gistfile1.groovy

[].eachWithPeek { current, peek ->
    assert false // shouldn't get here, nothing to iterate through
}

[1].eachWithPeek { current, peek ->
    assert current == 1
    assert peek == null  // only 1 element, nothing to peek at
}

def results = []
[1, 2, 3, 4, 5].eachWithPeek { current, peek ->
    results << [current, peek]
}
assert results == [[1, 2], [2, 3], [3, 4], [4, 5], [5, null]]