2. Overview

The basic way in which almost any foreign language interface works is this. The signature of each foreign-language procedure is expressed in some formal notation. From this signature, stub code is generated that marshals the parameters ``across the border'' between the two languages, calls the procedure using the foreign language's calling convention, and then unmarshals the results back across the border. Dealing with the different calling convetions of the two languages is usually the easy bit. The complications come in the marshaling of parameters, where data valuesed built by one language is transformed into a form that is comprehensible to the other.

A major design decision is the choice of notation in which to describe the signatures of the procedures that are to be called across the interface. There are three main possibilities:

• Use the host language (Haskell, in our case). That is, give the foreign function a Haskell type signature, and generate the stub code from it. Green Card uses this approach, as does J/Direct (Microsoft's foreign-language interface for Java).
• Use the foreign language (say C). In this case the stub code must be generated from the C prototype for the procedure. SWIG uses this approach.
• Use a separate Interface Definition Language (IDL), designed specifically for the purpose.

For H/Direct, we opt for the use of IDL as the notation to describe foreign-language interfaces (see the H/Direct ICFP paper for the motivations behind this choice.) Specifically, H/Direct uses IDL to help the programmer solve the following four tasks:

• Calling foreign procedures from Haskell. By describing the signatures of the foreign procedures using IDL, H/Direct generates Haskell stubs that correctly invokes these foreign procedures.
• Calling Haskell from the outside world. The dual to previous. By giving Haskell functions IDL signatures, H/Direct generates stubs that allows you to call Haskell functions from C.
• Calling COM components from Haskell. IDL is the de facto standard for textually describing the functionality supported by COM components. H/Direct translates the specification of a COM component into a set of Haskell stubs that allows you to create and use COM components from within Haskell.
• Implementing COM components in Haskell. H/Direct also uses IDL to guide the generation of stubs that takes care of all the details involved in exposing Haskell code as a COM component.

This document presents the programmer view of how H/Direct supports these four different uses of IDL, followed by a section that presents the design and implementation of the layers of stub code that H/Direct generates when given an IDL specification.