Template type deduction question

Basile B. b2.temp at gmx.com
Thu May 21 07:30:27 UTC 2020


On Thursday, 21 May 2020 at 07:16:11 UTC, Basile B. wrote:
> 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

I've found that even with a template constraint the diagnostic is 
sub-optimal, see issue  20851 [1].

[1] https://issues.dlang.org/show_bug.cgi?id=20851


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