[pp] Hide the internal representation of `std_ppcmds`.

Following a suggestion by @ppedrot in #390, we require `Pp` clients to
be aware that they are using a "view" on the `std_ppcmds` type.

This is not extremely useful as people caring about the documents will
indeed have to follow changes in the view, but it costs little to play
on the safe side here for now.

We also introduce a more standard notation, `Pp.t` for the main type.

1 files changed, 12 insertions, 4 deletions

diff --git a/lib/pp.ml b/lib/pp.ml
index 80c599274..9f33756df 100644
--- a/lib/pp.ml
+++ b/lib/pp.ml
@@ -25,17 +25,25 @@ type block_type =
   | Pp_hvbox  of int
   | Pp_hovbox of int
 
-type std_ppcmds =
+type doc_view =
   | Ppcmd_empty
   | Ppcmd_string of string
-  | Ppcmd_glue of std_ppcmds list
-  | Ppcmd_box  of block_type * std_ppcmds
-  | Ppcmd_tag of pp_tag * std_ppcmds
+  | Ppcmd_glue of doc_view list
+  | Ppcmd_box  of block_type * doc_view
+  | Ppcmd_tag of pp_tag * doc_view
   (* Are those redundant? *)
   | Ppcmd_print_break of int * int
   | Ppcmd_force_newline
   | Ppcmd_comment of string list
 
+(* Following discussion on #390, we play on the safe side and make the
+   internal representation opaque here. *)
+type t = doc_view
+type std_ppcmds = t
+
+let repr x = x
+let unrepr x = x
+
 (* Compute length of an UTF-8 encoded string
    Rem 1 : utf8_length <= String.length (equal if pure ascii)
    Rem 2 : if used for an iso8859_1 encoded string, the result is