On Wednesday, 29 July 2015 at 20:41:02 UTC, Tofu Ninja wrote:
On Wednesday, 29 July 2015 at 20:26:53 UTC, Tofu Ninja wrote:
If you write:
@satisfies!(isInputRange, MyRange) struct MyRange { ... }
the UDA can check it self, it really works as expected. Which
is why I suggested a way to get whatever the UDA is attached
to automatically in my other post.
A UDA can reference the thing it's being attached to with no
problems.
For example, this works 100% as expected right now...
@UDA!testS struct testS // UDA fails to instantiate because
testS is not an inputRange!
{
}
template UDA(alias a)
{
import std.range;
static assert(isInputRange!a);
}
The only thing that would be needed to make this a nice
solution is some syntax sugar to automatically get whatever the
UDA is attached to, which is why I suggested this:
template UDA(alias a = __UDA_ATTACHMENT__) { ... }
You still wouldn't get a better error message here than with:
struct MyRange {
...
static assert(isInputRange!MyRange);
}
It's less typing, but you still wouldn't know why the static
assertion failed. Now, if we make Adam's idiom in my
aforementioned pull request the go-to thing, then this would be
good:
@satisfies!(checkInputRange, MyRange) struct MyRange { ... }
There's still the problem of having two names for each
constraint: checkInputRange and isInputRange. But... we could
also establish the convention of checkXXX and use string mixins
to turn this:
@satifies!(isInputRange, MyRange)
into (which works cos I just tried it):
template satisfies(alias Constraint, R) {
enum check = "check" ~ Constraint.stringof[2..$-3] ~ "!(R)";
enum assert_ = "static assert("~ check ~ ");";
mixin(assert_); //mixes in "static assert(checkInputRange!R)"
}
@satisfies!(isInputRange, Zeroes)
struct Zeroes {
enum empty = false;
void popFront() {}
@property int front() { return 0; }
}
Now, _this_ I could go for.
Atila
