DEV Community

Cover image for Mutation is ok

Mutation is ok

Pragmatic Maciej on July 06, 2020

The phrase - "mutation" started to have almost negative connotation in our programming community. It's like something wrong to mutate. As if we mut...
Read full post
Collapse
 
kristijanfistrek profile image
KristijanFištrek •

Can't agree more! We should think logically about our software logic and not follow some patterns blindly. 😌

Collapse
 
phlash profile image
Phil Ashby •

Couple of thoughts while reading this excellent article Maciej:

  • Functional core, imperative shell: is a whole-application design ethos that you are not suggesting we break (thanks!), which differs in scale from your examples here of perfectly legitimate mutation within a method/function, however...
  • Internal mutation falls apart if others can attempt to extend/get-inside a thing, this largely impacts next-scale up entities such as classes or libraries that expose the state.
  • Ensuring that referential transparency is maintained is harder to prove when mutating anything, neat/efficient it may be, but proving it's safe for missile guidance may be harder :)
Collapse
 
macsikora profile image
Pragmatic Maciej •

Hi Phil,
Thanks for the comment.

Internal mutation falls apart if others can attempt to extend/get-inside

Extending function is not a common practice in FP. We more should strive for small replaceable units. Instead of adding new use cases to the unit we should compose it with another. Until we keep them small even imperative code should not be any issue.

Ensuring that referential transparency is maintained is harder to prove

Again the same counter argument from my side. Until we keep functions small with one responsibility, nothing like that should happen.

Collapse
 
richytong profile image
Richard Tong •

Be wary of people who tell you you are just plain "wrong". Usually they just wish you were wrong. I fully endorse your views in this article - they are telling of someone who likes functional programming and wants to get sh*t done.

Collapse
 
_hs_ profile image
HS •

OO is OK.
FP is OK.
Java is OK.
Kotlin is OK.
Everything is OK if it gets the job done heheh.
Just saw your series and made some suggestions

Collapse
 
adnanbabakan profile image
Adnan Babakan (he/him) •

Java is literally not OK. xD
(just kidding)

Collapse
 
_hs_ profile image
HS •

There's always one xD

Collapse
 
miketalbot profile image
Mike Talbot ⭐ •

Nice syntax and a feeling of "doing the right thing" lead to very inefficient code when it comes to immutability. This is a very nice article - I took a look at the costs of immutability in tight loops and code myself (which is another lens on this subject):

miketalbot

Tight Code 1: When immutability goes bad

Mike Talbot ⭐ ・ May 27 '20

#javascript #webdev #react
Collapse
 
phlash profile image
Phil Ashby •

My take on this - if the 'feels right' / 'elegant' solution isn't performant, then the language may have a problem - one of the reasons I like Python, the 'right way' is usually pretty quick too!

Collapse
 
miketalbot profile image
Mike Talbot ⭐ •

There is certainly a lot more sugar in JavaScript these days, and it's not always good for you ;)

Collapse
 
slifin profile image
Adrian Smith •

Most modern languages with immutability implement it with persistent data structures, ClojureScript for example, deep efficient immutability is the default

The closest thing you have to persistent data structures in JS is ImmutableJS, though honestly once you've cobbled together rambdajs, react, ImmutableJS, Google Clojure Compiler you should just use ClojureScript

Collapse
 
martinmuzatko profile image
Martin Muzatko • • Edited

Everything which does not belong to the function should not be mutated.

I have experienced that some devs love to write huge functions that span multiple pages. Which is something that is not ok for me. It might be convenient to just add to an ever increasing scope to never juggle state from one function to another, but it quickly becomes opaque which variable is mutated at which point or re-used somewhere else along the pages of code. So the rule of "mutation is ok within the same function" depends on how absurdly you stretch the rules, or in that case the function.

If it is compact, then maybe. But most of the time I prefer a re-usable function that you can apply to a map instead of writing a for loop that collects some data.

