cannot register generic function with template.Funcs

[Rory Campbell-Lange]

Hi. I have a generic function

func slicerG[A any](s []A, i int) []A {
fmt.Println(len(s), i)
if i > len(s) {
return s
}
return s[0:i]
}

When I try to register this in a template's FuncMap, it fails with

"cannot use generic function slicerG without instantiation"

https://go.dev/play/p/KpvrbbcTaM2

I guess this makes sense if the compiler is converting the function to function variants of particular types based on instantiation, but is there any way around this?

Thanks,
Rory

[Bruno Albuquerque]

You can easily get it to work, but not in a useful way I guess.

https://go.dev/play/p/mq0O8OhJ1-a

You could have a different template for each type you use I guess.

-Bruno

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[Rory Campbell-Lange]

On 06/03/23, Bruno Albuquerque (b...@gmail.com) wrote:
> You can easily get it to work, but not in a useful way I guess.
>
> https://go.dev/play/p/mq0O8OhJ1-a
>
> You could have a different template for each type you use I guess.

Thanks for that solution. I guess I could do

t.Funcs(template.FuncMap{
"slicerInt" : slicerG[int],
"slicerStr" : slicerG[string],
...
})

which is at least better than rewriting the function for each type.

Thanks again,
Rory

[Axel Wagner]

No, Go generics do not exist at runtime, so there is no way for this to possibly work in generality.

The best you can do is make a function that takes an `any` or a `reflect.Value` and use `reflect` to type-check at runtime, returning an error if type-checking fails: https://go.dev/play/p/c0jgBMxGdGq

Given that templates are necessarily type-checked at runtime and have to be rendered by the template using reflect anyways, the only downside is that the code is somewhat less readable.