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Error Handling

Exception Handling (Error Handling)

begin-rescue (try-catch).

# basic syntax
begin
  # ...
rescue => e
  # ...
end

# rescuing specific Error class name
begin
  # ...
rescue ArgumentError => e
  # ...
end

Full Chain

begin
    # may raise an exception
rescue AnException
    # exception handler
rescue AnotherException
    # exception handler
else
    # other exceptions
ensure
    # always executed
end

When/When NOT to Use

Exceptions shouldn't be used for validations.

You shouldn't traverse the stack for validations in general.

What you should NOT be doing

  • X is top level and can handle everything.
  • X calls Y. Y calls Z.
  • Z performs validations and does something after that, raising an exception if validation failed.

What you should be doing instead

  • X calls Y. Y calls V and X.
  • V performs validations and returns result based on if the operation was valid.
    • Y doesn't get to call X if V said the thing was invalid.
  • Y propagates the invalidness or the successful result to X
  • X does what it would have done with if/else on the validity, rather than rescue

Exception Chaining

In Ruby, rescuing multiple exceptions are easy.

begin
   ...
rescue Exception1, Exception2
  ...
rescue Exception1
  ...
rescue Exception2
  ...
end
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