John Carmack on mutable variables

After a 2 year Clojure stint I find it very hard to explain the clarity that comes with immutability for programmers used to trigger effects with a mutation.

I think it may be one of those things you have to see in order to understand.

I think the explanation is: When you mutate variables it implicitly creates an ordering dependency - later uses of the variable rely on previous mutations. However, this is an implicit dependency that isn't modeled by the language so reordering won't cause any errors.

With a very basic concrete example:

x = 7

x = x + 3

x = x / 2

Vs

x = 7

x1 = x + 3

x2 = x1 / 2

Reordering the first will have no error, but you'll get the wrong result. The second will produce an error if you try to reorder the statements.

Another way to look at it is that in the first example, the 3rd calculation doesn't have "x" as a dependency but rather "x in the state where addition has already been completed" (i.e. it's 3 different x's that all share the same name). Doing single assignment is just making this explicit.

Yet even Rust allows you to shadow variables with another one with the same name. Yes, they are two different variables, but for a human reader they have the same name.

I think that Rust made this decision because the x1, x2, x3 style of code is really a pain in the ass to write.

In idiomatic Rust you usually shadow variables with another one of the same name when the type is the only thing meaningfully changing. For example

   let x = "29"
   let x = x.parse::<i32>()
   let x = x.unwrap()

These all use the same name, but you still have the same explicit ordering dependency because they are typed differently. The first is a &str, the second a Result<i32, ParseIntError>, the third an i32, and any reordering of the lines would provide a compiler error. And if you add another line `let y = process(x)` you would expect it to do something similar no matter where you introduce it in these statements, provided it accepts the current type of x, because the values represent the "same" data.

Once you actually "change" the value, for example by dividing by 3, I would consider it unidiomatic to shadow under the same name. Either mark it as mutable for preferably make a new variable with a name that represents what the new value now expresses