[Haskell-cafe] Re: DDC compiler and effects; better than Haskell?

[Artem V. Andreev]

"John A. De Goes" <john at n-brain.net> writes:

> On Aug 15, 2009, at 6:36 AM, Jason Dusek wrote:
>> 2009/08/14 John A. De Goes <john at n-brain.net>:
>>> Hmmm, my point (perhaps I wasn't clear), is that different
>>> effects have different commutability properties. In the case
>>> of a file system, you can commute two sequential reads from
>>> two different files.
>>
>>  I think this is a bad example -- it's not something that's
>>  safe in general and discredits your idea. How would the
>>  compiler even know that two files are not actually the same
>>  file?
>
> I don't think the file system is the best example. However, I do think  it's a reasonable one.
>
> Let's say the type of the function getFilesInDir is annotated in such  a way as to tell the effect
> system that every file in the returned  array is unique. Further, let's say the type of the
> function  makeNewTempFile is annotated in such a way as to tell the effect  system that the
> function will succeed in creating a new temp file with  a name unique from any other existing
> file.
Sorry, but this example is ridiculuous. While file *names* in this case might be reasonably assumed 
to be unique, the *files* themselves may not. Any modern filesystem does support file aliasing,
and usually several forms thereof. And what does makeNewTempFile function do? Does it create a new
file like POSIX mktemp() and return its name, or does it rather behave as POSIX mkstemp()? 
The first case is a well known security hole, and the second case does not, as it seems to me, fit
well into the rest of your reasoning.

However, let's consider further file system tree traversal. In some cases you might not care, whether
some of the directories you descend into are actually the same directory, so your proposed optimization
would be `safe'. However, in other cases sequential traversal would work, while a parallelized version
would not, unless special additional measures are taken. E.g. consider a case of a build system. It
traverses a source tree, finds sources files and if corresponding object files are non-existent or
outdated, does something to regenerate them. Now if you have a directory that's actually a link to
another directory, and you do sequential traversal, everything is fine: you descend into the directory
the first time, build everything there and when you descend into it the second time, there's just nothing
to do. If you do parallel traversal, you may well end up in the situation where two threads check
simultaneously for an object file, discover it's outdated and run two build processes simultaneously,
with the most likely effect of corrupted object file.


> Then if you write a recursive function that loops through all files in  a directory, and for each
> file, it parses and compiles the file into a  new temp file, then a sufficiently sophisticated
> compiler should be  able to safely transform the recursion into parallel parsing and  compilation
> -- in a way that's provably correct, assuming the original  program was correct.
>
> The promise of a language with a purely functional part and a powerful  effect system for
> everything else is very great. And very important in  the massively concurrent world we are
> entering.
>
>>  Well, yes -- which sounds like, there are no guarantees
>>  in general. Something that works half the time leaves you with
>>  two responsibilities -- the old responsibility of the work you
>>  did when you didn't have it and the new responsibility of
>>  knowing when it applies and when it doesn't.
>
> In the other thread, I brought up the example of buffering reads.  Library authors make the
> decision to buffer for one reason: because if  some other program is messing with the data, you're
> screwed no matter  what.
>
> And yeah, "they might be screwing with the data in just the way you  need it to be screwed with,"
> (Sebastian), in which case my advice is  use C and hope for the best. :-)
>
> Regards,
>
> John A. De Goes
> N-Brain, Inc.
> The Evolution of Collaboration
>
> http://www.n-brain.net    |    877-376-2724 x 101
>
>
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					S. Y. A(R). A.