Against enforce()

[Andrei Alexandrescu]

On 3/18/11 11:07 AM, Steven Schveighoffer wrote:
> On Fri, 18 Mar 2011 11:35:22 -0400, spir <denis.spir at gmail.com> wrote:
>
>> On 03/18/2011 01:37 PM, Steven Schveighoffer wrote:
>>> This is a good example of why it's difficult to decide what "user
>>> input" is.
>>> One could consider that the 'user' in this case is the developer
>>> using the
>>> library, but I don't think that's the right choice.
>>>
>>> I'd say it's a bug, this is clearly a contract, since the data being
>>> passed
>>> into the ctor can easily not be user input (i.e. it's most likely two
>>> literals
>>> that will never depend on a user). If it is user input, the caller of
>>> the ctor
>>> should enforce the user input before passing it to iota.
>>
>> This is indeed a difficult topic. I'm a bit bluffed when reading
>> people confidently asserting apparently clear positions about the use
>> of enforce vs assert vs contracts and such, or whether such checks
>> should or not stay or not in various distribution builds (mainly
>> -release).
>> I can see at least 5 cases, and am far to be sure what the proper tool
>> is in every case, and in which builds it should stay. In each case,
>> there is potential "wrong" input; but note the variety of cases does
>> seems orthogonal (lol) to what kind of harm it may cause:
>>
>> * colleague: my code is called by code from the same app (same dev
>> team); typically, wrong input logically "cannot" happen
>> * friend: my code is called by code designed to cooperate with it;
>> there is a kind of moral contract
>> In both cases, wrong input reveals a bug; but in the first case, it's
>> my own (team's) bug. I guess, but am not sure, these cases are good
>> candidates for asserts (or contracts?), excluded from release build.
>>
>> * lib call: my code is a typical lib; thus, I have zero control on
>> caller.
>> I would let the check in release mode, thus use enforce. Or, use
>> assert if it remains when the *caller* is compiled in debug mode.
>> There is something unclear here, I guess. Maybe there are two sub-cases:
>> ~ the caller logically should be able to prove its args correct
>> ~ or not
>
> See, this is where I feel we have issues. The clear problem with
> *always* checking is the iota example. One may use iota like this:
>
> foreach(i; iota(0, 5))
>
> Why should checks in iota remain for iota to prove that 0 is less than
> 5? It always will be less than 5, and the check is not necessary.
[snip]

This is the kind of job that the compiler could and should do. Whether 
it's assert and enforce, an inlining pass followed by value range 
propagation should simply eliminate the unnecessary tests.


Andrei