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xmcqdpt2 3 days ago  [ - ]

Lists aren’t copied on prepend.

Tries (like scala’s Vector) or trie maps (the core map types of Scala, Clojure and probably Haskell?) aren’t copied on updates.

In fact, whether a data structure is an immutable or persistent data structure or merely an unmodifiable data structure (like Kotlin uses) is based on whether it requires full copies on most updates or not. In FP languages, immutable data structures aren’t “specialized” at all.

Y_Y 3 days ago  [ - ]

> whether a data structure is an immutable or persistent data structure or merely an unmodifiable data structure...

This hurt my brain. It seems that in some places (e.g. Java land) unmodifiable refers to something that you can't modify but could just be a wrapper around a structure that can be modified. In that case they use immutable to mean something that is nowhere modifiable.

I may be misrepresenting this idea, but I think the terminology is so poor that it deserves to be misunderstood.

mrkeen 3 days ago  [ - ]

Think about mutability in Java land this way:

  // Using mutability.
  // `increment` is void, and makes 2 bigger for everyone.
  increment(2); 

  // Typical Java "safety".
  // It's still void, but now it throws a RuntimeException
  // because the developers are saving you from making everyone's 2 bigger.
  increment(2);

  // Immutable
  // Returns 3
  increment(2);