In Io, all code is data. Below is an example of changing a functions behaviour (from addition to subtraction):

    Io> plus := block (a, b, a + b)
    ==> method(a, b, 
        a + b
    )
    Io> plus call (1, 2)
    ==> 3
    Io> plus message next setName ("-")
    ==> -(b)
    Io> plus
    ==> method(a, b, 
        a - b
    )
    Io> plus call (1, 2)
    ==> -1