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Chapter 7  Executing WhyML Programs

This chapter shows how WhyML code can be executed, either by being interpreted or compiled to some existing programming language.

Let us consider the program of Section 2.2 that computes the maximum and the sum of an array of integers. Let us assume it is contained in a file maxsum.mlw.

7.1  Interpreting WhyML Code

To test function max_sum, we can introduce a WhyML test function in module MaxAndSum

let test () = let n = 10 in let a = make n 0 in a[0] <- 9; a[1] <- 5; a[2] <- 0; a[3] <- 2; a[4] <- 7; a[5] <- 3; a[6] <- 2; a[7] <- 1; a[8] <- 10; a[9] <- 6; max_sum a n

and then we use the execute command to interpret this function, as follows:

> why3 execute maxsum.mlw MaxAndSum.test
Execution of MaxAndSum.test ():
     type: (int, int)
   result: (45, 10)

We get the expected output, namely the pair (45, 10).

7.2  Compiling WhyML to OCaml

An alternative to interpretation is to compile WhyML to OCaml. We do so using the extract command, as follows:

> why3 extract -D ocaml64 maxsum.mlw -o

The extract command requires the name of a driver, which indicates how theories/modules from the Why3 standard library are translated to OCaml. Here we assume a 64-bit architecture and thus we pass ocaml64. We also specify an output file using option -o, namely After this command, the file contains an OCaml code for function max_sum. To compile it, we create a file containing a call to max_sum, e.g.,

let a = Z.of_int [| 9; 5; 0; 2; 7; 3; 2; 1; 10; 6 |] let s, m = Max_sum.max_sum a (Z.of_int 10) let () = Format.printf "sum=%s, max=%s@." (Z.to_string s) (Z.to_string m)

It is convenient to use ocamlbuild to compile and link both files and

> ocamlbuild -pkg zarith main.native

Since Why3’s type int is translated to OCaml arbitrary precision integers using the ZArith library, we have to pass option -pkg zarith to ocamlbuild. In order to get extracted code that uses OCaml’s native integers instead, one has to use Why3’s types for 63-bit integers from libraries and mach.array.Array63.

Extraction Starting Point.

The extract command accepts three different targets for extraction: a WhyML file, a module, or a symbol (function, type, exception). To extract all the symbols from every module of a file named f.mlw, one should write

> why3 extract -D <driver> f.mlw

To extract only the symbols from module M of file f.mlw, one should write

> why3 extract -D <driver> -L <dir> f.M

To extract only the symbol s (a function, a type, or an exception) from module M of file f.mlw, one should write

> why3 extract -D <driver> -L <dir> f.M.s

Note the use of -L <dir>, for both extraction of a module and a symbol, in order to state the location of file f.mlw.


The following options can be added to the extraction command line:

performs a flat extraction, i.e., everything is extracted into a single file. This is the default behavior. The -o option should be given the name of a file or, if omitted, the result of extraction is printed to the standard output.
each module is extracted in its own, separated file. The -o option cannot be omitted, and it should be given the name of an existing directory. This directory will be populated with the resulting OCaml files.
recursively extracts all the dependencies of the chosen entry point. This option is valid for both modular and flat options.

We illustrate different ways of using the extract command through some examples. Consider the program of Section 2.6. If we are only interested in extracting function enqueue, we can proceed as follows:

> why3 extract -D ocaml64 -L . aqueue.AmortizedQueue.enqueue -o

Here we assume that file aqueue.mlw contains this program, and that we invoke extract from the directory where this file is stored. File now contains the following OCaml code:

let enqueue (x: 'a) (q: 'a queue) : 'a queue = create (q.front) (q.lenf) (x :: (q.rear)) (Z.add (q.lenr) (Z.of_string "1"))

Choosing a function symbol as the entry point of extraction allows us to focus only on specific parts of the program. However, the generated code cannot be type-checked by the OCaml compiler, as it depends on function create and on type ’a queue, whose definitions are not given. In order to obtain a complete OCaml implementation, we can perform a recursive extraction:

> why3 extract --recursive -D ocaml64 -L . \
    aqueue.AmortizedQueue.enqueue -o

This updates the contents of file as follows:

type 'a queue = { front: 'a list; lenf: Z.t; rear: 'a list; lenr: Z.t; } let create (f: 'a list) (lf: Z.t) (r: 'a list) (lr: Z.t) : 'a queue = if Z.geq lf lr then { front = f; lenf = lf; rear = r; lenr = lr } else let f1 = List.append f (List.rev r) in { front = f1; lenf = Z.add lf lr; rear = []; lenr = (Z.of_string "0") } let enqueue (x: 'a) (q: 'a queue) : 'a queue = create (q.front) (q.lenf) (x :: (q.rear)) (Z.add (q.lenr) (Z.of_string "1"))

This new version of the code is now accepted by the OCaml compiler (provided the ZArith library is available, as above).

Custom Extraction Drivers.

Several OCaml drivers can be specified on the command line, using option -D several times. In particular, one can provide a custom driver to map some symbols of a Why3 development to existing OCaml code. Suppose for instance we have a file file.mlw containing a proof parameterized with some type elt and some binary function f:

module M type elt val f (x y: elt) : elt let double (x: elt) : elt = f x x ...

When it comes to extract this module to OCaml, we may want to instantiate type elt with OCaml’s type int and function f with OCaml’s addition. For this purpose, we provide the following in a file mydriver.drv:

module file.M syntax type elt "int" syntax val f "%1 + %2" end

OCaml fragments to be substituted for Why3 symbols are given as arbitrary strings, where %1, %2, etc., will be replaced with actual arguments. Here is the extraction command line and its output:

> why3 extract -D ocaml64 -D mydriver.drv -L . file.M
let double (x: int) : int = x + x

When using such custom drivers, it is not possible to pass Why3 file names on the command line; one has to specify module names to be extracted, as done above.

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