this post was submitted on 15 Apr 2024
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You wouldn’t get it (sh.itjust.works)
submitted 6 months ago by sjmarf@sh.itjust.works to c/programmer_humor@programming.dev
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[–] Piafraus@lemmy.world 2 points 6 months ago (1 children)

So you have memory space which is reused... Which essentially makes it a mutable memory structure, where you update or add with new data keys... No?

[–] sudo@programming.dev 2 points 6 months ago

No. Persistent Data Structures are not mutable. The memory space of an older version is not rewritten, it is referenced by the newer version as a part of its definition. ie via composition. It can only safely do this if the data it references is guaranteed to not change.

x = 2 :: 1 :: Nil -- [2, 1]
y = 3 :: x -- [3, 2, 1]

In this example both x and y are single linked lists. y is a node with value 3 and a pointer to x. If x was mutable then changing x would change y. That's bad™ so its not allowed.

If you want to learn more about functional programming I suggest reading Structures and Interpretation of Computer Programs or Learn You a Haskell for Great Good