[vim/vim] Vim9: cannot declare and assign to variable constant separately (Issue #9797)
lacygoill
I was reading about const correctness via this comment, and thought about trying it in Vim9 script, but quickly realized that it was too annoying.
Suppose we have a variable whose value depends on some test:
vim9script var n: number if true n = 1 else n = 2 endif echomsg n
1
Now, suppose we want to make sure the variable does not mutate. We can't use :const:
vim9script const n: number if true n = 1 else n = 2 endif echomsg n
E1021: Const requires a value
Instead, we need :lockvar:
vim9script var n: number if true n = 1 else n = 2 endif lockvar n echomsg n
1
But if we allowed :const to not be followed by a value, the code would be easier to write (no need of :lockvar), and the intent would be easier to understand. Consider the fact that :lockvar could be written much later, and that while we're reading the code, as long as we don't find :lockvar, we are unaware that it's immutable.
Could we allow :const to be simply followed by a variable name and a type; then later lock the variable as soon as we assign to it some value?
FWIW, that's exactly what they seem to do in rust. Over there, variables are immutable by default. Which is why this code gives an error:
// written in /tmp/rs.rs fn main() { let x; { x = 1; x = x + 2; } println!("x is: {:?}", x); }
$ mkdir -p /tmp/build && rustc --out-dir /tmp/build/ /tmp/rs.rs && /tmp/build/rs
error[E0384]: cannot assign twice to immutable variable `x`
--> /tmp/rs.rs:5:9
|
2 | let x;
| - help: consider making this binding mutable: `mut x`
3 | {
4 | x = 1;
| ----- first assignment to `x`
5 | x = x + 2;
| ^^^^^^^^^ cannot assign twice to immutable variable
error: aborting due to previous error
For more information about this error, try `rustc --explain E0384`.
But they can still separate the declaration from the assignment:
// written in /tmp/rs.rs fn main() { let x; { x = 1; } println!("x is: {:?}", x); }
$ mkdir -p /tmp/build && rustc --out-dir /tmp/build/ /tmp/rs.rs && /tmp/build/rs
x is: 1
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bfrg
You can always do something like this:
const n: number = Is_true() ? Set_value(1) : Set_value(2)
And the complex logic is hidden inside the three functions. This also makes the code more readable.
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lacygoill
That's a nice workaround. I'll keep it in mind. But we still need to create at least one or two intermediate functions (here Is_true() and Set_value()). It would be nice for the Vim9 syntax to let us separate the declaration from the assignment.
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Bram Moolenaar
Rust is not a good example, it makes things more difficult (with the excuse that it's safer).
You should compute the value first, then declare the const:
vim9script
var val: number
if condition
val = 1
else
val = 2
endif
const n = val
It does require an extra temporary variable, but the big advantage is that it's clear where the constant is set and
there can be no mistakes with that.
If we would allow first declaring the constant without a value, and then later set the value, this can get very complicated if it's not a simple if-then-else. Also, setting the value can come much later, there could be a try/catch in between, etc.
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lacygoill
That's fair. I didn't think of all the implications. Closing the issue then.
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lacygoill
Closed #9797.
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