this post was submitted on 08 Oct 2023
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The definition of a variable is that it’s mutable. If it’s immutable it’s constant.
There's no ISO standardized definition for variable. People use that word with all kinds of meaning.
Waaaait a minute... isn't it called a variable because the contents are, you know, variable?
It started as a variable, then ended as a constant.
This is needlessly obtuse. The definition of the word is that it's non-constant. There isn't an ISO definition of the word no, but there are many reputable dictionaries out there that will serve as an alternative.
Well, starting with the definition from algebra, where it's not something allowed to vary...
I guess more people know about math than use imperative programing languages.
Except that's exactly what it is allowed to in algebra.
Sure, in most equations you solve in early algebra school there is only one possible value for the variables. But in many equations there can be multiple, or even infinite. It's an unknown, and the contents can vary (depending on other constraints, ie. The rest of the equation(s)).
There's no time in algebra for your variables to vary.
When you have a non-unitary set of solutions, you have a constant non-unitary set of solutions.
I feel like it's like pointers.
"Variable" refers to the label, i.e. a box that can contain anything (like *ptr is a pointer to [something we dont know anything about])
Immutable describes the contents, i.e. the stuff in the box cant change. (like int* ptr describes that the pointer points to an int)
Rust makes it very obvious that there's a difference between constants and immutable variables, mainly because constants must be compile time constants.
What do you call it when a variable cant change after its definition, but isnt guaranteed to be the same on each function call? (E.g. x is an array that's passed in, and we're just checking if element y exists)
It's not a constant, the contents of that label are "changing", but the label's contents cant be modified inside the scope of that function. So it's a variable, but immutable.