Re: implementing call/cc with only closures, no special forms ?

["Lalish-Menagh, Trevor" <trev@trevmex.com>]

Thanks Dustin! This is pretty cool stuff.

On Wed, Feb 22, 2012 at 10:04 PM, Dustin Getz <dustin.getz@gmail.com> wrote:
> i think i may have reconciled the problem - he implements callcc and
> continuation-monad without generators, but the actual usage of callcc as
> defined -- `return callcc(self.react)` -- self.react is itself a generator.
> this generator could (i think) be re-implemented with lambdas, but now the
> block passed to call/cc must conform to our interface. which means we can't
> use call/cc with arbitrary code. if i chase down the implications of this, i
> imagine we will end up concluding that, to turn blocking business logic into
> cooperative via low level monkey patches in the style of gevent, would also
> require altering the structure of the business logic itself.
>
>
> On Wed, Feb 22, 2012 at 9:02 PM, Dustin Getz <dustin.getz@gmail.com> wrote:
>>
>> this came up after redsnake last night - I mentioned to someone that I had
>> seen a "lambda" implementation of callcc in python. Which, if that's
>> actually possible, is pretty mind-blowing - because it doesn't require
>> patching the runtime/interpreter !! I'm not quite sure if its possible or
>> how close we can get, but here's the link I was talking about[1]:
>>
>>
>> [1]Âhttp://www.valuedlessons.com/2008/01/monads-in-python-with-nice-syntax.html
>>
>> He implements a few monads in python, including continuation monad. It
>> will take me significant study to understand this code - maybe you guys
>> would be interested in picking this apart at a future meetup - but here's
>> the author's conclusion:
>>
>> So, you can use monads with elegant syntax in any language that has
>> closures and any of the following:
>>
>> do syntax (Haskell, C#)
>> call/cc (Scheme, Ruby)
>> bidirectional generators (Python 2.5, a future JavaScript?)
>> coroutines (Lua, Io)
>>
>> my question is, can you implement continuation-monad with "un-elegant"
>> syntax with *only* closures, skipping the generators? I think maybe you can,
>> because 1) haskell's do-notation is just syntactic sugar over the closures,
>> and 2) you can at least implement one common generator pattern - lazy seqs -
>> in python without generators[2], which got me wondering if you can implement
>> a generalized generator using just lambdas, and now my head is spinning and
>> i'm saying stupid things like "i think you might be able to implement
>> call/cc with only closures" but that implies that we can do things like turn
>> third party blocking code into cooperative code per gevent[3] which is
>> absurd,Âthat's a ridiculous statement. I'm not even making sense anymore, so
>> I'm going to stop writing and click "send" now :)
>>
>> Dustin Getz
>>
>> [2] my implementation of lazy seqs with only
>> closures:Âhttps://github.com/dustingetz/sandbox/blob/master/etc/lazy.py
>> [3] lightning talk slides where we use callcc to make third party code
>> cooperative: https://docs.google.com/present/view?id=dv9r8sv_82cn7dngcq
>
>



-- 
Trevor Lalish-Menagh
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