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High-level bindings between .Net and OCaml

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CSML: high-levels bindings between .NET and OCaml
=================================================

CSML is a development tool for the community of OCaml developers under
Windows. It lets you write applications that freely mix OCaml and .NET
code.

It can be used in the following scenarios:

- Using .NET libraries (such as Winforms) from applications written
    in OCaml.

- Providing a .NET API to OCaml components.

- Developing mixed applications, where some parts are implemented in
    OCaml and other parts in .NET.

High-level overview
-------------------

CSML is based on an [Interface Definition Language](IDL.md) to declare
bi-directional bindings between OCaml and .NET.  Scripts in this IDL
specify both "pseudo-OCaml signatures" where each member is connected
to a .NET component and "pseudo-C# signatures" where each member is
connected to an OCaml component.  This means that the user is in
charge of organizing the bindings as they see fit. In particular,
it is possible to choose custom names for imported components and to
organize them into classes or namespaces in .NET and classes or
modules in OCaml.

See the CSML spec for the ["getting started"](examples/getting_started/getting_started.csml)
or ["winforms"](examples/winforms/winforms.csml) examples.

From such scripts, the [CSML compiler](USAGE.md) generates C# and
OCaml code to be compiled and linked separately.  This code exposes
components from one runtime to the other one (technically, this goes
to the low-level FFI that both system provide with C).  The main
program is always in .NET and initializes the OCaml engine explicitly
before calling OCaml components (which can then call back into .NET).

CSML takes care of all the *low-level details* of the bindings.  In
particular it takes care of making sure the two runtime systems work
happily together (memory management, in-memory layout of objects,
translation of values).  Basic types (int, string, etc) and built-in
structural types (lists, arrays, options, functions) are understood
directly by CSML.  Complex types can be transmitted by reference (as
opaque pointers from one heap to the other one) or by an automatic
structural mapping (OCaml datatypes translated to .NET classes).

*First-class functions* (in OCaml) and delegates (in .NET) are
considered seriously in CSML. The system relies on predefined
.NET generic delegates to mimic the structural aspect of OCaml arrow
types. A .NET delegate is wrapped into an OCaml closure when it enters
the OCaml world. Similarly, an OCaml closure is wrapped into a .NET
delegate when it goes to the C# world; if this delegate is later sent
back to OCaml, it is unwrapped to retrieve the original closure
instead of being wrapped again.

*Exceptions* raised in one language are wrapped in the other language as
regular exceptions, and round-tripping is supported (e.g. a C#
exception that escapes to OCaml and then back to C# will retain all
the information from the original one).

The interface generated by CSML does not compromise static *type-safety*
as provided by each language. If the binding script is ill-typed on
either side, this will be detected at compilation time.

Requirements
------------

- A native port of OCaml under Windows.  The current version has
    been tested with the MSVC 32-bit and 64-bit ports of OCaml 4.03,
    but two MinGW ports should work as well (perhaps with minor
    adjustments to the build system).

- The Microsoft .NET framework.  The current version has been tested
    with the version 3.5 of the framework, but CSML was initially
    developped against version 2.0.

- Alternatively, CSML has been tested under Linux with Mono 4.x, but
    not used in production.

- Moreover, it is expected that CSML could be made to work with little
    effort with the newest .NET Core framework both under Windows and
    other systems.

Installation
------------

The root Makefile in the distribution has a default target "all" that
builds the CSML compiler and runtime support. There is currently no
support for "installing" the resulting assets.

Documentation
-------------

- [CSML scripts](IDL.md)

- [Using CSML](USAGE.md)

Examples
--------

This repository contains a few sample applications:

- [getting_started](examples/getting_started): show the basics of using CSML.

- [opaque_binding](examples/opaque_binding): abstract binding .NET <-> OCaml
    (values are not copied but transmitted by reference).

- [structured_mapping](examples/structured_mapping): concrete mapping of structured OCaml values
    (.NET classes are generated from OCaml type definitions).

- [winforms](examples/winforms): a sample Windows Forms GUI application implemented in OCaml;
    illustrate importing .NET components as OCaml objects.

- [excel](examples/excel): expose OCaml function to Excel VBA (going through .NET and COM).

Look at [this Makefile](Makefile.examples) to get an idea of how to
compile and link applications built with CSML.

About
-----

CSML has been created by LexiFi and used in production since 2007,
initially purely as an internal tool.  It has been first released to
the community in 2008, but only the runtime support was open-source at
that time (the binding compiler was distributed in binary form).  In
2016, LexiFi decided to switch to a full open-source model for CSML
with an MIT license for the entire system.

Even though LexiFi only maintain CSML but does not put active
development resources into it these days, we are happy to receive
feedback, enhancement proposals or simply discuss future evolutions
and potential uses of the tool.

CSML is currently available as "technology preview" only.  No stable
public release have been created.

Contact: alain.frisch@lexifi.com

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