Discussion Thread: DIP 1034--Add a Bottom Type (reboot)--Community Review Round 1

[Paul Backus]

On Wednesday, 6 May 2020 at 11:04:06 UTC, Mike Parker wrote:
> This is the discussion thread for the first round of Community 
> Review of DIP 1034, "Add a Bottom Type (reboot)":
>
> https://github.com/dlang/DIPs/blob/15081980cd393e21218da6836321ed37ebc48dd3/DIPs/DIP1034.md

In one of the examples, it is asserted that there is no way to 
use `assert(0)` in a switch-like higher-order function that uses 
lambdas for its cases (such as sumtype's `match`).

> In a switch statement, it is possible to add a case default: 
> assert(0);. This is not possible when using lambda-handlers to 
> simulate a switch, like the sumtype package does:

This is not strictly true. It can be done by specifying the 
return type of the `assert(0)` lambda explicitly, rather than 
relying on type inference:

int round(Var v) {
     return v.match!(
         (int x) => return x,
         (double x) => return cast(int) x,
         function int (other) { assert(0);  } // compiles
     );
}