Extraction.v

(** * Extraction: Extracting ML from Coq *)

(* $Date: 2013-01-16 22:29:57 -0500 (Wed, 16 Jan 2013) $ *)

(** * Basic Extraction *)

(** In its simplest form, program extraction from Coq is completely straightforward. *)

(** First we say what language we want to extract into.  Options are OCaml (the
     most mature), Haskell (which mostly works), and Scheme (a bit out
     of date). *)

Extraction Language Ocaml.

(** Now we load up the Coq environment with some definitions, either
    directly or by importing them from other modules. *)

Require Import SfLib.
Require Import ImpCEvalFun.

(** Finally, we tell Coq the name of a definition to extract and the
    name of a file to put the extracted code into. *)

Extraction "imp1.ml" ceval_step.

(** When Coq processes this command, it generates a file [imp1.ml]
    containing an extracted version of [ceval_step], together with
    everything that it recursively depends on.  Have a look at this
    file now. *)

(* ############################################################## *)
(** * Controlling Extraction of Specific Types *)

(** We can tell Coq to extract certain [Inductive] definitions to
    specific OCaml types.  For each one, we must say
      - how the Coq type itself should be represented in OCaml, and
      - how each constructor should be translated. *)

Extract Inductive bool => "bool" [ "true" "false" ].

(** Also, for non-enumeration types (where the constructors take
    arguments), we give an OCaml expression that can be used as a
    "recursor" over elements of the type.  (Think Church numerals.) *)

Extract Inductive nat => "int"
  [ "0" "(fun x -> x + 1)" ]
  "(fun zero succ n ->
      if n=0 then zero () else succ (n-1))".

(** We can also extract defined constants to specific OCaml terms or
    operators. *)

Extract Constant plus => "( + )".
Extract Constant mult => "( * )".
Extract Constant beq_nat => "( = )".

(** Important: It is entirely _your responsibility_ to make sure that
    the translations you're proving make sense.  For example, it might
    be tempting to include this one
      Extract Constant minus => "( - )".
    but doing so could lead to serious confusion!  (Why?)
*)

Extraction "imp2.ml" ceval_step.

(** Have a look at the file [imp2.ml].  Notice how the fundamental
    definitions have changed from [imp1.ml]. *)

(* ############################################################## *)
(** * A Complete Example *)

(** To use our extracted evaluator to run Imp programs, all we need to
    add is a tiny driver program that calls the evaluator and somehow
    prints out the result.  

    For simplicity, we'll print results by dumping out the first four
    memory locations in the final state.

    Also, to make it easier to type in examples, let's extract a
    parser from the [ImpParser] Coq module.  To do this, we need a few
    more declarations to set up the right correspondence between Coq
    strings and lists of OCaml characters. *)

Require Import Ascii String.
Extract Inductive ascii => char
[
"(* If this appears, you're using Ascii internals. Please don't *) (fun (b0,b1,b2,b3,b4,b5,b6,b7) -> let f b i = if b then 1 lsl i else 0 in Char.chr (f b0 0 + f b1 1 + f b2 2 + f b3 3 + f b4 4 + f b5 5 + f b6 6 + f b7 7))"
]
"(* If this appears, you're using Ascii internals. Please don't *) (fun f c -> let n = Char.code c in let h i = (n land (1 lsl i)) <> 0 in f (h 0) (h 1) (h 2) (h 3) (h 4) (h 5) (h 6) (h 7))".
Extract Constant zero => "'\000'".
Extract Constant one => "'\001'".
Extract Constant shift =>
 "fun b c -> Char.chr (((Char.code c) lsl 1) land 255 + if b then 1 else 0)".
Extract Inlined Constant ascii_dec => "(=)".

(** We also need one more variant of booleans. *)

Extract Inductive sumbool => "bool" ["true" "false"].

(** The extraction is the same as always. *)

Require Import Imp.
Require Import ImpParser.
Extraction "imp.ml" empty_state ceval_step parse.

(** Now let's run our generated Imp evaluator.  First, have a look at
    [impdriver.ml].  (This was written by hand, not extracted.)

    Next, compile the driver together with the extracted code and
    execute it, as follows.
<<
	ocamlc -w -20 -w -26 -o impdriver imp.mli imp.ml impdriver.ml
	./impdriver
>>
    (The [-w] flags to [ocamlc] are just there to suppress a few
    spurious warnings.) *)

(* ############################################################## *)
(** * Discussion *)

(** Since we've proved that the [ceval_step] function behaves the same
    as the [ceval] relation in an appropriate sense, the extracted
    program can be viewed as a _certified_ Imp interpreter.  (Of
    course, the parser is not certified in any interesting sense,
    since we didn't prove anything about it.) *)