H/Direct: a binary foreign language interface for Haskell

H/Direct is a foreign-language interface for the purely functional language Haskell. Rather than rely on host-language type signatures, H/Direct compiles Interface Definition Language (IDL) to Haskell stub code that marshals data across the interface. This approach allows Haskell to call both C and COM, and allows a Haskell component to be wrapped in a C or COM interface. IDL is a complex language and language mappings for IDL are usually described informally. In contrast, we provide a relatively formal and precise definition of the mapping between Haskell and IDL.