Lambda the Ultimate

As I am working (slowly) on a compiler at the moment, I have started thinking about IDE integration. It occurs to me (as I am sure it has to everyone else) that a compiler should not be a command-line program, but a library that can be integrated with applications and IDEs. The command line interface for the compiler should be just another client of the library.

So my question is, in an ideal environment (starting from scratch) what should an API for a compiler (targeting IDE integration as well as other runtime uses) look like? Obviously some method to build the code, some methods for auto-completion, type checking, type inference.

Other thoughts are that all the above functions need to operate in a dynamic environment, where incomplete code fragments exist, without breaking the compiler, and hopefully still offering useful feedback.