Implementing typestate

[Ola Fosheim Grøstad via Digitalmars-d]

On Wednesday, 16 September 2015 at 17:15:55 UTC, Ola Fosheim 
Grøstad wrote:
> dostuff(r);
>
> (File<Open> f, FileRef<Void> r) = f.unborrow(r);

Of course, files are tricky since they can change their state 
themselves (like IO error). Doing that statically would require 
some kind of branching mechanism with a try-catch that jumps to a 
different location where the file type changes to "File<Error>"...

Sounds non-trivial to bolt onto an existing language.