Our goals are limited. We do not set out to solve the problem of language interoperation and foreign-language interfaces in general; rather, we intend to profit from the work of others' in this area. Specifically, we aim to provide a convenient way to call C procedures from Haskell.
The ability to call C from Haskell is an essential foundation. Through it we can access operating system services and mountains of other software libraries. It also provides a basis on which to tackle the problem of writing and accessing software components from Haskell.
In the other direction, should we be able to write a Haskell library that a C program can use? In principle this makes sense, but in practice there is little demand for it. The exception is the ability to support some sort of Haskell callbacks, but that is a very limited form of C calling Haskell.
Should we support languages other than C? The trite answer is that pretty much everything available as a library is available as a C library. For other languages the right thing to do is to interface to a language-independent software component architecture, rather than to a raft of specific languages. There's a couple of such architectures to choose from; COM and CORBA. This document, however, describes only the C interface mechanism.