interface.xml

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<page name="manual_interfacewithocaml">

<title>Interfacing CDuce with OCaml</title>

<box title="Introduction" link="intro">

<p>
This page describes the CDuce/OCaml interface. This interface allows
the programmer to:
</p>
<ul>
<li>call OCaml functions from a CDuce module;</li>
<li>export a CDuce model as an OCaml module, by giving it an explicit OCaml signature.</li>
</ul>

<p>
The intended usages for the interface are:
</p>
<ul>
<li>Piggyback existing OCaml libraries, such as database, 
    network, GUI, data structures;</li>
<li>Use CDuce as an XML layer (input/output/transformation) for OCaml
    projects;</li>
<li>Develop fully mixed OCaml/CDuce projects.</li>
</ul>

<p>
To see how to build CDuce with support for the OCaml interface, 
see the <a href="INSTALL">INSTALL</a> file from the CDuce distribution.
</p>


</box>

<box title="Translating types" link="transl">

<p>
The heart of the interface is a mapping from OCaml types to CDuce
types. An OCaml type <code>%%t%%</code> is translated to a CDuce type
<code>T(%%t%%)</code>, which is meant to be isomorphic to <code>%%t%%</code>:
there is a canonical function <code>%%t%%</code> &rarr; <code>T(%%t%%)</code>
from OCaml values of type <code>%%t%%</code> to CDuce values of type
<code>T(%%t%%)</code>, and another canonical function <code>T(%%t%%)</code> &rarr; <code>%%t%%</code>.
</p>

<ul>
<li>
Basic OCaml types <code>char</code>, <code>int</code>, <code>string</code>,
<code>unit</code> are translated respectively to
<code>Byte = '\0;'--'\255;'</code>, <code>-1073741824 --
1073741823</code>, <code>Latin1 = [ Byte* ]</code>, <code>[] = `nil</code>.
</li>

<li>
Tuple types <code>%%t%%1 * ... * %%t%%n</code> are translated to nested CDuce
product types <code>(T(%%t%%1),(...,T(%%t%%n))...)</code>. A function type
<code>%%t%% -> %%s%%</code> is translated to <code>T(%%t%%) -> T(%%s%%)</code>.
Labels and optional labels on the argument of the arrow are discarded.
</li>

<li>
A list type <code>%%t%% list</code> is translated to an homogeneous
sequence type <code>[ T(%%t%%)* ]</code>. An array type
<code>%%t%% array</code> has the same translation.
</li>

<li>
A variant type with a declaration <code>A1 of %%t%%1 | ... | An of
%%t%%n</code> is translated to a type <code>(`A1,T(%%t%%1)) | ... |
(`An,T(%%t%%n))</code>. If a constructor <code>Ai</code> has no argument, the resulting
term is <code>`Ai</code>, not <code>(`Ai,[])</code>.
Polymorphic variant types are treated similarly.
</li>

<li>
A record type with a declaration <code>{ l1 : %%t%%1; ...; ln : %%t%%n
}</code> is translated to a closed record type <code>{| l1 = T(%%t%%1);
... ; ln = T(%%t%%n) |}</code>. Mutable fields are just copied.
</li>

<li>
Private variant and record types are treated correctly: the interface
never tries to generate OCaml values of these types, but it will happily
translate them to CDuce values.
</li>

<li>
A reference type <code>%%t%% ref</code> is translated to the CDuce
reference type <code>ref T(%%t%%)</code>. When converting a Caml reference
to CDuce, the operation (set,get) on the resulting reference refers
to the original reference. However, when converting a CDuce reference
to OCaml, the content of the reference is fetched (set), and a fresh
OCaml reference is created (copy semantics).
</li>

<li>
The type <code>Cduce_lib.Value.t</code> is translated to the CDuce
type <code>Any</code>. The corresponding translation functions are the
identity. This can be used to avoid multiple copies when translating
a complex value back and forth between CDuce and OCaml.
The type <code>Cduce_lib.Encodings.Utf8.t</code> is translated to the CDuce
type <code>String</code>. 
The type <code>Big_int.big_int</code> is translated to the CDuce
type <code>Int</code>.
</li>

