Functional Groovy switch statement

In the previous post I showed how to replace chained if-else statements in Groovy with one concise switch. It was done for the special case of if-stement where every branch was evaluated using the same condition function. Today I want to make a generalization of that technique by allowing to use different conditionals.

Suppose your code looks like this:

if (param % 2 == 0) {
    'even'
} else if (param % 3 == 0) {
    'threeven'
} else if (0 < param) {
    'positive'
} else {
    'negative'
}

As long as every condition operates on the same parameter, you can replace the entire chain with a switch. In this scenario param becomes a switch parameter and conditions become case parameters of Closure type. The only thing we need to do is to override Closure.isCase() method as I described in the previous post. The safest way to do it is to create a category class:

class CaseCategory {
    static boolean isCase(Closure casePredicate, Object switchParameter) {
        casePredicate.call switchParameter
    }
}

Now we can replace if-statement with the following switch:

use (CaseCategory) {
    switch (param) {
        case { it % 2 == 0 } : return 'even'
        case { it % 3 == 0 } : return 'threeven'
        case { 0 < it }      : return 'positive'
        default              : return 'negative'
    }
}

We can actually go further and extract in-line closures:

def even = {
    it % 2 == 0
}
def threeven = {
    it % 3 == 0
}
def positive = {
    0 < it
}

After which the code becomes even more readable:

use (CaseCategory) {
    switch (param) {
        case even     : return 'even'
        case threeven : return 'threeven'
        case positive : return 'positive'
        default       : return 'negative'
    }
}