Tuples, CTFE, and Sliding Template Arguments

[Steven Schveighoffer]

On Saturday, 13 January 2024 at 02:16:06 UTC, Walter Bright wrote:
> On 1/12/2024 4:15 PM, Steven Schveighoffer wrote:
>> I don't view this as simpler than DIP1036e or DIP1027 -- a 
>> simple transformation is a simple transformation.
>
> Adding extra hidden templates isn't that simple. If a user is 
> not using a canned version, he'd have to be pretty familiar 
> with D to be able to write his own handler.

Yes, that is intentional. You should not be able to call 
functions with new syntax because the parameters happen to match. 
We have a type system for a reason.

> 1027 is simpler in that if the generated tuple is examined, it 
> matches just what one would have written using a format. 
> Nothing much to learn there.

In other words: "it matches just what one wouldn't have written, 
unless one is calling `writef`".

>> Certainly a hybrid DIP1027 with a format string passed at 
>> compile time is still not viable, due to reasons already 
>> stated.
>
> The other reasons:
>
> 1. preventing calls to functions passing an ordinary string as 
> opposed to an istring tuple

I don't see how this proposal fixes that. I'm assuming a function 
like `void foo(string s, int x)` will match `foo(i"something: 
$(1)")`

> 2. preventing nested istrings

Why do we want to prevent nested istrings? That's not a goal.

> have already been addressed. The compile time thing was the 
> only one left.

A compile time format string still needs parsing. Why would we 
want to throw away all the work the compiler already did?

If you want to call `writef`, you can construct a format string 
easily at compile time. Or just... call `writef` the normal way.

Compile-time string parsing is way more costly than compile-time 
string concatenation.

> > This does seem like it has the potential to break code:
>
> The shifted one would be a more costly match, and so the legacy 
> others would be favored first.

Ok. This does mean, for *intentional* overloading of a function 
to accept a compile-time first parameter, you will have to rename 
the function.

Possibly, if you have an opt-in syntax like Timon mentioned, then 
you can make the sliding template parameter more preferable.

-Steve