Exception transparency

[Reinier Zwitserloot]

Nope; anonymous inner classes are pure, and they can access locals from
outer scope. As long as they are final, though. The implementation shows why
this holds: When a variable is final, a copy is indistinguishable from the
original (and this is in fact how anonymous inner classes accessing outer
final locals is done by javac). Therefore, access to an immutable variable
from outer scope is no different from passing that variable along as a
parameter to the closure, which is conceptually similar to currying that
parameter into the closure. Which is clearly pure.

--Reinier Zwitserloot



On Tue, Jun 8, 2010 at 7:51 PM, Nathan Bryant
<nathan.bryant at linkshare.com>wrote:

>  Reinier:
>
>
>
> You mean A) directly or indirectly, right? A pure function may not access a
> mutable variable from another context… including statics, threadlocals, any
> big resource pool that’s hanging off of some static somewhere, or an object
> to which you have a reference… etc. In other words, a pure function may only
> call pure functions. No?
>
>
>
> If this is not what you mean, then it seems to me that you are talking
> about the “weaker, more complex definition of pureness” that I was alluding
> to.
>
>
>
> *From:* reinierz at gmail.com [mailto:reinierz at gmail.com] *On Behalf Of *Reinier
> Zwitserloot
> *Sent:* Tuesday, June 08, 2010 1:39 PM
> *To:* Nathan Bryant
> *Cc:* Paulo Levi; Brian Goetz; lambda-dev at openjdk.java.net
> *Subject:* Re: Exception transparency
>
>
>
> Nathan, I don't think pure means what you think it means. Or I'm horribly
> confused, also an option.
>
>
>
> "pure" at least in sofar I understand it within the confines of this
> discussion, simply means: "Does not assume that the runtime stack matches
> the lexical stack". No more, no less. So, everything is pure, unless it is a
> closure that does one or more of the following things:
>
>
>
> A) Access a variable from lexical scope that is outside the closure itself.
> The variable isn't final, and isn't effectively final.
>
>
>
> B) returns, breaks, or continues from inside the closure to outside the
> closure; i.e. the closure breaks a for loop in the containing method. (Not
> on the agenda for java7 closures so far, but nice to make possible, if not
> for java7, then for java 8)
>
>
>
> C) throws a checked exception which is not handled by the closure (as in,
> the closure type doesn't have a "throws TheCheckedException" in its
> signature), but it IS handled by the containing method (either because the
> closure is defined in a try/catch block that catches that exception, or the
> containing method has a "throws TheCheckedException" clause).
>
>
>
>
>
> Any closure that engages in none of those 3 things is pure. Pure does not
> mean "I have no side effects".
>
>
> --Reinier Zwitserloot
>
>
>  On Tue, Jun 8, 2010 at 5:47 PM, Nathan Bryant <
> nathan.bryant at linkshare.com> wrote:
>
> A simple example:
>
>
>
> public @pure int sum(int a, int b) {
>
> log.debug(“sum!”);
>
> return a + b;
>
> }
>
>
>
> log.debug can never be strictly pure, but surely we don’t want to disallow
> it!
>
>
>
> *From:* Paulo Levi [mailto:i30817 at gmail.com]
> *Sent:* Tuesday, June 08, 2010 11:44 AM
> *To:* Nathan Bryant
> *Cc:* Brian Goetz; Reinier Zwitserloot; lambda-dev at openjdk.java.net
> *Subject:* Re: Exception transparency
>
>
>
> Why is it not workable? Do programmers have trouble understanding that they
> can't use private methods outside of their class to give a foolish example?
>
>
> Why would they have problems understanding that they can only use "pure"
> methods in the body of a pure function -recursive obvs? (aside, here you can
> see why constructors, and SAM types are still useful, mark a method as pure,
> but leave the constructor to deal with the impurity).
>
> Seems like the most elegant solution, and frankly almost a requirement for
> that the new parallel api's if they are to maintain a little sanity. The
> alternative is to retrofit it later if the concept becomes available, that
> will break code, and thus be used as a excuse to do nothing, as usual.
>
>
>