Re: [Caml-list] Explicit Arity with Polymorphic Variants

[Jordan W <jordojw@gmail.com>]

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David Allsopp, could you please include me on the "Reply" so that the mail
threads make their way to my inbox?

David, are you certain that the explicit_arity attribute is only related to
type declarations? I do not believe that to be the case. I understand them
to apply to patterns and expressions as well.

(* Declares a variant that accepts a *single* parameter *)
type oneArgTuple = OneArg of (int*int)

(* Works because arity is massaged into whatever is necessary and doesn't
need to be expressed at parse time. *)
let OneArg (x,y) = OneArg (1,2)

(* But you can *explicitly* enforce arity and tell the type system that
this tuple should be treated as multiple arguments *)
(* That causes this to fail type checking on this tuple pattern! *)
let (OneArg (x,y) [@explicit_arity]) = OneArg (1,2);;
>> Error: The constructor OneArg expects 1 argument(s),
>> but is applied here to 2 argument(s)

(* Similarly, at parse time, you can tell the parser to parse an expression
as being multiple arguments *)
let result = OneArg (1,2)[@explicit_arity];;
>> Error: The constructor OneArg expects 1 argument(s),
>> but is applied here to 2 argument(s)

So the fact that you can inform the parser to treat a tuple as multiple
arguments to a Constructor, means that the same should be true of
polymorphic variants, but it is not. For example, this type checks but it
shouldn't:

 let (`MyThing (x,y) [@explicit_arity]) = `MyThing (2,2);;

On Fri, Jan 23, 2015 at 12:03 AM, Jacques Garrigue <
garrigue@math.nagoya-u.ac.jp> wrote:

> The answer is simple: polymorphic variants can only accept one argument
> (which may of course be a tuple). The other behavior would have required
> a specific syntax for multi-parameter polymorphic variants, since there is
> no information associated to the constructor for them.
>
> Jacques Garrigue
>
> On 2015/01/23 15:53, Jordan W wrote:
> >
> > The OCaml compiler allows distinguishing between variants that accept a
> single tuple and variant types that accept several parameters. What looks
> like a variant type accepting a tuple, is actually the later:
> >
> > type x = TwoSeparateArguments of int * int
> > let tuple = (10,10)
> > let thisWontWork = TwoSeparateArguments tuple;;
> > >> Error: The constructor TwoSeparateArguments expects 2 argument(s),
>                                                               but is
> applied here to 1 argument(s)
> >
> > (* Notice the extra parens around the two ints *)
> > type x = OneArgumentThatIsATuple of (int * int)
> > let thisActuallyWorks = OneArgumentThatIsATuple tuple
> >
> > The extra parens distinguish at type definition time which of the two is
> intended.
> >
> > But OCaml does some automatic massaging of the data that you supply to
> constructor values.
> > let _ = OneArgumentThatIsATuple (4, 5)
> > let _ = TwoSeparateArguments (4, 5)
> >
> > No extra parens are required in this case. But OCaml does give you the
> ability to annotate patterns and expressions with an "explicit_arity"
> attribute which allows syntactic distinction between supplying two separate
> parameters vs. one that happens to be a tuple. This is important for other
> parser extensions that wish to treat the two distinctly. What OCaml allows
> (explicit_arity attribute) works well enough.
> >
> > The only problem is that there doesn't seem to be a way to utilize the
> same explicit_arity attributes with polymorphic variants. Such attributes
> are not acknowledged by the type system. Is this intended?
> >
> > Taking a quick look at typecore.ml, explicit_arity appears to be
> acknowledged on standard constructors but not polymorphic variants.
> >
> https://github.com/ocaml/ocaml/blob/6e85c2d956c8fd5b45acd70a27586e44bb3a3119/typing/typecore.ml
> >
> > It seems these should be brought to consistency. I will file a mantis
> issue unless anyone believes this is intended.
> >
> > Thank you in advance.
> >
> > Jordan
> >
> >
>
>
>

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