DIP idea: q{}-inspired block mixins

[Q. Schroll]

On Thursday, 5 November 2020 at 12:51:38 UTC, Jacob Carlborg 
wrote:
> Isn't it quite rare to have the mixin location and the string, 
> to be mixed in, at the same place?

I'd say mixin(generate(args)) and mixin("..pattern..", hole, 
"..pattern..") are currently the two standard uses of string 
mixins. While the first is perfectly fine, the latter lacks 
readability greatly, since **code to be read is in string 
literals** where it shouldn't be. In the first use-case, generate 
may stitch together the generated code in a way that might be 
hard to follow, but that's hard to improve. My proposal makes 
code, well, code. Interpolated strings don't help that either. 
Interpolated strings' mission is to generate strings which has 
its merits and use-cases well beyond string mixins.

To convince myself I've just run

     grep 'mixin(.*"' *.d

and

     grep 'mixin.*q{' *.d

over Phobos (std/) and got 169 and 10 results respectively. (This 
search is surface-level but finds good candidates.) I reviewed 
every one of them manually and found 133 of 169 and 8 of 10 would 
probably have been written with my proposed syntax. Since Phobos 
is a meta-programming heavy library, I'd consider it a 
representative target, as mixin is without doubt a 
meta-programming tool. To be fair, 78 of the 133 matches follow 
one of the operator mixin patterns ("lhs" ~ op ~ "rhs" or 
op~"value") which is probably the most common and obvious 
use-case.