Better describe how groovy binding works

[samrocketman]

If a variable has an undeclared type it goes into the script binding. The binding is visible to all methods which means data is shared.

evaluate() is a helper method to allow the dynamic evaluation of groovy expressions using this scripts binding as the variable scope.

In a variable binding, you can declare a closure which accepts no argument and must be restricted to calls without arguments.

With all of that in mind, here is your script working as intended.

evaluate ("test = { -> println \"Test is successful!\" }")
test()