Why `case/when` does NOT work with just result codes?

Sometimes it may feel idiomatic to check the result code in the following way:

# wrong
if result.not_success?
  case result.code # `result.code` returns fancy object
  when :full_queue
    notify_devops
  when :duplicated_job
    notify_devs
  else
    # ...
  end
end

Although this snippet looks very appealing, it won't work as expected.

A lot of Ruby folks often have an assumption that a value of case is compared with a value of when.

case value === when value

In other words, the case/when from example above can be transformed to if/else like so:

# wrong
if result.not_success?
  if result.code === :full_queue
    notify_devops
  elsif result.code === :duplicated_job
    notify_devs
  else
    # ...
  end
end

But, that is NOT correct.

In reality, the value of when is always compared with the value of case.

when value === case value

Let's write the proper transformation:

# okish
if result.not_success?
  if :full_queue === result.code
    notify_devops
  elsif :duplicated_job === result.code
    notify_devs
  else
    # ...
  end
end

Symbol#=== is just an alias to Symbol#==.

There is no way to hook into any behavior like with strings (String#=== calls to_str on its argument).

As a result, just case result.code is not enough.

But once you use case result.code.to_sym, the natural Ruby charm is back.

# ok
if result.not_success?
  case result.code.to_sym
  when :full_queue
    notify_devops
  when :duplicated_job
    notify_devs
  else
    # ...
  end
end