
+
+  it "is executed even when a symbol is thrown in it's corresponding do block" do
+    begin
+      catch(:symbol) do
+        lambda do
+          ScratchPad << :begin
+          throw(:symbol)
+        rescue
+          ScratchPad << :rescue
+        ensure
+          ScratchPad << :ensure
+        end.call
+      end
+
+      ScratchPad.recorded.should == [:begin, :ensure]
+    end
+  end
+
+  it "is executed when nothing is raised or thrown in it's corresponding do block" do
+    lambda do
+      ScratchPad << :begin
+    rescue
+      ScratchPad << :rescue
+    ensure
+      ScratchPad << :ensure
+    end.call
+
+    ScratchPad.recorded.should == [:begin, :ensure]
+  end
+
+  it "has no return value" do
+    lambda do
+      :begin
+    ensure
+      :ensure
+    end.call.should == :begin
+  end
+end
~~~



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