Mutation is tricky, because basically the only way to do it sanely is to copy all data into the residual, but since most arrays aren't actually mutated, that's

Mutation is tricky, because basically the only way to do it sanely is to copy all data into the residual, but since most arrays aren't actually mutated, that's extremely wasteful. I've been hoping to address this by changing mutability in the language more generally (e.g. https://github.com/JuliaLang/julia/pull/42465 https://github.com/JuliaLang/julia/pull/42465) to make immutable arrays the default at which point there wouldn't be a penalty anymore. I've had request to do the mutable copying optionally, but it's a bit tricky, because it needs rule system integration and the rule system currently doesn't reason about mutation. As for exceptions and recursion, shouldn't be a problem, just needs to be implemented.