Storing a lambda alongside type-erased data

[Paul Backus]

On Thursday, 8 September 2022 at 03:18:08 UTC, Ali Çehreli wrote:
> I looked at how std.variant.VariantN prints the correct type 
> and failed to understand the magic there. :(
>
> Then I came up with storing a lambda that is created when the 
> exact type is known. The following simple variant can carry 
> arbitrary set of data because the data is provided as sequence 
> template parameters (aka variadic).

This is actually pretty much exactly what VariantN does, except 
instead of storing a pointer to a lambda, it stores a pointer to 
an instance of a template function.

The member variable `fptr` [1] is the equivalent of your 
`dataToStr`. It stores a pointer to an instance of the `handler` 
template [2]. Whenever a new value is assigned to the VariantN, 
`fptr` is updated to point to the template instance corresponding 
to the new value's type [3].

[1] 
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L217-L218
[2] 
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L260-L645
[3] 
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L731