[PPL-devel] Redundant elses

[Matthew Mundell]

Roberto Bagnara <bagnara at cs.unipr.it> writes:

> Matthew Mundell wrote:
>> The `else' repeats the information implied by the final return or
>> goto.
>
> But the `else' makes it clearer that we are talking about mutually
> exclusive conditions.

This should be clear enough from the code.

>                        And it is also more robust: what happens
> if someone changes the "then" branch without noticing that
> you removed the `else'?

It's better if the code breaks in this case, as the person will have
introduced the fall-through case so will have needed to check what
followed the if statement, even if there was an `else'.

>
>> Adding the else makes it less clear that the body of the "then" branch
>> of the `if' always ends in a return or goto.
>>
>>>If, in addition, you have something of the form
>>>
>>>    if (a)
>>>      ...
>>>    else
>>>      // comment
>>>      return ...
>> Did you mean this?
>>      if (a)
>>        return ...
>>      else
>>        // comment
>>        ...
>
> No, I meant what I wrote: in the original version, `else' and
> the comment are part of the same "phrase".

The question was whether the return was meant for the `then' block, as
that would be required to make the `else' redundant.  Anyway, what you
were saying was clear.

>                                             If you take out the
> `else' then the comment assumes an "unconditional flavor"
> that can only be recovered by noticing that the "then" branch
> forces a certain control-flow.  Taking out the `else' makes
> two sections of code that could/should be indepedent from one another
> strongly coupled: if you want to keep correctness you must
> make sure the "then" branch has the property to go somewhere
> else at the end.
>
> To come back to the subject: if you only look at semantics,
> it is true that such elses are redundant;  but if you look
> at the big picture (and this includes readability) things are
> more complex. Please notice that I am not arguing in favor
> of using this kind of elses everywhere: I only question that
> semantic redundancy is a good reason to take a random `else'
> and remove it.

If "to take a random `else'" refers to me making the changes in
question then have you looked at the surrounding code?  In both cases
the bodies of the `then' clauses are very small (2 statements, one of
which is tracing).  In the first case very similar code 24 lines above
has exactly the same structure.

>                 Whence my original question about the meaning
> of "redundant else".