Thread overview
Why D functions paramter can not implicit infer type of Variant?
Jan 13
Marcone
Jan 13
sighoya
Jan 13
sighoya
January 13
import std;

void a(int b){
}

void main()
{
  Variant c = 10;
  a(c); // Error
}

Need more sugar.
January 13
On 1/13/21 8:17 AM, Marcone wrote:

> import std;
>
> void a(int b){
> }
>
> void main()
> {
>    Variant c = 10;
>    a(c); // Error
> }
>
> Need more sugar.

That can't work in a strongly statically typed language. The call a(c) is decided at compile time but Variant is not an int at compile time.

It could only work if Variant were a user-defined type that had an automatic conversion to int with 'alias this'.

Ali

January 13
On Wednesday, 13 January 2021 at 16:17:02 UTC, Marcone wrote:
> import std;
>
> void a(int b){
> }
>
> void main()
> {
>   Variant c = 10;
>   a(c); // Error
> }
>
> Need more sugar.

Two problems:

1.) Variant is library defined, compared to the language level there isn't a default strategy to choose int32 here, it could be also, short, long, unsigned ... not to mention all the alias this types.
Though it may be possible to define a default strategy but part of the problem is how to tell D to init the type parameters appropriately.

2.) c is mutable, what is if you return c and assign other values of other types to it?
Determining all possibilities by traversing following assignments leads to global type inference which no one would ever want to have especially with support of subtyping.

A more natural conclusion would be to infer c to the most common supertype as other inferences would unnecessarily exclude future assignments to c.  But the most common supertype doesn't seem to exist, and I'm unsure if this type can be modeled at all in D?


January 13
On Wednesday, 13 January 2021 at 18:09:08 UTC, sighoya wrote:
> A more natural conclusion would be to infer c to the most common supertype as other inferences would unnecessarily exclude future assignments to c.  But the most common supertype doesn't seem to exist, and I'm unsure if this type can be modeled at all in D?

That's what Variant is--a struct that models the universal supertype (sometimes called "Top" or "Any").
January 13
On Wednesday, 13 January 2021 at 19:38:55 UTC, Paul Backus wrote:

> That's what Variant is--a struct that models the universal supertype (sometimes called "Top" or "Any").

Ahh right. Good point, so it already fits.