Improve error handling

[mheinzel]

Previously, in some places error and exitFailure were used to handle errors, so the whole program terminated.
Now the parsing uses Either (which was much simpler than I thought) and in case of an uncaught exception, the continuation is aborted.

To obtain the terminating continuation, we have to use callCC inside the Interpreter monad, which means it can no be set in the initial state. I'm not really happy with that solution, but can't think of anything more elegant at the moment.

The nice thing is, the REPL now doesn't close itself when encountering a parsing error or uncaught exception. It just ignores the current line, like the regular Python REPL.

This is the first time I'm working with monad transformer stacks, so it takes some time to grasp all the details, but it's fun and I already learned a lot. I want to do 1 or 2 more small fixes, then I am going to tackle generators.

Matthias

[mheinzel]

Put this one on hold for a moment, please. There's still something I need to check.

[mheinzel]

Okay, parsing errors now result in a non-zero exit code, but for uncaught exceptions it is a bit harder to do (without calling exitFailure in the handler).

Currently, encountering the default exception handler terminates execution of the continuation, but to recognize it terminated early, we'd need to add a stateFailed flag to InterpreterState or introduce something like ExceptT to the tranformer stack.

Any thoughts?

[mheinzel]

Now it works out. In case of an uncaught exception, name and text are written to stateErrorMsg, the continuation is used to terminate execution and runInterpreter returns Left (stateErrorMsg newState).

[mattgreen]

Looks good, thanks!