On 2/14/20 1:41 AM, cc wrote:
import std.meta;
enum A = AliasSeq!(1, 2, 3, 4);
THREELOOP: static foreach (idx, field; A) {
static if (field == 3) {
pragma(msg, "Got a 3!");
break THREELOOP;
}
static if (idx == A.length - 1) {
static assert(0, "Got no 3...");
}
}
What I'd like to achieve in this example is for compilation to fail if
the given tuple doesn't contain a matching item. Is there an easy way
to do this? Trying to break out of the static foreach gives me
Error:enclosing label `THREELOOP` for `break` not found
In this reduced example, I was able to accomplish this by setting some
enum within the successful comparison body and then checking e.g. static
if (!__traits(compiles, FOUND)) but this breaks down once I start
dealing with multiple levels of scope.
static foreach does not support break. Partly because the places where
static foreach is allowed do not allow break statements. In your example
above, if you are not in a function context, a break statement is not
allowed. Note that a naked break statement inside a static foreach
that's inside e.g. a switch statement needs a label to ensure the user
understands they are breaking the switch, not the static foreach.
foreach does allow breaks. If you use foreach on an AliasSeq, it will
work. But again, you must be in a function context.
But remember that static foreach and foreach implement all the bodies
given. So every single one has to be compiled, even if it's not used.
This can cause problems for things like return statements that make
further code not executable.
AND static foreach doesn't introduce a new scope (foreach does). Other
than recursive templates, I can't think of a great way to do this. Your
enum solution has limits, as you say.
-Steve
