Wanted: Format character for source code literal

[Paul Backus]

On Wednesday, 5 May 2021 at 08:46:05 UTC, Berni44 wrote:
> What is the difference between "generic" (which as far as I 
> understand you oppose) and adding `%D` for bool, integers, 
> floats, characters, strings, arrays and AAs (which you sound as 
> being OK with, and which is, what I plan to do)?
>
[...]
>
> `%D` has currently no meaning, so we cannot change it; we can 
> just add it.

`%D` *does* currently have a meaning, though. It means "custom 
format specifier."

Here's the scenario that could potentially lead to trouble:

1. Some existing library uses `%D` as a custom format specifier 
in their `toString` methods, with a meaning other than "format as 
D source code."

2. `%D` is added to `std.format` with the meaning "format as D 
source code," and a default implementation for types that do not 
have custom `toString` methods.

3. A new library is written that takes advantage of (2) and uses 
`%D` in generic code to format arbitrary values for the purpose 
of code generation.

4. Someone uses the library from (1) and the library from (3) in 
the same project, and library (3) ends up producing garbage, 
because library (1)'s `%D` doesn't work the way library (3) 
expects it to.

The "correct" place to fix this is in library (1), but doing so 
would require a breaking change. In practice, this means that 
libraries like the one in (3) will never be able to completely 
rely on the new standard for `%D`, and will always have to 
include some kind of workaround in case they are used with types 
like the ones in library (1).