In Caml Light, the rule is the following :- if you declare a *new* type, such as a sum type ( Foo | Bar | Baz ) or a record type ( { foo : t; bar : t' } ), use "="
- if you just add an alias, a new name for an existing type, use "=="
It is an alias if the right-hand side is a "type expression" (something you could use as is in a type annotation)
It is interesting to note that, though less syntactically visible, the difference has not vanished in OCaml.
OCaml type declarations may have two components, a type "equation" (~ alias for a type expression) and a "representation" (~ new ADT). For example the following is valid :
type test = A.ty = A | B | C
The type is declared to be an alias to the type "ty" in the module A, and also represented by the sum (A | B | C) : the definition is "re-exported", and an error will be thrown if A.ty is different from (A | B | C). It is occasionnaly nice to make representation explicit, while still keeping compatibility at the type level (the two types are considered equal by the type checker), and can also be avoid to "strenghten" an external dependency : if the upstream type definition changes, I'm sure my code will fail here instead of producing weird errors later.
On Wed, Dec 15, 2010 at 5:56 PM, Raphael Proust
<raphlalou@gmail.com> wrote:
Le mercredi 15 décembre 2010 17:15:23, sieira a écrit :
> Thanks for your replies. I'm now having some issue with Raphael's
> suggestion of using (string * string) list;; as the menu type.
>
>
> type menu = (string*string) list;;
>
> Results in a syntax error at the first parenthesis, while
>
> type menu = string*string;;
>
> fails too (at the asterisk)
>
> It seems like I'm missing something. Since according to the
>
http://caml.inria.fr/pub/docs/manual-caml-light/node3.5.html documentantion
> , this sintax should be right.
>
> I'm using Camllight 0.81 by François Boisson running in Ubuntu lucid lynx
type menu is just a shorter name for the complete type (string * string) list,
you have to use:
type menu == ( string * string ) list ;;
(with two '=' signs)
I don't know the exact rule about this. It might be close to the difference
between 'type' and 'data' in Haskell.
Good luck!
--
_______
Raphaël