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[Noob here] Can someone explain to me the advantage of mutable objects? (lemmy.world)

submitted 21 hours ago* (last edited 21 hours ago) by amon@lemmy.world to c/programming@programming.dev

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It makes the code icky and hard to debug, and you can simply return new immutable objects for every state change.

EDIT: why not just create a new object and reassign variable to point to the new object

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[–] tunetardis@lemmy.ca 12 points 18 hours ago (1 children)

Well, but then you're basically just pushing the mutability onto the container, since you need to be able to replace elements within it.

It's a good strategy at times though. Like say you're working in a language where strings are immutable and you want a string you can change. You can wrap it in a list along the lines s=['foo'] and pass references to the list around instead. Then if you go s[0]='bar' at some point, all the references will now see ['bar'] instead.

[–] Giooschi@lemmy.world 1 points 3 hours ago (1 children)

Well, but then you're basically just pushing the mutability onto the container

That's the point, when programming with immutable structures you always pass the mutability onto the enclosing structure.

It's a good strategy at times though. Like say you're working in a language where strings are immutable and you want a string you can change. You can wrap it in a list along the lines s=['foo'] and pass references to the list around instead. Then if you go s[0]='bar' at some point, all the references will now see ['bar'] instead.

A list is an antipattern here IMO. Just wrap it in some dedicated object (see e.g. Java's StringBuilder).

[–] tunetardis@lemmy.ca 1 points 50 minutes ago

That’s the point, when programming with immutable structures you always pass the mutability onto the enclosing structure.

I guess the point I was trying to make here was if the data type is already mutable, there is no point in sticking it in a list just so you can replace a reference with an identifier. You're just adding an extra level of indirection. But sure yeah, if the type is inherently immutable, you have to do something.

A list is an antipattern here IMO. Just wrap it in some dedicated object (see e.g. Java’s StringBuilder).

Interesting. I'm not aware of anything like StringBuilder in the standard library for either Python or JavaScript. Looks like it wraps a list of characters and tries to behave as string-like as possible? You could presumably write your own class like that or download an implementation from someplace.

I guess in most cases in my own code, where I need a mutable string is usually as part of a larger data structure which is the thing that gets passed around by reference, so it's easy enough to replace a field within that.

For building up a string, I would tend to use an io.StringIO in Python with file-writing calls, but those aren't meant for sharing. What you don't want to do is use the += operator a lot on strings. That gets expensive unless strings are mutable (like they are in say C++'s std::string).