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April 28, 2017 get parameters of a function | ||||
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Hi all, I have a question about the Parameters trait from https://dlang.org/phobos/std_traits.html#Parameters The following code does not compile. Why? import std.traits : Parameters; void main() { static assert(is(Parameters!S1 == Parameters!S2)); } struct S1 { auto opCall() {} auto opCall(int i){} } struct S2 { auto opCall(int i){} auto opCall() {} } So, of course, if I change the both opCall methods, e. g. in S1 it will. But I'm wondering, how to use the trait in this case? Is it mainly assumed to use it with functions without overloads? |
April 29, 2017 Re: get parameters of a function | ||||
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Posted in reply to Alex | On Friday, 28 April 2017 at 20:43:50 UTC, Alex wrote:
> Hi all,
> I have a question about the Parameters trait from
> https://dlang.org/phobos/std_traits.html#Parameters
>
> The following code does not compile. Why?
> Is it mainly assumed to use it with functions without overloads?
Rather, it is to be used on a concrete overload.
If you want to check if two structs are callable with the same parameters, you'll need to iterate opCall overloads for both and look for a match, e.g. like this:
void main()
{
foreach(o1; __traits(getOverloads, S1, "opCall"))
{
alias P1 = Parameters!o1;
foreach(o2; __traits(getOverloads, S2, "opCall"))
{
alias P2 = Parameters!o2;
static if (is(P1 == P2))
{
pragma(msg, "Matching params: "~P1.stringof);
}
}
}
}
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April 29, 2017 Re: get parameters of a function | ||||
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Posted in reply to Stanislav Blinov | On Saturday, 29 April 2017 at 05:58:35 UTC, Stanislav Blinov wrote:
> On Friday, 28 April 2017 at 20:43:50 UTC, Alex wrote:
>
> void main()
> {
> foreach(o1; __traits(getOverloads, S1, "opCall"))
> {
> alias P1 = Parameters!o1;
> foreach(o2; __traits(getOverloads, S2, "opCall"))
> {
> alias P2 = Parameters!o2;
> static if (is(P1 == P2))
> {
> pragma(msg, "Matching params: "~P1.stringof);
> }
> }
> }
> }
The problem is another one: say I have something like this:
import std.traits;
struct A(alias T) if(isCallable!T)
{
auto opCall(U...)(U args) if(is(Parameters!T == U)) //if(__traits(compiles, T(args)))
{
return T(args);
}
}
void main()
{
S s;
A!s c;
assert(c(4) == 42);
}
struct S
{
auto opCall(){ return 42; }
auto opCall(int i){return 42; }
}
This doesn't work because of Parameters trait and the both opCalls defined.
However, I see the point...
If I take the compiles-trait, it works as expected:
In this case the constrained is fulfilled, and with the second opCall commented out - it isn't.
Thanks :)
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April 29, 2017 Re: get parameters of a function | ||||
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Posted in reply to Alex | On Saturday, 29 April 2017 at 06:18:34 UTC, Alex wrote: > The problem is another one: say I have something like this: > > import std.traits; > > struct A(alias T) if(isCallable!T) > { > auto opCall(U...)(U args) if(is(Parameters!T == U)) //if(__traits(compiles, T(args))) > { > return T(args); > } > } > > void main() > { > S s; > A!s c; > assert(c(4) == 42); > } > > struct S > { > auto opCall(){ return 42; } > auto opCall(int i){return 42; } > } > > This doesn't work because of Parameters trait and the both opCalls defined. > However, I see the point... > If I take the compiles-trait, it works as expected: > In this case the constrained is fulfilled, and with the second opCall commented out - it isn't. > > Thanks :) Ah, that calls for something like a isCallableWith template. Pay extra care to how you pass parameters in that variadic opCall template though. Doing it like in the code above will pass everything by value, even though the original S.opCall might've expected a reference. To avoid that, auto ref + std.functional.forward can be used. The following code illustrates the difference. import std.traits; import std.functional : forward; /** Evaluates true if `func` is callable with arguments `args`. Can only be used as a function template constraint. */ enum isCallableWith(alias func, args...) = is(typeof(func(forward!args))); // Your original A struct A(alias func) if (isCallable!func) { auto opCall(Args...)