[IPython-dev] Line-based frontends - architecture

[Thomas Kluyver]

Thinking about the possibility of developing a web console to complement
the notebook, it struck me that the wire protocol doesn't yet provide for
line-based frontends to know when the input is complete, to decide whether
to execute or present another continuation prompt.

With the Qt console, we avoid the issue by running the same input
transformation machinery in the frontend, but that only works for frontends
written in Python.

I see three potential solutions to this:

1. Add a message type to the protocol, by which the frontend can send a
block of code, and the kernel can indicate whether it is 'finished'.
2. Reimplement the logic to determine whether a code block is finished in
the frontend.
3. Leave it up to the user, with separate key combinations for 'execute'
and 'new line', as it is in the notebook.

I prefer number 1. I think the console-like interaction is valuable enough
to make 3 undesirable. 2 sounds like a lot of work, especially as we get
both more frontends and more kernels for different languages, with
different rules about when a block is complete.

Thoughts?

Thanks,
Thomas
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