From 3ead957cf13ddd3b48ee85c008c6d56e44726eb4 Mon Sep 17 00:00:00 2001
From: Son Ho
Date: Fri, 4 Feb 2022 13:33:45 +0100
Subject: Work on decomposition of monadic let-bindings for F*

---
 src/ExtractToFStar.ml  | 12 +++++--
 src/Identifiers.ml     |  8 +++--
 src/PureMicroPasses.ml | 87 +++++++++++++++++++++++++++++++++++++++++++++++++-
 src/main.ml            |  3 +-
 4 files changed, 104 insertions(+), 6 deletions(-)

(limited to 'src')

diff --git a/src/ExtractToFStar.ml b/src/ExtractToFStar.ml
index 7e4a11fe..1d45b239 100644
--- a/src/ExtractToFStar.ml
+++ b/src/ExtractToFStar.ml
@@ -824,15 +824,23 @@ let rec extract_texpression (ctx : extraction_ctx) (fmt : F.formatter)
             F.pp_print_space fmt ();
             (* Open a box for the branch *)
             F.pp_open_hvbox fmt 0;
+            (* Print the `begin` if necessary *)
             let parenth = PureUtils.expression_requires_parentheses e_branch in
-            if parenth then F.pp_print_string fmt "(";
+            if parenth then (
+              F.pp_print_string fmt "begin";
+              F.pp_print_space fmt ());
+            (* Print the branch expression *)
             extract_texpression ctx fmt false e_branch;
-            if parenth then F.pp_print_string fmt ")";
+            (* Close the `begin ... end ` *)
+            if parenth then (
+              F.pp_print_space fmt ();
+              F.pp_print_string fmt "end");
             (* Close the box for the branch *)
             F.pp_close_box fmt ();
             (* Close the box for the then/else+branch *)
             F.pp_close_box fmt ()
           in
+
           extract_branch true e_then;
           extract_branch false e_else;
           (* Close the box for the whole `if ... then ... else ...` *)
diff --git a/src/Identifiers.ml b/src/Identifiers.ml
index dfcbb631..825b4ad9 100644
--- a/src/Identifiers.ml
+++ b/src/Identifiers.ml
@@ -20,6 +20,8 @@ module type Id = sig
 
   val fresh_stateful_generator : unit -> generator ref * (unit -> id)
 
+  val mk_stateful_generator : generator -> generator ref * (unit -> id)
+
   val incr : id -> id
 
   (* TODO: this should be stateful! - but we may want to be able to duplicate
@@ -103,8 +105,8 @@ module IdGen () : Id = struct
     let id = incr id in
     id
 
-  let fresh_stateful_generator () =
-    let g = ref 0 in
+  let mk_stateful_generator g =
+    let g = ref g in
     let fresh () =
       let id = !g in
       g := incr id;
@@ -112,6 +114,8 @@ module IdGen () : Id = struct
     in
     (g, fresh)
 
+  let fresh_stateful_generator () = mk_stateful_generator 0
+
   let fresh gen = (gen, incr gen)
 
   let to_string = string_of_int
diff --git a/src/PureMicroPasses.ml b/src/PureMicroPasses.ml
index d8bfe4cd..ceee71dd 100644
--- a/src/PureMicroPasses.ml
+++ b/src/PureMicroPasses.ml
@@ -8,14 +8,38 @@ open TranslateCore
 let log = L.pure_micro_passes_log
 