<li>
A <em>monomorphic</em> abstract type <code>t</code> is translated to
the CDuce type <code>!t</code>. This type just acts as a container for
values of the abstract type. CDuce never produces a value of this
type, and it cannot inspect the content of such a value (apart
from checking its type).
</li>
</ul>

<p>
The canonical translation is summarized in the following box:
</p>


<table border="1" style="align:middle">
<tr>
<th>OCaml type <tt><i>t</i></tt></th>
<th>CDuce type <tt>T(<i>t</i>)</tt></th>
</tr>
<tr><td><tt>char</tt></td><td><tt>Byte = '\0;'--'\255;'</tt></td></tr>
<tr><td><tt>int</tt></td><td><tt>-1073741824 -- 1073741823</tt></td></tr>
<tr><td><tt>string</tt></td><td><tt>Latin1 = [ Byte* ]</tt></td></tr>
<tr><td><tt>unit</tt></td><td><tt>[] = `nil</tt></td></tr>
<tr><td><tt>bool</tt></td><td><tt>Bool = `true | `false</tt></td></tr>

<tr><td><tt><i>t1</i> * ... * <i>tn</i></tt></td>
<td><tt>(T(<i>t1</i>),(...,T(<i>tn</i>))...)</tt></td></tr>

<tr><td><tt><i>t</i> -> <i>s</i></tt></td>
<td><tt>T(<i>t</i>) -> T(<i>s</i>)</tt></td></tr>

<tr><td><tt><i>t</i> list</tt></td>
<td><tt>[ T(<i>t</i>)* ]</tt></td></tr>

<tr><td><tt><i>t</i> array</tt></td>
<td><tt>[ T(<i>t</i>)* ]</tt></td></tr>

<tr><td><tt>A of <i>t</i> | B of <i>s</i> | C</tt></td>
<td><tt>(`A, T(<i>t</i>)) | (`B, T(<i>s</i>)) | `C</tt></td></tr>

<tr><td><tt>[ `A of <i>t</i> | `B of <i>s</i> | `C ]</tt></td>
<td><tt>(`A, T(<i>t</i>)) | (`B, T(<i>s</i>)) | `C</tt></td></tr>

<tr><td><tt>{ x : <i>t</i>; y : <i>s</i> }</tt></td>
<td><tt>{| x = T(<i>t</i>); y = T(<i>s</i>) |}</tt></td></tr>

<tr><td><tt><i>t</i> ref</tt></td>
<td><tt>ref T(<i>t</i>)</tt></td></tr>

<tr><td><tt>Cduce_lib.Value.t</tt></td><td><tt>Any</tt></td></tr>
<tr><td><tt>Cduce_lib.Encodings.Utf8.t</tt></td><td><tt>String</tt></td></tr>
<tr><td><tt>Big_int.big_int</tt></td><td><tt>Int</tt></td></tr>
</table>

<p>
Only monomorphic types are handled by the interface. It is allowed to
use polymorphic constructors as an intermediate, as long as the final
type to be translated is monomorphic. Recursive types, including
unguarded ones (option <code>-rectypes</code> of the OCaml compiler)
are accepted. In the following example:
</p>

<sample>
type 'a t = A of int | B of 'a t
type s = int t

type 'a u = A of ('a * 'a) u | B
type v = int u
</sample>

<p>
the type <code>s</code> can be translated, but the type <code>v</code>
can't, because its infinite unfolding is not a regular type.
</p>

<p>
OCaml object types are not supported.
</p>

<p>
Note that values are copied in depth (until reaching an abstract type,
a function types, etc...). In particular, translating an OCaml cyclic
values to CDuce will not terminate (well, with a stack overflow!).
</p>

</box>

<box title="Calling OCaml from CDuce" link="call_ocaml">

<p>
If an OCaml value has a type that can be translated, it is possible to
use it from CDuce (see the <a href="#link">How to compile and link</a> section for
more details).
</p>

<p>
In a CDuce module, you can write <code>M.f</code>
to denote the result of translating the OCaml value <code>M.f</code>
to CDuce.
</p>

<p>
If the value you want to use has a polymorphic type, you can make
the translation work by explicitly instantiating its type
variables with CDuce types. The syntax is <code>M.f with { t1
... tn }</code> where the <code>ti</code> are CDuce types. The type
variables are listed in the order they appear in a left-to-right
reading of the OCaml type. Example:
</p>

