https://dlang.org/changelog/2.094.0.html#preview-in
My question is whether to get the optimal `const scope` behavior
with `in`, do we still need to add:
`-preview=in` to the CLI when compiling our D program?
The program sample is derived from Programming in D, page 152.
source/app.d
```
import std.stdio;
void main()
{
const(int[]) constSlice = [10, 20, 30, 40];
print(constSlice); // ← compilation ERROR with first and third
print functions
}
// This won't compile as the parameter allows mutation, even
though no mutation is occurring
// void print(int[] slice) {
// writefln("%s elements: ", slice.length);
// foreach (i, element; slice) {
// writefln("%s: %s", i, element);
// }
// }
// This will compile for all of int[] slice, const(int[])slice
and immutable(int[]) slice
// This is "const-correct"
// `in` is equivalent to `const scope`
// This is the preferred D way to state that the slice parameter
won't be mutated, transitively, that is, "all the way down"
void print(in int[] slice)
{
// slice ~= 50; // ← compilation ERROR: Can't append to
slice
// slice[0] = 11; // ← compilation ERROR: Can't change any
element
// slice.length = 2; // ← compilation ERROR: Can't change length
writefln("%s elements: ", slice.length);
foreach (i, element; slice)
{
writefln("%s: %s", i, element);
}
}
// This won't compile as immutable is too strong
// void print(immutable int[] slice) {
// writefln("%s elements: ", slice.length);
// foreach (i, element; slice) {
// writefln("%s: %s", i, element);
// }
// }
```
Console output
```
4 elements:
0: 10
1: 20
2: 30
3: 40
```