(Args args) if (__traits(compiles, func(args))) { // all arguments are copied, any "ref/out" semantics are lost return func(args); } } // Modified to forward arguments struct B(alias func) if (isCallable!func) { // auto ref: preserve lvalues auto opCall(Args...)(auto ref Args args) if (isCallableWith!(func, args)) { // forward: preserve lvalues return func(forward!args); } } struct S { auto opCall() { return 42; } auto opCall(int i) { return 42; } // Note: parameter passed by reference! auto opCall(out float f) { f = 42.0f; } } void main() { S s; { A!s c; int i = 4; static assert(is(typeof(c(i)))); static assert(is(typeof(c(4)))); static assert(is(typeof(c()))); assert(c(i) == 42); assert(c(4) == 42); assert(c() == 42); // Should pass but doesn't. //static assert(!is(typeof(c(3.5f)))); float f; static assert(is(typeof(c(f)))); c(f); // Will assert: assert(f == 42.0f); } { B!s c; int i = 4; static assert(is(typeof(c(i)))); static assert(is(typeof(c(4)))); static assert(is(typeof(c()))); assert(c(i) == 42); assert(c(4) == 42); assert(c() == 42); // Passes static assert(!is(typeof(c(3.5f)))); float f; static assert(is(typeof(c(f)))); c(f); // Won't assert assert(f == 42.0f); } } |
April 29, 2017 Re: get parameters of a function | ||||
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Posted in reply to Stanislav Blinov | On Saturday, 29 April 2017 at 08:15:06 UTC, Stanislav Blinov wrote: > Ah, that calls for something like a isCallableWith template. Pay extra care to how you pass parameters in that variadic opCall template though. Doing it like in the code above will pass everything by value, even though the original S.opCall might've expected a reference. To avoid that, auto ref + std.functional.forward can be used. :) > The following code illustrates the difference. > > import std.traits; > import std.functional : forward; > > /** > Evaluates true if `func` is callable with arguments `args`. > Can only be used as a function template constraint. > */ > enum isCallableWith(alias func, args...) = is(typeof(func(forward!args))); // That's cool! Much shorter, as I thought. > > // Your original A > struct A(alias func) if (isCallable!func) > { > auto opCall(Args...)(Args args) if (__traits(compiles, func(args))) > { > // all arguments are copied, any "ref/out" semantics are lost > return func(args); > } > } > > // Modified to forward arguments > struct B(alias func) if (isCallable!func) > { > // auto ref: preserve lvalues > auto opCall(Args...)(auto ref Args args) if (isCallableWith!(func, args)) > { > // forward: preserve lvalues > return func(forward!args); > } > } > > struct S > { > auto opCall() { return 42; } > auto opCall(int i) { return 42; } > // Note: parameter passed by reference! > auto opCall(out float f) { f = 42.0f; } > } > > void main() > { > S s; > > { > A!s c; > int i = 4; > static assert(is(typeof(c(i)))); > static assert(is(typeof(c(4)))); > static assert(is(typeof(c()))); > > assert(c(i) == 42); > assert(c(4) == 42); > assert(c() == 42); > // Should pass but doesn't. > //static assert(!is(typeof(c(3.5f)))); > float f; > static assert(is(typeof(c(f)))); > c(f); > // Will assert: > assert(f == 42.0f); > } > > { > B!s c; > int i = 4; > static assert(is(typeof(c(i)))); > static assert(is(typeof(c(4)))); > static assert(is(typeof(c()))); > > assert(c(i) == 42); > assert(c(4) == 42); > assert(c() == 42); > // Passes > static assert(!is(typeof(c(3.5f)))); > float f; > static assert(is(typeof(c(f)))); > c(f); > // Won't assert > assert(f == 42.0f); > } > } Ha, that is exactly what I am after :) Thanks a lot! |
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