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As someone who's implementing closures in a JVM-based language, I can tell you there's no technical limitations on the JVM side: there are decisions to be made

by akeefer 17y ago

As someone who's implementing closures in a JVM-based language, I can tell you there's no technical limitations on the JVM side: there are decisions to be made as to how you want to implement them, but it's certainly doable with enough compiler magic. That said, the BGGA proposal is a bit of a disaster in my opinion: the distinction between "restricted" and "unrestricted" closures, along with non-local returns, just makes no sense. My assumption is that they want to use closures for things like control structures, which they're just totally inappropriate for, rather than as just anonymous functions, which is what they really ought to be. Basically, it's what they did with generics: take a relatively sensible concept, then add a bunch of baroque edges to it for no good reason. Honestly: => and ==> are almost equivalent things with totally different semantics as far as control structures? Really? People are going to understand that about as well as they understand why they can't add an Object to List<? extends Object>, or why they can't pass a List<String> back from a method that says it returns List<Object>. By which I mean, not at all.

guelo 17y ago
The info is sketchy at this point but it seems that Stephen Colebourne's First Class Method proposal is the one in the running. http://docs.google.com/Doc?id=ddhp95vd_0f7mcns http://docs.google.com/Doc?id=ddhp95vd_0f7mcns
jrockway 17y ago
The design principle behind Java is "don't add features that could confuse programmers". (This confuses me, a programmer, but that is a bit too meta for this post.) Despite this principle, Java's users asked for generics, and so the language designers added them. But with a twist; they intentionally made the simple concept very confusing. This is so that the next time someone asks for a feature, they can point at generics and say "look at how badly we broke that! do you really want us adding more features?" A few episodes like this (closures will be next), and people won't ask for features anymore. With no more feature requests, it will become clear that Java Is Perfect, and the designers can retire with a very smug feeling of self-superiority. I think.
wgren 17y ago
"That said, the BGGA proposal is a bit of a disaster in my opinion: the distinction between "restricted" and "unrestricted" closures, along with non-local returns, just makes no sense." The BGGA proposal was recently updated: "The concepts of restricted and unrestricted function types and closures have been removed. Now, all lambda expressions are restricted. The equivalent of the previous specification's unrestricted closure may be passed to a method by using the control invocation syntax." It is rumored that it is something like a compromise between the BGGA and the FCM proposals that Sun is pushing.

by akeefer 17y ago

guelo 17y ago

The info is sketchy at this point but it seems that Stephen Colebourne's First Class Method proposal is the one in the running. http://docs.google.com/Doc?id=ddhp95vd_0f7mcns http://docs.google.com/Doc?id=ddhp95vd_0f7mcns

jrockway 17y ago

The design principle behind Java is "don't add features that could confuse programmers". (This confuses me, a programmer, but that is a bit too meta for this post.) Despite this principle, Java's users asked for generics, and so the language designers added them. But with a twist; they intentionally made the simple concept very confusing. This is so that the next time someone asks for a feature, they can point at generics and say "look at how badly we broke that! do you really want us adding more features?" A few episodes like this (closures will be next), and people won't ask for features anymore. With no more feature requests, it will become clear that Java Is Perfect, and the designers can retire with a very smug feeling of self-superiority. I think.

wgren 17y ago

"That said, the BGGA proposal is a bit of a disaster in my opinion: the distinction between "restricted" and "unrestricted" closures, along with non-local returns, just makes no sense." The BGGA proposal was recently updated: "The concepts of restricted and unrestricted function types and closures have been removed. Now, all lambda expressions are restricted. The equivalent of the previous specification's unrestricted closure may be passed to a method by using the control invocation syntax." It is rumored that it is something like a compromise between the BGGA and the FCM proposals that Sun is pushing.