<sample>
let listmap = List.map with { Int String }
</sample>

<p>
will return a function of type <code>(Int -> String) -> ([Int*] -> [String*])</code>
</p>

</box>

<box title="Calling CDuce from OCaml" link="call_cduce">

<p>
We have seen in the section above how OCaml values can be used from a
CDuce module. It is also possible to use CDuce values from OCaml. To
do so, you must give an OCaml interface (.mli) for the CDuce module
(.cdo). The interface can define arbitrary types, and declare
monomorphic values. These values must be defined in the CDuce module
with a compatible type (subtype of the translation).
</p>

<p>
As an example, suppose you have this CDuce module (foo.cd):
</p>

<sample>
type s = (`A,int) | `B
let double (x : Latin1) : Latin1 = x @ x
let dump (x : s) : Latin1 = string_of x
</sample>

<p>
You can define an OCaml interface for it (foo.mli):
</p>

<sample>
type t = A of int | B
val double: string -> string
val dump: t -> string
</sample>

<p>
When the foo.cdo module is compiled, CDuce will look for the foo.cmi
compiled interface (hence, you must first compile it yourself with
OCaml), and generate stub code, so as to define an OCaml module
<code>Foo</code> with the given interface. This module can then be
linked together with other "regular" OCaml modules, and used from them.
</p>

<p>
Notes:
</p>

<ul>
<li>
It is not mandatory to export all the values of the CDuce module in
the OCaml interface.
</li>
<li>
The types defined in the interface cannot (currently) be used
within the CDuce module.
</li>
</ul>

</box>

<box title="How to compile and link" link="link">

<p>
Here is the protocol to compile a single CDuce module:
</p>

<ul>
  <li>
    Create a <code>.cmi</code> from your OCaml file with 
    <code>ocamlc -c foo.mli</code>.
  </li>
  <li>
    Compile your CDuce file <code>cduce --compile foo.cd</code>. This command
    will create a CDuce bytecode file <code>foo.cdo</code>, which
    also contains the OCaml glue code to export CDuce values as OCaml
    ones, and to bind OCaml values used within the CDuce module.
  </li>
  <li>
    Compile the OCaml glue code 
    <code>ocamlfind ocamlc -c -package cduce -pp cdo2ml -impl foo.cdo</code>.
    The<code>cdo2ml</code> tool extracts the OCaml glue code from the
    CDuce bytecode file.
  </li>
</ul>

<p>
  You can then link the resulting OCaml module, maybe with other
  modules (either regular ones, or wrapping a CDuce module):
  <code>ocamlfind ocamlc -o {{...}} -package cduce -linkpkg foo.cmo {{...}}</code>.
  When the program is run, the CDuce bytecode file
  <code>foo.cdo</code> is looked in the <em>current directory</em>
  only, and loaded dynamically (with a checksum test).
</p>

<p>
  It might be preferable to include the CDuce bytecode directly into
  the OCaml glue code. You can do this by giving <code>cdo2ml</code>
  the <code>-static</code> option:
  <code>ocamlfind ocamlc -c -package cduce -pp "cdo2ml -static" -impl foo.cdo</code>.
  Modules which have been compiled this way don't need the
  corresponding <code>.cdo</code> at runtime.
</p>

<p>
  If you choose static linking, you have to use a correct ordering
  when linking with OCaml. Note that it is possible to mix static and
  dynamic linking for various CDuce modules in a same program.
</p>

<p>
  Everything works <i>mutatis mutandis</i> with the native OCaml compiler ocamlopt.
</p>

<p>
  You might need to pass extra <code>-I</code> flags to CDuce so that
  it could find the referenced <code>.cmi</code> files.
</p>

<p>
  It is possible to run a CDuce module with <code>cduce --run
  foo.cdo</code>, but only if it doesn't use OCaml values.
</p>

<p>
  Interested users can look at the output of <code>cdo2ml</code> to
  better understand how the interface works.
</p>

</box>

<box title="Calling OCaml from the toplevel" link="topl">

<p>
The tool <code>cduce_mktop</code> creates custom versions of the CDuce 
toplevel with built-in support for some OCaml modules / functions.
</p>

