Re: Can’t use UFCS to create InputRange?
On Wednesday, 29 April 2020 at 12:23:11 UTC, Simen Kjærås wrote: Of course, nothing stops us from defining our own front, popFront and friends that combine the two approaches above: [...] We could conceivably add these to std.range.primitives (probably adding some constraints first), and suddenly UFCS ranges are possible! (I am as of yet not convinced that we should, though) -- Simen This is basically what C++ calls "argument-dependent lookup." Discussion about adding this sort of thing to D has come up before on the forums [1], and iirc Walter has generally been opposed to it. If it were to be added as a library feature, it would probably have to be opt-in. [1] https://forum.dlang.org/post/mailman.123.1472818535.2965.digitalmar...@puremagic.com
Re: Can’t use UFCS to create InputRange?
On Wednesday, 29 April 2020 at 09:16:58 UTC, user1234 wrote:
The static checker doesn't see your free funcs because to do so
it would have to import the whole module. (is it possible to do
that ? no idea.)
Of course it's possible! :) We can find the context of R (in this
case, the module) with __traits(parent), and import that:
mixin("import "~__traits(parent, R).stringof["module
".length..$]~";");
However, doing that in isInputRange doesn't help much. First, all
other range functions would have to do it, and second, just
importing into function scope doesn't enable UFCS lookup.
Also your signature for the primitives are quite unusual (i.e
not idiomatic). Usually they dont take param. Usually we pass a
type that contains the member funcs matching to IsIntputRange.
You can see a good counterexample to this in
https://dlang.org/library/std/range/primitives/pop_front.html,
which defines popFront for regular arrays. However, that is the
one and only counterexample I know of.
Of course, nothing stops us from defining our own front, popFront
and friends that combine the two approaches above:
template front(R) {
auto front(R r) {
return __traits(getMember, __traits(parent, R),
"front")(r);
}
}
template popFront(R) {
auto popFront(R r) {
return __traits(getMember, __traits(parent, R),
"popFront")(r);
}
}
template empty(R) {
auto empty(R r) {
return __traits(getMember, __traits(parent, R),
"empty")(r);
}
}
We could conceivably add these to std.range.primitives (probably
adding some constraints first), and suddenly UFCS ranges are
possible! (I am as of yet not convinced that we should, though)
--
Simen
Re: Can’t use UFCS to create InputRange?
On Wednesday, 29 April 2020 at 08:34:53 UTC, Ogi wrote:
struct R {}
int front(R r) { return 42; }
void popFront(R r) {}
bool empty(R r) { return false; }
void main() {
import std.range.primitives : isInputRange;
static assert(isInputRange!R);
}
Error: static assert: `isInputRange!(R)` is false
Whats really weird is that if I replace isInputRange with its
definition from std.range.primitives, it returns true:
import std;
struct R {}
int front(R r) { return 42; }
void popFront(R r) {}
bool empty(R r) { return false; }
void main() {
static assert(is(typeof(R.init) == R)
&& is(ReturnType!((R r) => r.empty) == bool)
&& is(typeof((return ref R r) => r.front))
&& !is(ReturnType!((R r) => r.front) == void)
&& is(typeof((R r) => r.popFront)));
}
This compiles.
What’s going on here?
The static checker doesn't see your free funcs because to do so
it would have to import the whole module. (is it possible to do
that ? no idea.)
Also your signature for the primitives are quite unusual (i.e not
idiomatic). Usually they dont take param. Usually we pass a type
that contains the member funcs matching to IsIntputRange.
Re: Can’t use UFCS to create InputRange?
On Wednesday, 29 April 2020 at 08:34:53 UTC, Ogi wrote:
struct R {}
int front(R r) { return 42; }
void popFront(R r) {}
bool empty(R r) { return false; }
void main() {
import std.range.primitives : isInputRange;
static assert(isInputRange!R);
}
Error: static assert: `isInputRange!(R)` is false
What’s going on here?
The template IsInputRange is in the std.range.primitives module,
and thus can't see the front, popFront and empty definitions in
your module.
--
Simen
Can’t use UFCS to create InputRange?
struct R {}
int front(R r) { return 42; }
void popFront(R r) {}
bool empty(R r) { return false; }
void main() {
import std.range.primitives : isInputRange;
static assert(isInputRange!R);
}
Error: static assert: `isInputRange!(R)` is false
Whats really weird is that if I replace isInputRange with its
definition from std.range.primitives, it returns true:
import std;
struct R {}
int front(R r) { return 42; }
void popFront(R r) {}
bool empty(R r) { return false; }
void main() {
static assert(is(typeof(R.init) == R)
&& is(ReturnType!((R r) => r.empty) == bool)
&& is(typeof((return ref R r) => r.front))
&& !is(ReturnType!((R r) => r.front) == void)
&& is(typeof((R r) => r.popFront)));
}
This compiles.
What’s going on here?
