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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. jibal · 1789899200 · focus · HN ↗

      > Types are properties of values, not variables.

      In statically typed languages, variables and expressions have a compile-time type and values have a run-time type.

      > Mutability is a property of variable, not of a value.

      In which languages?

      In D, mutability is a property of a type. And variables and expressions have a compile-time type and values have a run-time type, so mutability is also a property of variables, expressions, and values.

      (Immutability is also transitive in D, so an immutable value cannot have mutable parts, including mutable references, and a variable with an immutable type cannot contain a value or reference with any mutable parts.)

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