[lib] Split auxiliary libraries into Coq-specific and general.

Up to this point the `lib` directory contained two different library
archives, `clib.cma` and `lib.cma`, which a rough splitting between
Coq-specific libraries and general-purpose ones.

We know split the directory in two, as to make the distinction clear:

- `clib`: contains libraries that are not Coq specific and implement
  common data structures and programming patterns. These libraries
  could be eventually replace with external dependencies and the rest
  of the code base wouldn't notice much.

- `lib`: contains Coq-specific common libraries in widespread use
  along the codebase, but that are not considered part of other
  components. Examples are printing, error handling, or flags.

In some cases we have coupling due to utility files depending on Coq
specific flags, however this commit doesn't modify any files, but only
moves them around, further cleanup is welcome, as indeed a few files
in `lib` should likely be placed in `clib`.

Also note that `Deque` is not used ATM.

1 files changed, 52 insertions, 0 deletions

diff --git a/clib/cEphemeron.mli b/clib/cEphemeron.mli
new file mode 100644
index 000000000..d8a1f2757
--- /dev/null
+++ b/clib/cEphemeron.mli
@@ -0,0 +1,52 @@
+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2017     *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+(* Use case:
+     You have a data structure that needs to be marshalled but it contains
+     unmarshallable data (like a closure, or a file descriptor).  Actually
+     you don't need this data to be preserved by marshalling, it just happens
+     to be there.
+     You could produced a trimmed down data structure, but then, once
+     unmarshalled, you can't used the very same code to process it, unless you
+     re-inject the trimmed down data structure into the standard one, using
+     dummy values for the unmarshallable stuff.
+     Similarly you could change your data structure turning all types [bad]
+     into [bad option], then just before marshalling you set all values of type
+     [bad option] to [None].  Still this pruning may be expensive and you have
+     to code it.
+
+   Desiderata:
+     The marshalling operation automatically discards values that cannot be
+     marshalled or cannot be properly unmarshalled.
+
+   Proposed solution:
+     Turn all occurrences of [bad] into [bad key] in your data structure.
+     Use [create bad_val] to obtain a unique key [k] for [bad_val], and store
+     [k] in the data structure.  Use [get k] to obtain [bad_val].
+
+     An ['a key] can always be marshalled.  When marshalled, a key loses its
+     value.  The function [get] raises Not_found on unmarshalled keys.
+     
+     If a key is garbage collected, the corresponding value is garbage
+     collected too (unless extra references to it exist).
+     In short no memory management hassle, keys can just replace their
+     corresponding value in the data structure.  *)
+
+type 'a key
+
+val create : 'a -> 'a key
+
+(* May raise InvalidKey *)
+exception InvalidKey
+val get : 'a key -> 'a
+
+(* These never fail. *)
+val iter_opt : 'a key -> ('a -> unit) -> unit
+val default : 'a key -> 'a -> 'a
+
+val clear : unit -> unit