Template type deduction question

Basile B. b2.temp at gmx.com
Thu May 21 07:16:11 UTC 2020


On Thursday, 21 May 2020 at 04:46:02 UTC, data pulverizer wrote:
> I'd like to pass kernel functions using:
>
> ```
> auto calculateKernelMatrix(K, T)(K!(T) Kernel, Matrix!(T) data)
> {
>   ...
> }
>
> ```
>
> and call it using `calculateKernelMatrix(myKernel, myData);` 
> but I get a type deduction error and have to call it using 
> `calculateKernelMatrix!(typeof(myKernel), float)(myKernel, 
> myData);`
>
> How do I resolve this?

The problem is that "K" is a template type parameter [1].
When the compiler deduces the parameter that ends up with a 
symbol, i.e not a type.
To permit a symbol to be deduced you can use a template alias 
parameter[2] instead:

---
struct Matrix(T) {}
struct Kernel(T) {}

void calculateKernelMatrix(alias K, T)(K!T kernel, Matrix!T data) 
{ }

void main()
{
     Matrix!float m;
     Kernel!float k;
     calculateKernelMatrix(k,m); // OK
}
---

However I think that there could be an useful error message, as 
the current one is of no usefulness. Note that maybe that under 
the hood there's one but the compiler uses a system of gagging 
when trying uncertain operations, so that other things can be 
tried, in case of failure.

[1] https://dlang.org/spec/template.html#template_type_parameters
[2] https://dlang.org/spec/template.html#aliasparameters




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