Reloading GHCi

Starting GHCi with Cabal is fairly easy, as the interpreter takes most of the same arguments we've been feeding GHC. But reloading it is a little more difficult. Admittedly, there are a number of things we don't have to do when reloading:

• All the options given to the interpreter initially seem to come along for the ride with each :reload command, so rendering GHC command line options isn't necessary.
  
• Preparing the build directory and auto-generated files already took place when the interpreter was started, so that stuff isn't required for a reload.

In fact, there are only really two things that need doing. First, there's preprocessing. This is a high level operation not specific to a compiler. At the same time, there's C sources, a step in building restricted to GHC.

I propose adding a flag (--reload, perhaps) to the repl command to handle these two operations, handled by the high level Build.hs code to perform preprocessing and the compiler specific GHC.hs code to perform C source building. Macros would be defined in GHCi to shadow those commands resulting in code reloading, calling cabal repl FILENAME --reload first.

To define these macros, I think I'll need to modify GHCi to take a custom .ghci file. I can't just sneakily add to the user one, and piping in the commands through cat or something would look ugly. As it is, I think the best approach is to add an option for an extra .ghci file. Cabal would write its set of macros to shadow default commands to some place in the dist directory, telling GHCi about the the file.