Passing multiple error classes to ruby's rescue clause in a DRY fashion

Question

I have some code that needs to rescue multiple types of exceptions in ruby:

begin   a = rand   if a > 0.5     raise FooException   else     raise BarException   end rescue FooException, BarException   puts "rescued!" end 

What I'd like to do is somehow store the list of exception types that I want to rescue somewhere and pass those types to the rescue clause:

EXCEPTIONS = [FooException, BarException] 

and then:

rescue EXCEPTIONS 

Is this even possible, and is it possible without some really hack-y calls to eval? I'm not hopeful given that I'm seeing TypeError: class or module required for rescue clause when I attempt the above.

Answer 1

You can use an array with the splat operator *.

EXCEPTIONS = [FooException, BarException]  begin   a = rand   if a > 0.5     raise FooException   else     raise BarException   end rescue *EXCEPTIONS   puts "rescued!" end 

If you are going to use a constant for the array as above (with EXCEPTIONS), note that you cannot define it within a definition, and also if you define it in some other class, you have to refer to it with its namespace. Actually, it does not have to be a constant.

---

Splat Operator

The splat operator * "unpacks" an array in its position so that

rescue *EXCEPTIONS 

means the same as

rescue FooException, BarException 

You can also use it within an array literal as

[BazException, *EXCEPTIONS, BangExcepion] 

which is the same as

[BazException, FooException, BarException, BangExcepion] 

or in an argument position

method(BazException, *EXCEPTIONS, BangExcepion) 

which means

method(BazException, FooException, BarException, BangExcepion) 

[] expands to vacuity:

[a, *[], b] # => [a, b] 

One difference between ruby 1.8 and ruby 1.9 is with nil.

[a, *nil, b] # => [a, b]       (ruby 1.9) [a, *nil, b] # => [a, nil, b]  (ruby 1.8) 

Be careful with objects on which to_a is defined, as to_a will be applied in such cases:

[a, *{k: :v}, b] # => [a, [:k, :v], b] 

With other types of objects, it returns itself.

[1, *2, 3] # => [1, 2, 3]