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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

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
> 
>