Discussion Thread: DIP 1036--String Interpolation Tuple Literals--Community Review Round 2

[Andrei Alexandrescu]

> Try to please everybody and you end up pleasing nobody. 
...
> DIP1027 works with writef, and I consider that to be the goal, not printf.

Unpopular opinion follows (at this point I have no idea which side of 
the debate this puts me on).

I think working with either writef or printf is a bit of a side quest, 
and to the extent it affects the fundamental design it becomes a nongoal.

The printf family (which includes writef, too) has a fundamental 
characteristic: the formatting specification are SEPARATE from the data. 
You pass the formatting template, you pass the data, and the function 
puts them together and produces a string nice. This way of doing things 
has pluses and minuses that are well known.

Interpolated strings fundamentally do the converse: they MIX the 
formatting specification with the data. You see bits of literal strings 
mixed with pieces of data. That, too, has pluses and minuses.

 From that vantage point, I look at the effort of supporting 
{print,write}f in interpolated strings leading to any number of 
contortions and come and ask: cui prodest? Indeed, cui the heck prodest?

The main client of string interpolation by A MILE, is code generation. 
Whether it's D code, SQL, your own embedded language - that's the ONE 
use case that any DIP proposing this should drive with. Supporting 
(print|write)f is some weird invention - I already have these functions 
that separate formatting from data, why do I want to transform the 
converse way of doing things into this?

But wait! one might say in response. What if I need on occasion some 
special formatting of one argument in one of them nice interpolated 
strings? No problem, I say. Do what Perl, PHP, and Python programmers 
have done for millenia - apply the formatting function separately for 
the desired argument:

i"Hello, ${format(`%45s`, var)} world"!

That's how the cookie crumbles. To have to pay syntactically and in 
complexity WHENEVER I EVER use an interpolated string on the off chance 
that I might need special formatting for ONE argument is unconscionable.

Define interpolated strings to be simple, cheap, obvious, and well fit 
for their MAIN USE CASE, which is code generation.