FP has the promise of a declarative style that should make it more readable. Sometimes that is true, when you know what the words mean:

const isOddNumber = n => n % 2
const filterOddNumbers = numbers => numbers.filter(isOddNumber)
Enter fullscreen mode Exit fullscreen mode

or better:

const filterList = fn => arr => arr.filter(fn)
const isOddNumber = n => n % 2
const filterOddNumbers = filterList(isOddNumber)
Enter fullscreen mode Exit fullscreen mode

It needs some setup, but once you have the function assembled, you don't need as much boilerplate than with if and for statements EVERY TIME you use it. that is the entire point of FP. You can compact a lot of behavior into one function, give it a fitting name and re-use it as much as you want.

vs

const getOddNumbers = numbers => {
    let result = []
    for (const n of numbers) {
        if (n % 2) result.push(n)
    }
    return result
}
Enter fullscreen mode Exit fullscreen mode

Ok, to be fair, we can apply at least the isOddNumber function within the if condition.
What bugs me the most, is that all these imperative statements (if, switch, for) always come with their own boilerplate. If you work on the result of getOddNumbers you have to do a for loop once more, which adds to the boilerplate.

I know why I do FP and I know why state mutation and statements are most oftenly not ideal to express your desired computation or behavior.

Collapse
 
macsikora profile image
Pragmatic Maciej • • Edited

I would just answer - it depends. We state our opinions, in contrary to you I see also problem in extending dictionary for such simple cases, in my experience creating so many one liner function can end in ton of functions which nobody will use because they just prefer to write them inline. The example which shows that is famous id function, I personally prefer to just use x => x than import id from magic utils, it is just too simple.

As having functions and composing them is a good thing, we need to take into consideration that more one liners mean also more jumping over the code. I was writing Elm for a while and I found myself in preferring using inline if expressions, or inline case of instead of constantly creating new function. As I loose eye contact with what is really going on, and having many functions force me to jump through them. And no, readable name not always is enough to say that we never want to see what is going on.

Also when I see this:

const filterList = fn => arr => arr.filter(fn)
Enter fullscreen mode Exit fullscreen mode

It is 🚩 for me, as this is nothing more like duplicating the already existing tools, such function in my opinion has no value, and even worst introduce smth which is additional in the codebase. So for me your first implementation:

const isOddNumber = n => n % 2
const filterOddNumbers = numbers => numbers.filter(isOddNumber)
Enter fullscreen mode Exit fullscreen mode

is better, and probably I would even wrote that in that way, but this example is simplified and doesn't shows the whole picture.

I don't force anybody to use mutation, I am just saying that it is fine to mutate locally, it is fine to use "for" loops, nothing wrong with that. Of course if we just reimplement map or filter then we should think twice. But if function makes more than that, transformation for example needs reduce then reaching for loops is fully ok. I don't feel pain looking at statements, also "boilerplate" can happen with expressions, Elm is famous for having a lot of boilerplate, even though it has no statements at all.

And yes I prefer expressions over statements, but I in the same way I hate ternary expression and prefer if statement, as it is just more readable. Python has <expr1> if <conditional_expr> else <expr2> and this is awesome. But in JS, nah, ternary is really not nice.

Collapse
 
nosknut profile image
nosknut • • Edited

Generally i choose whatever code is more readable. Readabillity over performance is generally a good rule. Unless you get a meaningful performance drop that actually impacts the application, or alredy when making it know that the code will be executed constantly many times a second, readabillity is king in my book. Generally i prefer the code i dont need to understand. If the method signature is enough, i prefer not looking too much into the implementation. If the implementation is nice clean english such as users.filter, i won't have to give the rest a detailed read to feel comfortable using or changing it. You should understand the code enough that you arent putting gibberish into your codebase of course, however the best code is the one you can understand without reading it.

Collapse
 
kajalelohai profile image
Elohai Kajal •

Nice topic of discussion. I liked it.