 type config = {
+  decompose_monadic_let_bindings : bool;
+      (** Some provers like F* don't support the decomposition of return values
+          in monadic let-bindings:
+          ```
+          // NOT supported in F*
+          let (x, y) <-- f ();
+          ...
+          ```
+
+          In such situations, we might want to introduce an intermediate
+          assignment:
+          ```
+          let tmp <-- f ();
+          let (x, y) = tmp in
+          ...
+          ```
+       *)
   unfold_monadic_let_bindings : bool;
       (** Controls the unfolding of monadic let-bindings to explicit matches:
+          
           `y <-- f x; ...`
+
           becomes:
+          
           `match f x with | Failure -> Failure | Return y -> ...`
-     
+
+          
           This is useful when extracting to F*: the support for monadic
           definitions is not super powerful.
+          Note that when [undolf_monadic_let_bindings] is true, setting
+          [decompose_monadic_let_bindings] to true and only makes the code
+          more verbose.
        *)
   filter_unused_monadic_calls : bool;
       (** Controls whether we try to filter the calls to monadic functions
@@ -714,6 +738,51 @@ let eliminate_box_functions (_ctx : trans_ctx) (def : fun_def) : fun_def =
   let body = obj#visit_texpression () def.body in
   { def with body }
 
+(** Decompose the monadic let-bindings.
+
+    See the explanations in [config].
+ *)
+let decompose_monadic_let_bindings (_ctx : trans_ctx) (def : fun_def) : fun_def
+    =
+  (* Set up the var id generator *)
+  let cnt = get_expression_min_var_counter def.body.e in
+  let _, fresh_id = VarId.mk_stateful_generator cnt in
+  (* It is a very simple map *)
+  let obj =
+    object (self)
+      inherit [_] map_expression as super
+
+      method! visit_Let env monadic lv re next_e =
+        if not monadic then super#visit_Let env monadic lv re next_e
+        else
+          (* If monadic, we need to check if the left-value is a variable:
+           * - if yes, don't decompose
+           * - if not, make the decomposition in two steps
+           *)
+          match lv.value with
+          | LvVar _ ->
+              (* Variable: nothing to do *)
+              super#visit_Let env monadic lv re next_e
+          | _ ->
+              (* Not a variable: decompose *)
+              (* Introduce a temporary variable to receive the value of the
+               * monadic binding *)
+              let vid = fresh_id () in
+              let tmp : var = { id = vid; basename = None; ty = lv.ty } in
+              let ltmp = mk_typed_lvalue_from_var tmp None in
+              let rtmp = mk_typed_rvalue_from_var tmp in
+              let rtmp = mk_value_expression rtmp None in
+              (* Visit the next expression *)
+              let next_e = self#visit_texpression env next_e in
+              (* Create the let-bindings *)
+              (mk_let true ltmp re (mk_let false lv rtmp next_e)).e
+    end
+  in
+  (* Update the body *)
+  let body = obj#visit_texpression () def.body in
+  (* Return *)
+  { def with body }
+
 (** Unfold the monadic let-bindings to explicit matches. *)
 let unfold_monadic_let_bindings (_ctx : trans_ctx) (def : fun_def) : fun_def =
   (* It is a very simple map *)
@@ -813,6 +882,22 @@ let apply_passes_to_def (config : config) (ctx : trans_ctx) (def : fun_def) :
       log#ldebug
         (lazy ("filter_unused:\n\n" ^ fun_def_to_string ctx def ^ "\n"));
 
+      (* Decompose the monadic let-bindings *)
+      let def =
+        if config.decompose_monadic_let_bindings then (
+          let def = decompose_monadic_let_bindings ctx def in
+          log#ldebug
+            (lazy
+              ("decompose_monadic_let_bindings:\n\n" ^ fun_def_to_string ctx def
+             ^ "\n"));
+          def)
+        else (
+          log#ldebug
+            (lazy
+              "ignoring decompose_monadic_let_bindings due to the configuration\n");
+          def)
+      in
+
       (* Unfold the monadic let-bindings *)
       let def =
         if config.unfold_monadic_let_bindings then (
diff --git a/src/main.ml b/src/main.ml
index 7f7ed96c..0eaff02d 100644
--- a/src/main.ml
+++ b/src/main.ml
@@ -88,7 +88,8 @@ let () =
       let test_unit_functions = true in
       let micro_passes_config =
         {
-          Micro.unfold_monadic_let_bindings = true;
+          Micro.decompose_monadic_let_bindings = false;
+          unfold_monadic_let_bindings = true;
           filter_unused_monadic_calls = true;
         }
       in
-- 
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