2011-05-28 11:35, Heinrich Apfelmus skrev:
Emil Axelsson wrote:
Hello!
Lacking a proper blog, I've written some notes about Data.Unique here:
http://community.haskell.org/~emax/darcs/MoreUnique/
This describes a real problem that makes Data.Unique unsuitable for
implementing observable sharing.
The document also proposes a solution that uses time stamps to generate
symbols that are "more unique".
Does anyone have any comments on the proposed solution? Are there any
alternatives available?
I don't know how Data.Unique is implemented. For observable sharing, I
usually implement my own variant of Data.Unique with a global
variable/counter.
This is how Data.Unique is implemented too. Would your implementation
pass the test I posted at the link above?
Since every module of my DSL depends on the same
global variable, only two things should happen:
* Reloading a module does not reload the global counter. Everything is fine.
* Reloading a module does reload the global counter. This forces *all*
modules to be reloaded, which removes all expressions that have used the
old variable from scope.
But Simon Marlow just said that CAFs are reset upon reload. So the first
situation should never occur(?). And as my example shows, resetting the
counter does not force all dependent modules to get reloaded.
However, note that the problem is not so much with Data.Unique; the real
problem is that you can't say "I want this to be unique", you have to
say "I want this to be unique within this or that *context*". Imagine
that I give you two expressions with some Uniques inside, some of them
equal, some of them not. How do you know that two equal Uniques denote
the same subexpressions? For that, you have to know that I created them
in the same context, in the same *environment*.
Using Data.Unique assumes that the environment is one program run. It is
clear that reloading modules in GHCi leads to different environments.
OK, so I guess you could say that my solution makes the context explicit
by associating it with a time stamp. Of course, this is just an
approximation, because it's not impossible for two different contexts to
get the same time stamp, especially in a parallel setting. However, it
seems that this somewhat ugly trick might actually be what makes this
approach to observable sharing work in practice.
Concerning observable sharing, I very much like the approach from
Andy Gill. Type safe observable sharing.
http://www.ittc.ku.edu/csdl/fpg/sites/default/files/Gill-09-TypeSafeReification.pdf
which uses StablePointers . Unfortunately, it forces Typeable
contraints on polymorphic combinators.
As far as I've heard, the semantics of stable pointers is not always
well-defined either (but I could be wrong). But I should look into
whether this technique could be used with the syntactic library. It
would be nice to be able to gather several techniques under a common
interface.
/ Emil
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