[phobos] Pureness of enforce()

[Andrei Alexandrescu]

On 11/9/10 7:15 AM, Robert Jacques wrote:
> On Tue, 09 Nov 2010 06:21:38 -0500, Lars Tandle Kyllingstad
> <lars at kyllingen.net> wrote:
>
>> enforce(x) is basically a substitute for
>>
>> if (x) throw new Exception;
>>
>> which doesn't violate the purity rules. It is not possible to annotate
>> enforce() with 'pure', however, because it takes a lazy parameter, which
>> is just shorthand for a (possibly impure) delegate.
>>
>> enforce() is used virtually everywhere in Phobos, and this means that a
>> lot of functions that could otherwise be marked as pure, currently
>> can't.
>>
>> One example is std.conv.to(), which should definitely be pure. (In
>> fact, I tried marking it as such, which is what got me thinking about
>> this in the first place.)
>>
>> What to do?
>>
>> -Lars
>
> Well, long term, we need modifiers to apply to delegates. i.e. it should
> be possible to declare a pure delegate. (Or a const delegate. Or an
> immutable delegate. Or a shared delegate, etc.) For example, right now,
> it's not possible to (type-) safely use a delegate in shared code. And
> David's std.parallelism is a prime case where many methods could be
> marked @safe instead of @here_be_dragons if delegate modifiers existed.

I agree. For the time being, if pressed to choose between @pure and 
lazy, I'd choose the latter. Use of enforce() in Phobos is probably not 
representative for application code, where error messages are more 
elaborate and may be loaded from a string table etc.

Andrei