Multiple dispatch again

yaipa h. yaipa at yahoo.com
Thu Jan 2 18:48:17 CET 2003


David,

First let me thank you for all your great articles and your
on-line book in progress.

I am currently writing a multi-disk sector editor for 
use with Storage devices such as digital film cards. These
devices are Flash Memory based and require complex business
logic to move data around because jpg files tend to be smaller
than erase blocks and people tend to erase files one at a time.
Because erase cycles in flash tend to be slow, files not to
be deleted are mapped to other sectors before the sector is
erased.  This all leads to why it is interesting to read/write
the same data to multiple drives and then have a sector editor
that can work with multiple drives referencing the same physical
sector for each device ( think map() et. al. ). Drives are slow 
in respect to CPU time and therefor, if one is working with a 
'bunch' of devices at the same time I believe your Multiple Dispatch 
logic will be of some help.  As the project progresses I will 
incorporate your code and report back my findings to this thread,
although it maybe a month or two before I do so.

Thanks again,

yaipa.h


mertz at gnosis.cx (David Mertz) wrote in message news:<mailman.1041492423.21345.python-list at python.org>...
> I've got to thinking about multimethods/multiple dispatch lately.  I
> wonder if Pythonistas have some futher opinions on the use of these.
> Actually, part of it is that I'd -really- like to better understand use
> cases for multiple dispatch.  As a trick, I can see it has a certain
> elegance, but little rock/paper/scissors toys are not, finally,
> compelling.
> 
> An inspiration for the code I am about to mention is simple and
> beautiful code posted a bit over a month ago by Tim Hochberg.  I played
> with that, and enhanced it fairly greatly at:
> 
>     http://www.gnosis.cx/secret/multimethods.py
> 
> I welcome comments of any sort on the code.
> 
> There are a few things that I had particularly in mind.  In reading
> about Dylan, I found that it has a method named 'next-method( )' to
> propogate dispatch from the closest match to more distant ones.  You can
> control whether you want this to happen by including that method, or
> not.  Take a look at:
> 
>     http://www.tpk.net/~ekidd/dylan/multiple-dispatch.html
> 
> In my code, I added the facility to propogate dispatch.  Using the
> simple example that was a topic of conversation here 6 weeks ago, you
> can make a call like:
> 
>     beats.add_rule((Fire,Thing), firepower, next_meth=AT_END)
> 
> I give you options to propogate dispatch either before or after the code
> within the current method execution.
> 
> As well, I decided to make dispatch resolution order configurable.
> Hochberg's example was perhaps naive in fixing resolution order
> according to definition order.  That seems a little fussier than I would
> want.  So I made that configurable as well, and provide four sample
> resolution functions.  Two are naively definition-order based (either
> forward or reversed).  But two others are perhaps better candidates:
> 'lexicographic_mro()' and 'weighted_mro()'.  If I understand correctly
> (which I may not), the first follows Dylan, the second follows Damian
> Conway' Class::Multimethods.  I would be interested to understand other
> examples, particularly CLOS.  I have little intuition about what is the
> "right" answer.
> 
> Since I allow propogated dispatch, I had to decide what to do with
> results from the function calls.  I decided to accumulate all returned
> values into a list, and let the user pick what they want.  So for
> example:
> 
>     <fire, fire>      ['Fire always wins!', 'Fire always wins!', 0]
> 
> But for a non-propogating rule, the list has a single member:
> 
>     <rock, scissors>  [1]
> 
> If you stick to either 'SKIP' or 'AT_END' propogation, you can count on
> index 0 of the return being the most specific function's value.  Or if
> conversely, you like 'AT_START', you can use index -1.  If you mix the
> two styles, it could become complicated to find relevant return values.
> 
> Yours, David...




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