> So you'd need a runtime instanceof test for Class, and use the
> fastpath if true, reflection if not.
>
> Perf could be harder to pin down, as adding an import could cause
> previously fast code to get slow.
Actually, I was thinking of performing the inference based on *all*
classes on the classpath, not just the ones which have been imported.
However, considering the fact that Clojure allows dynamic additions to
the classpath, coupled with the fact that not all available classes
need to be loaded, this might not be the best idea. The idea of call-
point import scoping is an interesting one, I'm not sure how solid you
could make it though. I mean, don't you look at the FnExpr types
prior to analyzing its usage? (at least, that's how I've always
written my compilers) At the very least, it might make library
functions a little more interesting (certainly ruling out any pre-
compilation).
With respect to the separate paths issue, I assume that you're talking
about something like this:
(defn bad-boy [c x]
(if c
(.getMethods x)
(.length x)))
I think in this case the proper inference would be #{} -- in other
words, reflection. The overhead of trying to fastpath and then
fallback in certain branches would probably outweigh any benefits in
this sort of edge case.
> The best you could do is make a type-inferred fastpath, since you
> could always be wrong
Yeah, that is annoying. I hadn't thought of the fact that you could
infer at declaration point and get a totally unambiguous type but
still be wrong (since calls in different scopes could pass unexpected
types that happen to fulfill the structural signature). How much
overhead does #getClass() impose these days? I heard they were
optimizing some of that in Java 6, but I haven't actually benchmarked
anything. I suppose a simple check of that sort wouldn't ever be more
expensive that a full-blown reflective call, but still.
At what point are functions actually compiled? Is it possible to
apply some of the techniques used in tracing VMs (e.g. V8) to get some
improved context before performing the inference? In that case, you
could actually create several different fast-path compilations
+fallback. There are just so many complicated places you could go
with this, it's just a question of implementation details (and of
course, time to do it).
Daniel
On Nov 6, 4:26 pm, Rich Hickey <[EMAIL PROTECTED]> wrote:
> On Nov 6, 4:31 pm, Daniel Spiewak <[EMAIL PROTECTED]> wrote:
>
>
>
> > I've been perusing Stu Halloway's beta of "Programming Clojure" and I
> > came across the following example:
>
> > (defn describe-class [c]
> > {:name (.getName c)
> > :final (java.lang.reflect.Modifier/isFinal (.getModifiers c))})
>
> > As demonstrated by the *warn-on-reflection* flag, Clojure is unable to
> > determine a type for c, therefore all (or rather, almost all) of the
> > calls made upon it must be handled reflectively. The performance
> > "solution" suggested in the book is just to annotate the c parameter
> > with a type:
>
> > (defn describe-class [#^Class c] ...)
>
> > This is a perfectly valid solution, but it seems a little ugly to me.
> > The thought that I had is perhaps the Clojure compiler could actually
> > infer this type automatically. Languages like ML and Haskell have
> > been doing this for decades. Traditionally, Hindley-Milner isn't
> > applied to object-oriented languages (or rather, languages which deal
> > with uncontrolled OO constructs), but from what I understand, most of
> > the reasoning behind this does not apply to Clojure. For example,
> > Clojure doesn't have structural or parametric types because of its
> > dynamic nature. There would be no *need* to infer any type parameters
> > on calls out to Java, so the type system could be treated as nominal.
>
> > The algorithm I'm thinking of would be something like this:
>
> > * Maintain a multi-map of method and field names to classes (this
> > could be done incrementally based on what classes the compiler knows
> > are in scope)
> > * For each parameter with an unfixed type:
> > * Traverse the FnExpr AST and find all Java member access to that
> > parameter
> > * For each access, obtain a set of possible classes from the scope
> > multi-map
> > * At each step, intersect the previous set of possible classes with
> > the subsequent one (think: a fold)
> > * Final set can be optimized by choosing the most-specific type
> > (eliminating the inheritance case)
> > * If the final set has size of 1, then the type has been inferred
> > and can be used to avoid reflection
> > * If the set has size 0, then fallback on reflection
> > * If the set has size >1, either fallback on reflection or get more
> > sophisticated
>
> > Improvements on this could be added to the >1 case where the second
> > pass could look for methods of a given name *and* a specific arity.
> > Subsequent passes could also incorporate inferred types for the method
> > parameters.
>
> > It's not a complete inference algorithm, but it doesn't have to be.
> > There's nothing stopping Clojure from falling back on reflection for
> > unfixed types. The whole logic behind this dance is to avoid
> > reflection as much as possible, producing a fairly serious
> > optimization in Java-interop without requiring those ugly type
> > annotations.
>
> > I briefly looked at implementing this myself, but Clojure's compiler
> > code is...well, it's a compiler. :-) At first glance, I couldn't
> > even figure out where to place the appropriate hooks to kick off this
> > process. A better question though is: has this been tried/considered
> > before? What are your overall thoughts with regards to this?
>
> It's not a bad idea, and I'm sure Clojure could do more inference than
> it does, but the devil's in the details as usual.
>
> You'd want the least specific type.
>
> The best you could do is make a type-inferred fastpath, since you
> could always be wrong:
>
> In the case above, let's say you had imported Class (actually
> Clojure did that for you), and nothing else you imported had getName
> and getModifiers methods. Making the inference 'must be Class' could
> be wrong, since the function should work fine on
> java.lang.reflect.Method, for instance, and maybe that was the intent.
>
> So you'd need a runtime instanceof test for Class, and use the
> fastpath if true, reflection if not.
>
> Perf could be harder to pin down, as adding an import could cause
> previously fast code to get slow.
>
> Calls can be made only in specific conditional branches, which some
> higher-level logic might understand maps to non-overlapping types, so
> the inference should be of the call site, not the parameter itself -
> i.e. it's perfectly fine to write fns that expect a set of unrelated
> types, without being duck-typed. In fact it's even ok to type hint it
> even though the hint is only correct in one path.
>
> You have to allow for by-name duck-type calls or call them out
> specifically. Right now they are allowed.
>
> I think a simple version of this would be easy to do, when I get some
> time.
>
> I think it hasn't been a priority as it takes very few hints at the
> moment to remove reflection, given that types are tracked from calls
> to new and static calls, and from any other hinted calls. But it would
> be great to not need them at all!
>
> Rich
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