Thread

1. js8 · 1789889511 · focus · HN ↗

   I feel like the explanation is overcomplicated. Types are properties of values, not variables. Mutability is a property of variable, not of a value.

   So it's kind of a categorical error. (I want to joke here that all categorical errors are just type errors in category theory.) When we speak of "type of a variable", we mean this variable can only be assigned (bound to) values of certain type. This has nothing to do with whether it can be reassigned (i.e. mutability).

   So you don't even need the notion of subtyping to explain this.

   Also, one could probably define variable as a monad over its type.

   1. kaoD · 1789898445 · focus · HN ↗

      > Mutability is a property of variable, not of a value. [...] This has nothing to do with whether it can be reassigned (i.e. mutability).

      I feel like you're using "mutable" too narrowly, as it's used colloquially in some programming languages. E.g. in JS, MDN itself talks about "reassignment"[0] and not "mutability" even though people often use "mutability" as a word to refer to the distinction between `const` and `let`. The only mention of mutability there is:

      > Others may prefer `let` for non-primitives that are mutated

      I.e. explicitly using `let` for mutated arrays, even if not reassigned, to make the signal mutability (which JS cannot express).

      Note that you used "reassignment" which is the specific word for "mutating a binding", but the article is not referring to bindings at all.

      [0] <a href="https:&#x2F;&#x2F;developer.mozilla.org&#x2F;en-US&#x2F;docs&#x2F;Web&#x2F;JavaScript&#x2F;Reference&#x2F;Statements&#x2F;let" rel="nofollow">https:&#x2F;&#x2F;developer.mozilla.org&#x2F;en-US&#x2F;docs&#x2F;Web&#x2F;JavaScript&#x2F;Refe...

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