RE: [Caml-list] Ocaml compiler documentation

[David Allsopp <dra-news@metastack.com>]

Yotam Barnoy wrote:
> 2. What is demonstrated in lines 114-117 regarding polymorphic variant row fields:
> 
>  | Rtag of label * bool * core_type list
>        (* [`A]                   ( true,  [] )
>           [`A of T]              ( false, [T] )
>           [`A of T1 & .. & Tn]   ( false, [T1;...Tn] )
>           [`A of & T1 & .. & Tn] ( true,  [T1;...Tn] )
>         *)
>
> What does the bool value represent?

It indicates that the constructor is constant, or that one of the types in the conjunction is "empty" (i.e. constant - I don't know what Jacques' terminology for that is!)

> Why are the type separators in the comments using the & symbol?

For example:

type t = [ `A of string & int ]
Error: The present constructor A has a conjunctive type

(see typing/typetexp.ml)

but, adapting "Advanced use" example in section 4.2 of the manual:

# let f x = (function `A x -> x = 1 | `B -> true) x && (function `A x -> x = "a" | `B -> true) x;;
val f : [< `A of string & int | `B ] -> bool = <fun>

in order to write an annotation:

# let f : [< `A of string & int | `B] ] -> bool = fun x -> ...

> What is the difference between the 3rd and 4th example?

The conjunction may include the constant constructor case:

# let f x = (function `A -> false | `B -> true) x && (function `A x -> x = "a" | `B -> true) x;;
val f : [< `A of & string | `B ] -> bool = <fun>

The constructor Reither will give you other pointers into where this information is needed in typing/typecore.ml.

HTH,


David