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From "Paul King (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (GROOVY-8301) break/return/continue support in "Appended Block Closures"
Date Thu, 22 Mar 2018 01:08:00 GMT

    [ https://issues.apache.org/jira/browse/GROOVY-8301?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16408877#comment-16408877
] 

Paul King edited comment on GROOVY-8301 at 3/22/18 1:07 AM:
------------------------------------------------------------

There is a back-door mechanism for doing break that was originally baked into Closures (you
can set a directive to DONE or SKIP) but SKIP isn't implemented at all and DONE is rarely
supported in the codebase. It has to be hard-coded into each DGM method. There are a few that
support it:
{code}
100.times {
  if (it == 4) directive = Closure.DONE
  println it
}

def collector = []
(1..100).collect(collector){ if (it == 4) directive = Closure.DONE; it ** 2 }
println collector
{code}
We haven't advertised this mechanism because we wondered whether we should delete it since
it is hardly supported. And that may still be the best thing to do - but I mention it here
while ideas are being discussed.



was (Author: paulk):
There is a back-door mechanism for doing break that was originally baked into Closures (you
can set a directive to DONE or SKIP) but SKIP isn't implemented at all and DONE is rarely
supported widely. It has to be hard-coded into each DGM method. There are a few that support
it:
{code}
100.times {
  if (it == 4) directive = Closure.DONE
  println it
}

def collector = []
(1..100).collect(collector){ if (it == 4) directive = Closure.DONE; it ** 2 }
println collector
{code}
We haven't advertised this mechanism because we wondered whether we should delete it since
it is hardly supported. And that may still be the best thing to do - but I mention it here
while ideas are being discussed.


> break/return/continue support in "Appended Block Closures"
> ----------------------------------------------------------
>
>                 Key: GROOVY-8301
>                 URL: https://issues.apache.org/jira/browse/GROOVY-8301
>             Project: Groovy
>          Issue Type: New Feature
>          Components: Compiler
>            Reporter: mgroovy
>            Priority: Major
>              Labels: break, closure, continue, inline, return
>
> This proposal revisits the idea to change the semantics of the break, return, and continue
keyword for closures that are used in a way that mimics Java block constructs such as if,
for, or while.
> Example 1:
> {code}
> // Iterate over all PERSON rows in the result set
> sqe.forEachRow("select * from PERSON") { final row ->
>   if(row.TAG == 0) { break } // Leave forEachRow loop
>   if(row.TAG == 1) { return } // return from the method enclosing the forEachRow loop
>   if(row.TAG == 2) { continue } // Move to next iteration in the forEachRow loop
> }
> {code}
> Example 2:
> {code}
> // Encapsulate with statically imported helper method that catches exceptions and ignores
them, if the passed parameter is true
> execWithOptionalIgnoreErrors(ignoreErrorsQ) {
>   final x = dangerousOperation() 
>   if(x == 1) { return } // return from the enclosing method
>   // Note: break & continue are not allowed here
> }
> {code}
> Support for continue can trivially be mapped to a return statement inside the closure,
so what one needs to look at is how to support leaving the iteration ("break" semantics) and
returning from the enclosing method ("return" semantics). With current Groovy closures there
are two known potential approaches to achieve this behavior:
> # Use try-catch to simulate a non-local goto to "jump" to the right spot.
> ** Described e.g. here http://blackdragsview.blogspot.co.at/2006/09/non-local-transfers-in-groovy-90.html
by [~blackdrag].
> ** Drawbacks of the approach:
> ### If the user wraps a try-catch-everything (i.e. catching Exception or Throwable base
class) around the code, the mechanism will break in an unexpected way.
> ### Exceptions incur a certain performance overhead.
> # Use special return values that indicate whether to break or return.
> ** This evidently works for all cases which have no return value (e.g. forEach)
> ** For all other cases one could make the closure return an Object, which would again
allow to discern regular return values from break/return-semantics return values, if specific
classes (e.g. GroovyClosureBreak) are returned
> *** Since casting is required in this case, as small overhead is again incurred
> ** A big drawback is that for both cases for "return" semantics all surrounding methods
would need to have / made to have Object as return type.
> ** This approach could be used to supply "continue" and "break" semantics only.
> *** Note that having more than one return in a method is also often considered bad style,
so one could argue that "continue" and "break" semantics are the more important of the three
to support.
> A different approach would be to introduce "inline closures" to Groovy. These closures
would effectively be treated as inline block constructs, the same as they already exist for
e.g. regular for loops. This feature would make contine/break/return support for "closures"
trivial, since the keywords would just automatically work as expected if the closure code
is inlined.
> * This approach is used e.g. by the Kotlin programming language https://kotlinlang.org/docs/reference/inline-functions.html
> * Kotlin uses a seperate "inline" keyword for this. I feel like supplying an annotatio
in Groovy to indicate inlining would be the more Groovy/flexible (can be combined with other
annotations) approach (Kotlin to me generally seems to suffer from an overabundance of keywords)
> * Allowing inlining is also a feature that could also be applied for general methods.
> * Leaving implementation effort aside, this approach to me looks like the best way forward,
it
> ## never breaks
> ## never incurs any performance overhead
> ## is elegant
> ## potentially can be used for applications that go beyond the topic of this issue
> * Note: The general version of this feature has already been suggested in GROOVY-6880
with regards to performance improvements. I agree that potential performance improvements
are not a strong enough argument to introduce this, automatic break/contine/return support
for inlined closures to me is.



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