Revised abstract types
From Gallium
The syntax for abstract type declarations has been reverted in 3.10 to match the original syntax.
Here is an explanation...
In the revised syntax, abstract types are expressed by defining them with an unbound type variable:
(* Original *) type t;; (* Revised *) type t = 'a;
The motivation is that looks like an existential type, which is a way of seeing abstract types.
However I found this motivation misapplied since it doesn't look like an existential type, there is no exists keyword?!? (type t = exists 'a. 'a;)
It's like a hot-dog without sausage?!?
In fact, consequences of that choice are worst. If forces the parser/printer to do some semantical operation to convert back and forth between syntaxes.
type t 'a = 'a; (* not abstract *) type t 'a = 'b; (* abstract *)
It was considered acceptable, since the test for the freeness of a single type variable seemed simple because very local.
Indeed only the list of parameters was consulted to compute the freeness of the type variable.
It seems very weak since highly dependent of future evolution of the language.
Nowadays it's no longer sufficient. Constraints can be added with a type declaration to constrain the type of parameters.
type c 'a = 'b constraint 'a = < b : 'b; .. >; (* Thanks to Till Varoquaux for it's bug report. *)
Clearly I don't want to push that wrong choice further by making more semantic analysis in the parser/printer.
So I revert back to << type t; >> for abstract types.
Now, what's the new representation for abstract types. OCaml use a option type, where None represent the abstract type. We can't afford that, since we want a concrete syntax for everything. However we have a nil type that can be used as a default case (for lists of types or optional types).
<:sig_item< type t >> === <:sig_item< type t = $ <:ctyp<>> $ >>
Not that this will also concern abstract module types.
Alas, this will affect existing code using the revised syntax, but will be easily caught at compilation.
From a pragmatic point of view, a grep to show the usage of such types:
grep -E "^[ \t]*type.*=[ \t]*'[^ \t]*[ \t]*;[ \t]*$" **/*.ml*