John Carmack on mutable variables

If you need new values you just make new things.

If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

The nice thing here is you can pass around pointers to fooA and never worry that anything is going to change it underneath you.

You don't need to protect private variables because your internal workings cannot be mutated. Other code can copy it but not disrupt it.

> If you need new values you just make new things. > If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

The beautiful thing about this is you can stop naming things generically, and can start naming them specifically what they are. Comprehension goes through the roof.

> If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

This is the bit I don't get.

Why would I do that? I will never want a fooA and a fooB. I can't see any circumstances where having a correct fooB and an incorrect fooA kicking around would be useful.

As Carmack points out, naming the intermediate values aides in debugging. It also helps you write code as you can give a name to every mutation.

But also keep in mind that correct and incorrect is not binary. You might want to pass a fooA to another class that does not want the fooB mutation.

If you just have foo, you end up with situations where a copy should have happened but didn't and then you get unwanted changes.

It is about being able to think clearly about your code logic. If your code has many places where a variable can change, then it is hard to go back and understand exactly where it changed if you have unexpected behavior. If the variable can never change then the logical backtrace is much shorter.

But that's just it, why would a copy ever happen? Why would you want a correct and an incorrect version of your variable hanging about?

Yes, but you can do that without having loads of stale copies of incorrect data lying around, presumably.

Taking your point of view: you assigned a value1 to a name. Then you assigned a value2 to the same name.

You say that value2 is correct. It logically follows that value1 was incorrect. Why did you assign it then?

The names are free, you can just use a correct name every single time.