Load files into REPL

[jstolarek]

I want to be able to load a file when starting a REPL. In theory haskeline has some kind of support for doing this but it didn't work out-of-the-box when I initially hacked the REPL. Need to investigate - this would be a very useful feature.

[jstolarek]

Note to self:

• [x] change desugar to return gamma
• [x] modify parseDesugarEval and desugarEval in Language.Slicer.Run to return gamma
• [x] modify runRepl in REPL monad to allow passing initial state of the REPL (data decls + gamma + store)
• [x] modify main to accept file names and --repl flag at the same time. When file passed parseDesugarEval it and start repl setting the initial state to what was returned.

[jstolarek]

I implemented all of the above (commit 373677c on branch js-slicing-h4x) and it turns out not as simple as I thought it will be. The problematic part is that scopes are nested, so if I say:

let x = 5
in ...

then x is not in scope after that expression has been desugared. The result is that when we load a file into a REPL we end up with an empty environment, so we can't refer to any top-level definitions inside a file. That's probably because we don't even have a concept of top level definitions.

[jstolarek]

Done. I found a much easier and much simpler way of doing this (~30 lines of code). You can now start a REPL by passing a list of REPL scripts to run during startup. Each file should contain single-line definitions (multi-line definitions are not allowed) declared with let bindings (so essentially, just as if you typed them into a REPL). See also #33 for a bug that allows to use program-style let bindings rather than repl-style let bindings.