Improvements to switch

[Meta]

On Tuesday, 16 April 2024 at 10:34:21 UTC, ryuukk_ wrote:
> It is time to make them nice to use
>
>
> - allow them as expression
>
> ```D
> int myvalue = switch(code) {
>     // ...
> };
>
> ```
>
> - more concise
>
> ```D
> switch(code) {
>     case 1 -> "what";
>     case 2, 3 -> "ok";
>     else -> "no";
> }
> ```
>
> - pattern match
>
> ```D
> switch(tagged) {
>     case A, B -> do_that();
>     case C myc -> do_this(myc);
>     else -> nothing();
> }
> ```

A couple more things:

- branch guards

```D
auto input = readln().strip().to!int();
auto s = switch (input) {
     case n if (n == 0) -> "zero";
     case n if (n > 0)  -> "greater than zero";
     case n if (n < 0)  -> "less than zero";
};

switch (readln().strip()) {
     case string: "" -> writeln("empty string"); // Naming the 
value is optional
     case string s: "asdf" -> writeln(s[2..$]);  // Prints "df"
}
```

- pattern match on arrays, slices:

```D
int[3] staticArr = [1, 2, 3];
switch (staticArr) {
    case int[N], size_t N -> writeln("Static int array of size ", 
N);
    case int[N], size_t N: 1 -> writeln("This branch won't be 
taken");
    case int[N], size_t N if (N == 1) -> writeln("Equivalent to 
previous branch");
    case int[2] -> writeln("This branch won't be taken either");
    case string[N], N -> writeln("Nope");
}

string[] slice = ["D", "rocks", "!"];
switch (slice) {
     case string[]  : [] -> writeln("empty slice");
     case string[] s: ["D"] -> writln(s[0]);
     case string[] s: ["D", rest...] -> writeln(rest);     // 
Prints ["rocks", "!"]
     case string[] s: ["D", mid..., "!"]  -> writeln(mid); // 
Prints "rocks"
     case string[] s: ["D", "rocks", "!"] -> writeln(s);   // 
Prints ["D", "rocks", "!"]
}
```

It'd be really nice to be able to pattern match on ranges as 
well, but I don't know exactly how we'd do that given how they 
work.