DIP 1027--String Interpolation--Final Review Discussion Thread

[Dennis]

On Wednesday, 5 February 2020 at 05:26:30 UTC, Walter Bright 
wrote:
> The reason for formatted strings is for formatting that is 
> other than the default, and to interface with common functions 
> that use formatted strings.

So the proposed design makes it impossible for a library to 
verify arguments are correctly corresponding to format 
specifiers, not even at runtime. The programmer is expected to 
know the format conventions of the function.

Considering that, what is the purpose of putting the format 
specifier inside brackets {}?
Take for example this format string using range formatting 
specifiers from Phobos:

```
byte[] arr = [0x10, 0x1A, 0x1F];
writefln("arr = ${%(%X; %)}arr"); // entire format specifier in {}
writefln("arr = %(${%X}arr; %)"); // only element specifier in {}
```

The %( %) format specifiers are not the most beautiful to begin 
with, but the mandatory {} make it only worse. I would prefer to 
write:
```
writefln("The array is %(%s; %)$arr");
```

It's not like the {} add any delimiting / error checking of any 
kind, so I might as well put the format specifier in the string 
myself. This would also be required if the format string itself 
contained unbalanced {} in them, since no means of escaping { is 
provided.

But this still would not work with the current proposal, since a 
stray %s will be written into the string because of the absence 
of {}. Which, as mentioned before, is a direct contradiction with 
the design goal "the language does not now anything about format 
specifiers".

One idea is to just write nothing instead of %s, and removing the 
{}, which would be less typing and remove the hard-wired Phobos 
convention from the language.

Of course, with Steven/Adam's proposal, this can be made to work:
```
writefln("arr = ${X; }arr");
```

Here the {} can actually do the delimiting instead of %( and %), 
and possibly do error checking as well!

> Strings and expressions are stuck with the default format. If 
> you're happy with the default formatting,
>
>     writefln("I want $i bananas");
>
> is not much of any improvement over:
>
>     writeln("I want ",i," bananas");

That second one is more prone to mistakes with whitespace, e.g.:

> text("a is", a, ", b is ", b, " and the sum is: ", a + b);

From: 
https://forum.dlang.org/post/jahvdekidbugougmyhgb@forum.dlang.org