Emil Axelsson wrote:
Hello Heinrich! Thanks for your interest!
In principle, this is exactly the kind of thing you can do with
syntactic -- if you can make your data type fit into the framework, that
is. A compilation phase would then look something like this:
phase :: AST (C1 :+: TheRest) -> AST (C2 :+: C3 :+: TheRest)
This phase destroys the constructor C1 and introduces C2 and C3.
Now, the question is whether your data type can be made to fit into the
framework. The only problem I can see (but there may be more!) is that
you'd have to treat EventD and BehaviorD as parts of the same data
structure. I've pasted an example of how to do that here:
http://hpaste.org/46525/
Here, the difference between Event and Behavior is denoted by the type
constructors E and B respectively:
type Event a = ASTF (Never :+: B2E :+: Pure :+: E2B) (E a)
type Behavior a = ASTF (Never :+: B2E :+: Pure :+: E2B) (B a)
You decide if this seems like an acceptable solution...
Ah, interesting trick. I think it's ok because none of the data types
contains superfluous constructors.
I'm also unhappy about some of the boilerplate. For instance, have a
look at the function goE in compileAccumB (line 210), it's just a
generic applicative traversal through the data type.
Most likely, this boilerplate could be simplified using syntactic.
Hm, does the boilerplate get removed or only simplified? I was hoping
that one could use a completely generic traversal; but is that actually
the case?
Would your library make me happier? Note that I have an additional
complication: I need to observe sharing and I need to support
recursive AST.
I plan to add observable sharing soon. Once that's is in place, it
should -- I think/hope -- be easy to support cycles. This is not
something we need in Feldspar, so it might take a while before I get
around to supporting cycles, though.
I'd be happy to give more assistance should you choose to try out
syntactic for your library!
Thanks for your offer!
On closer inspection, I'm discovering another issue, namely the need for
the Typeable class. This is quite unfortunate, because it would mean
that I won't be able to make an API built on type classes like Functor
or Applicative. Some discussion on that can be found at the end of
http://apfelmus.nfshost.com/blog/2011/04/24-frp-push-driven-sharing.html
So, it looks like I can't make use of syntactic at the moment. Then
again, my library is probably one of the strongest tests of expressivity
for endeavors like syntactic , so that's fine. Another example of
similar difficulty would be D. Swierstra's recent parser/grammar
combinators that can handle left-recursive grammars. Once syntactic
can deal with those, you're the king! :)
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
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