On Thu, Sep 29, 2022 at 10:58:36AM +0100, Richard W.M. Jones wrote: > It seems to be possible with phantom types & GADTs which are a very > dark corner of OCaml and one which we shouldn't use, but here's how: > > ---------------------------------------------------------------------- > > type nullable > type not_nullable
Probably would have been better if I'd called those phantom types "primitive" and "boxed" :-) Rich. > type _ arg = > | Char : not_nullable arg > | Int : not_nullable arg > | String : nullable arg > | List : nullable arg > > (* > # Char ;; > - : not_nullable arg = Char > # String ;; > - : nullable arg = String > *) > > let only_for_nullables = function > | String -> "do something" > | List -> "do something else" > | not_nullable -> . > (* val only_for_nullables : nullable arg -> string = <fun> *) > > ---------------------------------------------------------------------- > > Notes: > > (1) The “| not_nullable -> .” line can be omitted. It's just there to > document the unreachable case, but (in this case) the compiler can > infer this. > > (2) If you omit either of the “| String“ or “| List“ cases then the > compile will warn about incomplete matching, which is good. > > Extending the example further, you can see how GADTs make the common > case (second one below) possible, but harder to write: > > ---------------------------------------------------------------------- > # let only_not_nullables = function | Char -> "char" | Int -> "int" ;; > val only_not_nullables : not_nullable arg -> string = <fun> > > # let all : type a. a arg -> string = function > | Char -> "char" > | Int -> "int" > | String -> "string" > | List -> "list";; > val all : 'a arg -> string = <fun> > ---------------------------------------------------------------------- > > Rich. > > -- > Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones > Read my programming and virtualization blog: http://rwmj.wordpress.com > nbdkit - Flexible, fast NBD server with plugins > https://gitlab.com/nbdkit/nbdkit -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html _______________________________________________ Libguestfs mailing list Libguestfs@redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs
