Yes, i knew the variant constructor but, somehow i didn't realize i was
precisely using it for my mind was focused on the polymorphic variant list
:)

In fact, i wondered if a generic result type like this

type ('a, 'b) result = Ok of 'a | Error of 'b

that we can see in several library could be used to specify a "safe" result
which could have type something like ('a, []) result. One could encode 'b
as some error list at type level but it needs some complicated type
management and i'm targeting OCaml beginners for which i just want them to
be careful about non nominal results.


2016-11-25 12:22 GMT+01:00 David Allsopp <dra-news@metastack.com>:

> Julien Blond wrote:
> > 2016-11-25 9:39 GMT+01:00 Julien Blond <mailto:julien.blond@gmail.com>:
> > Hi,
> > Let's try something :
> > $ ocaml
> >         OCaml version 4.03.0
> >
> > # let _ : [] list = [];;
> > Characters 9-10:
> >  let _ : [] list = [];;
> > Error: Syntax error
> > # type empty = [];;
> > type empty = []
> > # let _ : empty list = [];;
> > - : empty list = []
> > #
> >
> > Does anyone know if there is a reason to forbid the empty polymorphic
> variant
> > set in type expressions or if it's a bug ?
>
> As you've observed, [] is a variant constructor since 4.03.0 - see GPR#234
> (https://github.com/ocaml/ocaml/pull/234). The GPR contains references
> and comments as to the motivation for this.
>
> What's your desired use for the type of the non-extensible empty
> polymorphic variant?
>
> Possibly related, you can define a general type for a list of polymorphic
> variants:
>
> let (empty : [> ] list) = []
>
> or
>
> let (length : [> ] list -> int) = List.length;;
> length [`Foo; `Bar];;
> length [42];;
>
> if that's what you were after?
>
>
> David
>