Running { println "foo" } in the groovyConsole results in the compiler error message "Closure expression looks like it may be an isolated open block.; solution: Add an explicit parameter list, as in {it -> ...}, or label it as L:{...}.". For me as a groovy newbie it wasn't clear that the first solution marks the code as closure while the second marks it as block, I thought that both would result in a closure. Perhaps the wording should be changed to something like this: "Expression could be either a closure expression or an isolated open block. Solution: Add an explicit parameter list, as in {it -> ...}, to create a closure or label it as L:{...} to mark it as block."