Discussion Thread: DIP 1037--Add Unary Operator ...--Community Review Round 1

[Andrei Alexandrescu]

On 10/29/20 9:48 AM, Steven Schveighoffer wrote:
>  From the Feedback Thread:
> 
>> * Major bug: the "Rationale" discusses only expression, whereas
>> staticMap does not use expressions. It just processes tuples, which may
>> contain types. Types cannot appear in expressions. C++ goes to great
>> lengths to distinguish between template parameter packs (which may be
>> one of type parameter pack, value parameter pack, and template template
>> parameter pack) and function parameter packs (which may only be
>> parameter declarations). By the Rationale nothing except expressions
>> will be accessible to D's proposed "...". That means no staticMap for
>> non-valies (e.g. staticMap!(Unqual, types)), which probably wasn't the
>> intent of the DIP.
> 
> I am confused by this. Isn't it fine to work with expressions only?
> 
> I mean staticMap's implementation can be replaced with a ... expansion, 
> because even though there might be types in the list, there are no types 
> in staticMap, just tuples.

I don't think it works that way, no.

> And a type can appear in an expression, as a template parameter:
> 
> alias everyOther = AliasSeq!(0, 2, 4);
> 
> alias everyOtherType = (AliasSeq!(int, uint, char, byte, ubyte, 
> int)[everyOther])...;
> 
> I think the DIP is intending for staticMap to be wholesale replaced with 
> this, and seems to be saying this to me. I don't understand the above 
> complaint, and why static map wouldn't work with types.

What would the implementation of staticMap with the DIP look like? As 
far as I can tell from the DIP it cannot be done.

> Perhaps it's simply vocabulary that is the problem?

May as well. The way the DIP is written, there's no discussion of tuples 
containing anything else but values.