I like the idea. Here's another that seems similar to it: an immutable language wherein you could create a mutable, write-only copy of an object. After writing

I like the idea. Here's another that seems similar to it: an immutable language wherein you could create a mutable, write-only copy of an object. After writing all that was required, the copy could be finalized, at which point it becomes read-only (hence immutable). This is a bit like the builder pattern, but I've never seen a language with built-in support for it. Thoughts? Is it much different than what the OP proposes?