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digitalmars.D.learn - function type parameter inference not working

reply XavierAP <n3minis-git yahoo.es> writes:
It's not working for my case, while I see no special reason why 
it couldn't. Also I can't find specific inference rules at 
http://dlang.org/spec/function.html#function-templates

Is it a problem that the types to be inferred are in turn also 
templated? Any workaround that can make inference work? Otherwise 
I would re-consider my design rather than having to specify types 
already available in the runtime arguments :(

void assembleMass1D(Mat, Vec)(ref Mat M, const ref Vec x)
{ /* ... */ }
Matrix!(2,2) M = /* ... */;
Vector!2     V = /* ... */;
assembleMass1D(M, V); // ERROR template cannot deduce function 
from argument types
Apr 23
parent reply ag0aep6g <anonymous example.com> writes:
On 04/23/2017 09:33 PM, XavierAP wrote:
 void assembleMass1D(Mat, Vec)(ref Mat M, const ref Vec x)
 { /* ... */ }
 Matrix!(2,2) M = /* ... */;
 Vector!2     V = /* ... */;
 assembleMass1D(M, V); // ERROR template cannot deduce function from
 argument types
Please post self-contained code. When I fill the gaps you left, it works for me: ---- struct Matrix(uint a, uint b) {} struct Vector(uint a) {} void assembleMass1D(Mat, Vec)(ref Mat M, const ref Vec x) { /* ... */ } void main() { Matrix!(2,2) M; Vector!2 V; assembleMass1D(M, V); /* no error */ } ----
Apr 23
next sibling parent XavierAP <n3minis-git yahoo.es> writes:
On Sunday, 23 April 2017 at 19:40:39 UTC, ag0aep6g wrote:
 Please post self-contained code. When I fill the gaps you left, 
 it works for me:
Interesting, thanks a lot. I'll test and narrow down what's in my code preventing this from working (I can't really think of anything) and I'll report back.
Apr 23
prev sibling parent reply XavierAP <n3minis-git yahoo.es> writes:
On Sunday, 23 April 2017 at 19:40:39 UTC, ag0aep6g wrote:
 Please post self-contained code. When I fill the gaps you left, 
 it works for me:
Found it! It stops working (DMD v2.073.0 for Windows) if it has to infer the type of a temporary local variable -- constructed in place of the argument: struct Matrix(size_t nr, size_t nc) {} struct Vector(size_t n) {} void assembleMass1D(Mat, Vec)(ref Mat M, const ref Vec x) { /* ... */ } Matrix!(2,2) M; Vector!2 V; assembleMass1D(M, V); // OK assembleMass1D(M, Vector!2()); // ERROR template cannot deduce function Is this a bug?
Apr 25
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 04/25/2017 12:19 PM, XavierAP wrote:

 void assembleMass1D(Mat, Vec)(ref Mat M, const ref Vec x)
 { /* ... */ }

 Matrix!(2,2) M;
 Vector!2     V;
 assembleMass1D(M, V);          // OK
 assembleMass1D(M, Vector!2()); // ERROR template cannot deduce function


 Is this a bug?
This is an intentional limitation of D. It's not possible to bind rvalues (temporaries) to reference parameters. The best option here is 'auto ref': void assembleMass1D(Mat, Vec)(auto ref Mat M, auto ref const(Vec) x) { /* ... */ } Ali
Apr 25
parent reply XavierAP <n3minis-git yahoo.es> writes:
On Tuesday, 25 April 2017 at 19:57:30 UTC, Ali Çehreli wrote:
 This is an intentional limitation of D. It's not possible to 
 bind rvalues (temporaries) to reference parameters. The best 
 option here is 'auto ref':
Aha I had forgotten about the ref (obviously, otherwise I wouldn't have passed a temporary even in the unit test -- I'm embarrassed). If that's the reason why it doesn't work I'm satisfied. It would be helpful if the error message talked about the ref qualifier as the root cause instead of the consequent failure to infer template parameters.
Apr 25
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 04/25/2017 01:02 PM, XavierAP wrote:

 It would be helpful if the error message talked about the ref qualifier
 as the root cause instead of the consequent failure to infer template
 parameters.
That's a common request but the solution is not implemented yet because it's not trivial if you consider multiple overloads. The compiler must remember which parameters failed and present it in a readable format: "I considered these overloads and this failed for parameter 1 and this other one failed for parameter 2 (although it might succeed if it were not ref), etc." Ali
Apr 25