How could I design the following code in a more fu...

# announcements

[Philipp Mayer]

How could I design the following code in a more functional, immutable way?
There's a single string where I want to throw multiple regexes on it. The size of the list of regex is flexible, so I can't operate over a set range.
I'm actually refactoring some guys code and it grinds my gears that I cannot come up with a simpler solution for that:

var label = when {
    ..someConditions.. -> "someString"
    else -> ""
}

regexes.forEach {
    label = label.replace(it.toRegex(), "").trim()
}

return label

I should maybe just step away from it for some time.. 😄
any hint is appreciated, thanks!

[ephemient]

label = regexes.fold(label) { label, regex -> label.replace(regex, "") }

[nanodeath]

any

list.forEach {
    x = f(x)
}

is often best modeled as a fold/reduce-type thing.
only exception is probably if you're working with collections...List#add makes a copy each time, which can have atrocious performance, so gotta watch out for that.

[ephemient]

in which case you can use a .mapTo() 🙂

[Joel]

@nanodeath @ephemient can you give a brief example of the problem and solution? This reminds me of a day back in Java 101 but I can't seem to recall the rest of the lesson 🤣

[nanodeath]

the problem is "how do I avoid using var when applying a list of operations to a value"

[nanodeath]

and the solution is to fold over the list

[Joel]

Sorry, the List#add performance issue

[nanodeath]

ah

[nanodeath]

tbh I didn't follow the mapTo suggestion

[ephemient]

instead of or val result = list.fold(listOf()) { result, it -> result + it } or val result = mutableListOf(); list.forEach { result.add(f(it)) }

[ephemient]

use list.mapTo(result) { f(it) }

[ephemient]

well, of course in a simple case like this you could just use .map(), but in general .mapTo() has its uses too

[Joel]

Is the issue with ArrayList#add that it creates a new array under the hood each time?

[ephemient]

yes, exactly

[Philipp Mayer]

Thanks for your help! Finally a good point to start looking into fold. I was always wondering what a usecase for it could be.

[Matteo Mirk]

you have a use for fold() any time you need to apply some function to a list to compute a single value out of it