[pp] Make feedback the only logging mechanism.

Previously to this patch, Coq featured to distinct logging paths: the
console legacy one, based on `Pp.std_ppcmds` and Ocaml's `Format`
module, and the `Feedback` one, intended to encapsulate message inside a
more general, GUI-based feedback protocol.

This patch removes the legacy logging path and makes feedback
canonical. Thus, the core of Coq has no dependency on console code
anymore.

Additionally, this patch resolves the duplication of "document" formats
present in the same situation. The original console-based printing code
relied on an opaque datatype `std_ppcmds`, (mostly a reification of
`Format`'s format strings) that could be then rendered to the console.

However, the feedback path couldn't reuse this type due to its opaque
nature. The first versions just embedded rending of `std_ppcmds` to a
string, however in 8.5 a new "rich printing" type, `Richpp.richpp` was
introduced.

The idea for this type was to be serializable, however it brought
several problems: it didn't have proper document manipulation
operations, its format was overly verbose and didn't preserve the full
layout, and it still relied on `Format` for generation, making
client-side rendering difficult.

We thus follow the plan outlined in CEP#9, that is to say, we take a
public and refactored version of `std_ppcmds` as the canonical "document
type", and move feedback to be over there. The toplevel now is
implemented as a feedback listener and has ownership of the console.

`richpp` is now IDE-specific, and only used for legacy rendering. It
could go away in future versions. `std_ppcmds` carries strictly more
information and is friendlier to client-side rendering and display
control.

Thus, the new panorama is:

- `Feedback` has become a very module for event dispatching.
- `Pp` contains a target-independent box-based document format.
  It also contains the `Format`-based renderer.
- All console access lives in `toplevel`, with console handlers private
  to coqtop.

_NOTE_: After this patch, many printing parameters such as printing
width or depth should be set client-side. This works better IMO,
clients don't need to notify Coq about resizing anywmore. Indeed, for
box-based capable backends such as HTML or LaTeX, the UI can directly
render and let the engine perform the word breaking work.

_NOTE_: Many messages could benefit from new features of the output
format, however we have chosen not to alter them to preserve output.

A Future commits will move console tag handling in `Pp_style` to
`toplevel/`, where it logically belongs.

The only change with regards to printing is that the "Error:" header was
added to console output in several different positions, we have removed
some of this duplication, now error messages should be a bit more
consistent.

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/pp.mli b/lib/pp.mli
index 5bf5391d3b..12747d3a1d 100644
--- a/lib/pp.mli
+++ b/lib/pp.mli
@@ -8,6 +8,30 @@
 
 (** Coq document type. *)
 
+(** Pretty printing guidelines ******************************************)
+(*                                                                      *)
+(* std_ppcmds is the main pretty printing datatype in he Coq. Documents *)
+(* are composed laying out boxes, and users can add arbitrary metadata  *)
+(* that backends are free to interpret.                                 *)
+(*                                                                      *)
+(* The datatype is public to allow serialization or advanced uses,      *)
+(* regular users are _strongly_ encouraged to use the top-level         *)
+(* functions to manipulate the type.                                    *)
+(*                                                                      *)
+(* Box order and number is indeed an important factor. Users should try *)
+(* to create a proper amount of boxes. Also, the ++ operator provides   *)
+(* "efficient" concatenation, but directly using a list is preferred.   *)
+(*                                                                      *)
+(* That is to say, this:                                                *)
+(*                                                                      *)
+(* `hov [str "Term"; hov (pr_term t); str "is defined"]`                *)
+(*                                                                      *)
+(* is preferred to:                                                     *)
+(*                                                                      *)
+(* `hov (str "Term" ++ hov (pr_term t) ++ str "is defined")`            *)
+(*                                                                      *)
+(************************************************************************)
+
 (* XXX: Improve and add attributes *)
 type pp_tag = string list