Re: [Haskell-cafe] Hot-Swap with Haskell

Thomas Schilling writes:

What would be the semantics of hot-swapping? For, example, somewhere in memory you have a thunk of expression e. Now the user wants to upgrade e to e'. Would you require all thunks to be modified? A similar problem occurs with stack frames.

You'd also have to make sure that e and e' have the same (or compatible types). e' most likely has different free variables than e, how can you translate thunk one to the other?

Now assuming you don't try to do this, how would you handle the case when something goes wrong? Good question.

Infact, currently, i just use same technology with Yi/XMonad. Re-compile modules, and save some initial state before re-launch, and do re-execute some command to re-store those state after re-launch. Re-compile will drop old code, and restore state in new code. But re-compile solution not neat way since some state will lost. About your "upgrade expression" question, i haven no answer, that's why i post this topic to looking for a better solution. I know Emacs use some "indirection symbol table" for hot-swapping, but it's not safe since it not do type-check. Maybe we can wrap 'Dynamic' check type for safe re-load in runtime, if type match then upgrade, otherwise stop upgrade and keep old state. I just test re-compile technology, not try dynamic linking/load technology that Dons introduce in "Plugging Haskell In.pdf" since 'plugins' package is broken at the moment. Anyway, i believe this have a neat way to make static typing language can extension dynamic. Now, i'm reading Dons' new paper "Dynamic Extension of Typed Functional Languages" (http://www.cse.unsw.edu.au/~dons/papers/dons-phd-thesis.pdf) I hope i can find answer after read this paper... Any discuss are welcome! -- Andy

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On 16 July 2010 04:05, Andy Stewart wrote:

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Hi all,

I'm research to build a hot-swap Haskell program to developing itself in Runtime, like Emacs.

Essentially, Yi/Xmonad/dyre solution is "replace currently executing" technology:

  re-compile new code with new binary entry

  when re-compile success      $ do          save state before re-launch new entry          replace current entry with new binary entry (executeFile)          store state after re-launch new entry

There are some problems with re-compile solution:

1) You can't save *all* state with some FFI code, such as gtk2hs, you can't save state of GTK+ widget. You will lost some state after re-launch new entry.

2) Sometimes re-execute is un-acceptable, example, you running some command in temrinal before you re-compile, you need re-execute command to restore state after re-launch, in this situation re-execute command is un-acceptable.

I wonder have a better way that hot-swapping new code without re-compile/reboot.

Thanks,

 -- Andy

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