DDC compiler and effects;
better than Haskell? (was Re: [Haskell-cafe] unsafeDestructiveAssign?)
John A. De Goes
john at n-brain.net
Sat Aug 15 19:05:33 EDT 2009
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
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.
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
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. :-)
John A. De Goes
The Evolution of Collaboration
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