move Decode from support/ to src/ for more visibility

Suggested-by: Neven Villani <neven.villani@crans.org>

README.md

@@ -373,7 +373,7 @@ you prefer).
 
 - `src/`: the bulk of the code, that place where we expect you to
    work. `src/*.{ml,mli}` are the more important modules that you will
-   have to modify or use directory. `src/support/*.{ml,mli}` has the
+   have to modify or use directly. `src/support/*.{ml,mli}` has the
    less interesting support code. (Again, feel free to modify the
    support code, but you do not need to.)
 

src/Decode.ml

@@ -1,3 +1,8 @@
+(* There is nothing that you have to implement in this file/module,
+   and no particular need to read its implementation. [Decode.mli]
+   explains its purpose. *)
+
+
 type env = Unif.Env.t
 
 type slot =

src/Decode.mli

@@ -0,0 +1,21 @@
+type env = Unif.Env.t
+
+(** Suppose the unification environment [env] contains the following equations:
+
+    ?w = ?w1 -> ?w2
+    ?w1 = int
+    ?w2 = bool
+    ?w3 = ?w1 -> ?w4
+    ?w4 : no structure
+
+    Then [decode env ?w] will return the type [int -> bool].
+
+    Notice on the other hand that [?w3] is of the form [?w1 -> ?w4]
+    for a still-undertermined variable [?w4]. Any type of the form
+    [int -> foo] might work, for any [foo], but further progress in
+    the inference process could end up in a solution incompatible with
+    any specific choice of [foo]. We decide to decode this into
+    a fresh (rigid) type variable (which is assumed distinct from
+    everything else, but nothing else): [?w3] decodes into [int -> α].
+*)
+val decode : env -> Constraint.variable -> STLC.ty

src/Unif.ml

@@ -1,6 +1,6 @@
 (* There is nothing that you have to implement in this file/module,
    and no particular need to read its implementation. On the other hand,
-   you want to understand the interface exposed in [Unif.mli] has it
+   you want to understand the interface exposed in [Unif.mli], as it
    is important to implement a constraint solver in Solver.ml. *)
 
 module UF = UnionFind.Make(UnionFind.StoreMap)

src/support/Decode.mli

@@ -1,3 +0,0 @@
-type env = Unif.Env.t
-
-val decode : env -> Constraint.variable -> STLC.ty