CLEANUP: Context.{Rel,Named}.Declaration.t

  Originally, rel-context was represented as:

    Context.rel_context = Names.Name.t * Constr.t option * Constr.t

  Now it is represented as:

    Context.Rel.t = LocalAssum of Names.Name.t * Constr.t
                  | LocalDef of Names.Name.t * Constr.t * Constr.t

  Originally, named-context was represented as:

    Context.named_context = Names.Id.t * Constr.t option * Constr.t

  Now it is represented as:

    Context.Named.t = LocalAssum of Names.Id.t * Constr.t
                    | LocalDef of Names.Id.t * Constr.t * Constr.t

Motivation:

  (1) In "tactics/hipattern.ml4" file we define "test_strict_disjunction"
      function which looked like this:

        let test_strict_disjunction n lc =
          Array.for_all_i (fun i c ->
            match (prod_assum (snd (decompose_prod_n_assum n c))) with
            | [_,None,c] -> isRel c && Int.equal (destRel c) (n - i)
            | _ -> false) 0 lc

      Suppose that you do not know about rel-context and named-context.
      (that is the case of people who just started to read the source code)
      Merlin would tell you that the type of the value you are destructing
      by "match" is:

        'a * 'b option * Constr.t                    (* worst-case scenario *)

      or

        Named.Name.t * Constr.t option * Constr.t    (* best-case scenario (?) *)

      To me, this is akin to wearing an opaque veil.
      It is hard to figure out the meaning of the values you are looking at.
      In particular, it is hard to discover the connection between the value
      we are destructing above and the datatypes and functions defined
      in the "kernel/context.ml" file.

      In this case, the connection is there, but it is not visible
      (between the function above and the "Context" module).

      ------------------------------------------------------------------------

      Now consider, what happens when the reader see the same function
      presented in the following form:

        let test_strict_disjunction n lc =
          Array.for_all_i (fun i c ->
            match (prod_assum (snd (decompose_prod_n_assum n c))) with
            | [LocalAssum (_,c)] -> isRel c && Int.equal (destRel c) (n - i)
            | _ -> false) 0 lc

      If the reader haven't seen "LocalAssum" before, (s)he can use Merlin
      to jump to the corresponding definition and learn more.

      In this case, the connection is there, and it is directly visible
      (between the function above and the "Context" module).

  (2) Also, if we already have the concepts such as:
      - local declaration
      - local assumption
      - local definition
      and we describe these notions meticulously in the Reference Manual,
      then it is a real pity not to reinforce the connection
      of the actual code with the abstract description we published.

1 files changed, 6 insertions, 5 deletions

diff --git a/library/heads.ml b/library/heads.ml
index 8124d3474..4c9b78976 100644
--- a/library/heads.ml
+++ b/library/heads.ml
@@ -15,6 +15,7 @@ open Environ
 open Globnames
 open Libobject
 open Lib
+open Context.Named.Declaration
 
 (** Characterization of the head of a term *)
 
@@ -63,9 +64,9 @@ let kind_of_head env t =
       (try on_subterm k l b (variable_head id)
        with Not_found ->
         (* a goal variable *)
-        match pi2 (lookup_named id env) with
-        | Some c -> aux k l c b
-        | None -> NotImmediatelyComputableHead)
+        match lookup_named id env with
+        | LocalDef (_,c,_) -> aux k l c b
+        | LocalAssum _ -> NotImmediatelyComputableHead)
   | Const (cst,_) ->
       (try on_subterm k l b (constant_head cst)
        with Not_found ->
@@ -132,8 +133,8 @@ let compute_head = function
      | None -> RigidHead (RigidParameter cst)
      | Some c -> kind_of_head env c)
 | EvalVarRef id ->
-    (match pi2 (Global.lookup_named id) with
-     | Some c when not (Decls.variable_opacity id) ->
+    (match Global.lookup_named id with
+     | LocalDef (_,c,_) when not (Decls.variable_opacity id) ->
            kind_of_head (Global.env()) c
      | _ ->
            RigidHead (RigidVar id))