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

Andy Stewart lazycat.manatee at gmail.com
Fri Jul 16 00:06:02 EDT 2010

Don Stewart <dons at galois.com> writes:

> lazycat.manatee:
>> 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.
> Well, the other approach to reloadable modules, using either object code
> plugins, or bytecode plugins, giving you module-level granularity.
Thanks for your reply.

I have read your papers : "Dynamic Application From the Group Up" and  "Plugging Haskell In"

In "Dynamic Application From the Group Up", you introduction how to use
re-compile technology implement source-code level hot-swapping.

In "Plugging Haskell In", you introduction to how to buld hot-swapping
with object-code level.

Yes, Dynamic linking can add new code to a running program, but how to
replace existing binding with new ones? 
Looks you still need some reboot when you do *replace* and not just *add*.

Infact, reboot is okay, only problem is *keep state*, some *static state*
is easier to re-build, example, if you want restore editor buffer state, you
just need save (filepath, cursorPosition), you can re-open file and
restore cursor position after reboot process.

Difficult is *Stream State*, such as:
  delete operation in file-manager
  command running in temrinal
  network communications in browser
It's really difficult to restore those state, and re-execute is
un-acceptable sometimes.

You can found the screenshot of my project at http://www.flickr.com/photos/48809572@N02/

Currently, the closest library to implement dynamic linking is your
plugins package (http://hackage.haskell.org/package/plugins-1.4.1), 
i really want to write some code to test it, unfortunately, it's
broken with Cabal- that can't compile with ghc-6.12.x/ghc-6.12.3, 
can you fix it if you have time? It's so great package...

I'm looking for some paper about "Haskell and hot-swapping".
Any paper or suggestion are welcome!

  -- Andy

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