D Core Guidelines?

[Adam D Ruppe]

On Sunday, 3 September 2023 at 05:52:12 UTC, Walter Bright wrote:
> String concatenation tends to be highly useful in CTFE code, 
> and runtime performance considerations don't exist.

Well, it depends there too since CTFE string concat has 
absolutely terrible performance if you do a lot of it.

A small x ~ y in CTFE is virtually invisible, I'd be fine with 
that. But a loop of string ~= thing in ctfe quickly becomes a 
compile time killer - it is at least quadratic time.

Redoing that as a single pre-allocated char[] that is slice 
assigned can take very noticeable times off builds. For example, 
my jsvar.d used to have one that ballooned builds to about 6 
seconds. I rewrote it with a length calculation loop, single 
allocation, slice assign loop instead of a ~= loop, and it 
brought it back down to 2 seconds. (Still a bit slow, but 
obviously 2 is much faster than 6.)

Or my jni.d when running over android, it used to use a CTFE 
string.replace and it took *minutes* to compile along with dozens 
of gigabytes of ram.  Prerunning that, so it was just two 
separate string literals (yes, this ctfe use only ended up in two 
permutations when all was said and done) brought that back to the 
land of reality too.

So CTFE concat I almost think is *more* dangerous than runtime 
concat. At least the runtime concat has *consistently* decent 
performance.

That said tho, anybody reading this, if your strings are short, 
CTFE concat works perfectly well. Don't overcomplicate code 
unless the strings are long and there's an append loop, then you 
MIGHT notice the difference.