Decorator

Decorators are a feature built on top of ABS' functions -- they're not a type per se but they do have their own syntactic sugar.

A decorator is a function that "wraps" another function, allowing you to enhance the original function's functionality with the decorator's one.

An example could be a decorator that logs how long a function takes to execute, or delays execution altogether.

Simple decorators

A decorator is a plain-old function that accepts the original function and returns a new function that wraps the original one with its own behaviour. After defining it, you can "decorate" other functions through the convenient @ syntax:

f uppercase(fn) {
    return f() {
        return fn(...).upper()
    }
}

@uppercase
f stringer(x) {
    return x.str()
}

stringer({}) # "{}"
stringer(12) # "12"
stringer("hello") # "HELLO"

As you see, stringer's behaviour has been altered: it will now output uppercase strings.

Decorators with arguments

As we've just seen, a decorator simply needs to be a function that accepts the original function and returns a new one, "enhancing" the original behavior. If you wish to configure decorators with arguments, it is as simple as adding another level of "wrapping":

f log_if_slow(treshold_ms) {
    return f(original_fn) {
        return f() {
            start = `date +%s%3N`.int()
            res = original_fn(...)
            end = `date +%s%3N`.int()

            if end - start > treshold_ms {
                echo("mmm, we were pretty slow...")
            }

            return res
        }
    }
}

That's as simple as that: a named function that returns a new function that executes the decorated one (original_fn) and returns its result, while logging if it takes longer than a few milliseconds.

Now that we've declared our decorator, it's time to use it, through the @ notation:

@log_if_slow(500)
f return_random_number_after_sleeping(seconds) {
    `sleep $seconds`
    return rand(1000)
}

and we can test our decorator has taken the stage:

⧐  return_random_number_after_sleeping(0)
493
⧐  return_random_number_after_sleeping(1)
mmm, we were pretty slow...
371

Decorators are heavily inspired by Python (opens new window) -- if you wish to understand how they work more in depth we'd recommend reading this primer on Python decorators (opens new window).