Just another example of missing string interpolation

[Walter Bright]

On 10/18/2023 1:27 PM, Adam D Ruppe wrote:
> On Wednesday, 18 October 2023 at 03:57:52 UTC, Walter Bright wrote:
>> What does writeln() do with __header?
> 
> "The __header value, to be discussed later, is generated by the compiler and 
> contains compile-time information about the interpolated string."
> 
> <a few moments later>
> 
> "The header object has a trivial toString method that expands to the null 
> string. This method makes it possible to pass interpolated strings directly to 
> functions such as writeln and text because these functions detect and use 
> toString to convert unknown data types to strings."
> 
> It just uses the existing rules for a struct. No special case here. It doesn't 
> spell this out, but you can think about what happens to normal D functions if 
> you pass some `struct` to a function expecting a `string` - an ordinary type 
> mismatch error. This is what gives library authors such power with this 
> proposal: they can overload a function to provided specialized behavior.

Ok.

The lowering to arguments to InterpolatedExpression seem to all be string 
versions of the argument identifiers, rather than what the types are.

"the header gives the callee complete access to the strings corresponding to the 
expressions passed in."

I don't know what that is useful for. Note that if write() is a template, it 
already has access to all the arguments and their types.


> Library authors overload functions on the type of what was passed, including an 
> interpolated type if desired, same as any other overload. End users call a 
> particular function uses the given arguments to create their desired result; a 
> generic function forwarder still works, preserving the full interpolated 
> argument, since it works like any other set of function arguments.
> 
> This gives *enormous* capability to both sides - mostly by using already 
> existing features in the D language.

I'd like to see an example of code of how this customization might work, because 
I don't see it.

P.S. there are several errors in the document, such as '$' appearing in the 
lowered code. What "normalization" means is unclear.