<sample>
cduce_mktop [target] [primitive file]
</sample>

<p>
The first argument is the file name of the resulting toplevel.
The second points to a file whose contents specify a set of built-in
OCaml values to be embedded in the toplevel. Each line must either
be a qualified value (like <code>List.map</code>) or
the name of an OCaml unit (like <code>List</code>). Empty lines
and lines starting with a sharp character are ignored.
</p>

<p>
In a custom toplevel, the directive <code>#builtins</code> prints the name
of embedded OCaml values.
</p>

</box>

<box title="Examples" link="examples">

<section title="Getting the value of an environment variable">

<sample>
let home = Sys.getenv "home";;
</sample>

</section>

<section title="Ejecting your CD with CDuce">

<p>
This example demonstrates how to use OCamlSDL library.
</p>

<sample>
Sdl.init `None [ `EVERYTHING ];;
let cd = Sdlcdrom.cd_open 0;; 
Sdlcdrom.cd_eject cd;;
</sample>

<p>
If you put these lines in a file <code>cdsdl.cd</code>, you can
compile and link it with:
</p>

<sample>
cduce --compile cdsdl.cd -I `ocamlfind query ocamlsdl`
ocamlfind ocamlc -o cdsdl -pp "cdo2ml -static" -impl cdsdl.cdo   \ 
  -package cduce,ocamlsdl -linkpkg
</sample>


</section>

<section title="Accessing MySQL">

<p>
This example demonstrates how to use ocaml-mysql library.
</p>

<sample>
let db = Mysql.connect Mysql.defaults;;

match Mysql.list_dbs db `None [] with
 | (`Some,l) -> print [ 'Databases: ' !(string_of l) '\n' ]
 | `None -> [];;

print [ 
  'Client info: ' !(Mysql.client_info []) '\n'
  'Host info: ' !(Mysql.host_info db) '\n'
  'Server info: ' !(Mysql.server_info db) '\n'
  'Proto info: ' !(string_of (Mysql.proto_info db)) '\n'
];;
</sample>

<p>
If you put these lines in a file <code>cdmysql.cd</code>, you can
compile and link it with:
</p>

<sample>
cduce --compile cdmysql.cd -I `ocamlfind query mysql`
ocamlfind ocamlc -o cdmysql -pp "cdo2ml -static" -impl cdmysql.cdo   \ 
  -package cduce,mysql -linkpkg
</sample>

</section>



<section title="Evaluating CDuce expressions">

<p>
This example demonstrates how to dynamically compile
and evaluate CDuce programs contained in a string.
</p>

<sample>
<![CDATA[
let pr = Cduce_lib.Value.print_utf8

try 
 let l = Cduce_lib.Cduce.eval 
  "let fun f (x : Int) : Int = x + 1;;
   let fun g (x : Int) : Int = 2 * x;;
   f;; g;; 
   let a = g (f 10);;
  "
 in
 transform l with
  | ((`Some,id),v) -> 
	pr [ !id ' = ' !(string_of v) '\n' ]
  | (`None, f & (Int -> Int)) ->
        pr [ !(string_of (f 100)) '\n' ]
  | (`None,v) -> 
	pr [ !(string_of v) '\n' ]
with (exn & Latin1) ->
  print [ 'Exception: ' !exn  '\n' ]
]]>
</sample>

<p>
If you put these lines in a file <code>eval.cd</code>, you can
compile and link it with:
</p>

<sample>
cduce --compile eval.cd -I `ocamlfind query cduce`
ocamlfind ocamlc -o eval -pp "cdo2ml -static" -impl eval.cdo   \ 
  -package cduce -linkpkg
</sample>

</section>

<section title="Use CDuce to compute the factorial on big integers">

<sample>
(* File cdnum.mli: *)

val fact: Big_int.big_int -> Big_int.big_int


(* File cdnum.cd: *)

let aux ((Int,Int) -> Int)
 | (x, 0 | 1) -> x
 | (x, n) -> aux (x * n, n - 1)

let fact (x : Int) : Int = aux (Big_int.unit_big_int, x) 
  (* Could write 1 instead of Big_int.unit_big_int. Just for fun. *)

</sample>

</section>

</box>

</page>