Discussion Thread: DIP 1033--Implicit Conversion of Expressions to Delegates--Community Review Round 1

[Steven Schveighoffer]

On 4/23/20 7:13 AM, Patrick Schluter wrote:
> On Thursday, 23 April 2020 at 09:33:35 UTC, aliak wrote:
>> On Wednesday, 22 April 2020 at 11:56:44 UTC, kinke wrote:
>>> I don't think this is a good idea. One of the problems with `lazy` 
>>> IMO is that one needs good IDE support to somehow highlight that the 
>>> argument expression isn't evaluated before the call, and might not be 
>>> evaluated at all:
>>>
>>> // in some other module:
>>> void foo(lazy int i) {}
>>>
>>> void main()
>>> {
>>>     int i = 1;
>>>     foo(++i);
>>>     assert(i == 1); // what? not incremented above? ah yeah, lazy and 
>>> not evaluated at all by foo
>>> }
>>>
>>> The proposal would extend that problem to regular delegate params 
>>> too. So I find the explicit version (`() => ++i`) better to make that 
>>> clear. I don't think the 'overhead' of putting an additional `() =>` 
>>> in front of the expression justifies introducing the proposed 
>>> implicit convertibility.
>>
>> I agree with this. Implicit conversion has always been argued against 
>> in D - why is it ok now? And this is not the good kind of implicit 
>> conversion where the user explicitly asks for it, but the bad kind 
>> where the language decides it should happen and the user doesn't know 
>> it can even happen.
>>
>> Thinking about it more now feels like the real rationale for the DIP 
>> is "in order to deprecate lazy, we need an alternative" ... ???
> 
> I always thought that lazy should also be marked at the call site as it 
> is a very different semantic on the caller and on the callee. As it is 
> now, it is true that the example above is unfortunate and a real source 
> of unpleasant surprizes.
> 
> 
> void foo(lazy int i) {}
> 
>   void main()
>   {
>       int i = 1;
>       foo(lazy ++i);
>       assert(i == 1); // No surprize here, the required annotation tells 
> it.
>   }
> 

If lazy binds to delegates then this just becomes

foo(() => ++i);

And then we can require lazy to require a delegate at the call site, and 
then get rid of lazy.

-Steve