Re: [Caml-list] Newbie comment on constructor syntax

[Jeremy Yallop <yallop@gmail.com>]

On 9 November 2015 at 22:27, Andreas Rossberg <rossberg@mpi-sws.org> wrote:
>> On Nov 9, 2015, at 22:11 , Gabriel Scherer <gabriel.scherer@gmail.com> wrote:
>> I like the revised syntax choice of writing
>>  type t = A of int and int list
>> instead of
>>  type t = A of int * int list
>> which removes the beginner surprise without introducing other
>> unpalatable design side-effects. (It is still awkward for GADTs, but
>> such is life.)
>
> I am not fond of the revised syntax, because it does not explain why the term-level syntax for constructor application uses tuple notation.

In fact, the revised syntax doesn't use tuple notation for constructor
application!  The revised type definition syntax also has a few
additional differences, such as prefix type constructor application,
besides the one Gabriel mentioned.  Here's an example, showing how to
define a data type with a multi-argument constructor and to construct
and match against values, with curried notation:

   # type t = [ A of int and list int ];
   type t = [ A of int and list int ]
   # value x = A 3 [];
   value x : t = A 3 []
   # fun [ A x y -> (x, y) ];
   - : t -> (int * list int) = <fun>

However, partial application of constructors is not supported:

   # A 3;
   Characters 0-3:
     A 3;
     ^^^
   Error: The constructor A expects 2 argument(s),
          but is applied here to 1 argument(s)


Jeremy.