How does one return from a groovy closure and stop its execution?

I think you want to use find instead of each (at least for the specified example). Closures don’t directly support break.

Under the covers, groovy doesn’t actually use a closure either for find, it uses a for loop.

Alternatively, you could write your own enhanced version of find/each iterator that takes a conditional test closure, and another closure to call if a match is found, having it break if a match is met.

Here’s an example:

Object.metaClass.eachBreak = { ifClosure, workClosure ->
    for (Iterator iter = delegate.iterator(); iter.hasNext();) {
        def value = iter.next()
        if (ifClosure.call(value)) {
            workClosure.call(value)
            break
        }        
    }
}

def a = ["foo", "bar", "baz", "qux"]

a.eachBreak( { it.startsWith("b") } ) {
    println "working on $it"
}

// prints "working on bar"