Complex data structures in Haskell - how do they work?

Question

As I figured out, variables in Haskell are immutable (thus, they are not really `variables\').

In this case, if we have a complex and big data structure, like a red-bla

Answer 1

Your intuition about copying is exactly the right direction you should be thinking. However, the 'copying' intelligently implemented by the Haskell runtime. For example, given

data Tree = Empty | Branch Tree Integer Tree

You could implement a function to replace the left branch:

replaceLeft :: Tree -> Tree -> Tree
replaceLeft (Branch _ i r) newLeft = Branch newLeft i r

The result isn't that you create a entire copy of the new tree, as that isn't necessary. The value i and the trees r and newLeft are unchanged so we don't have to copy their contents into fresh memory.

The new Branch that is created (this is the only real allocation in this example: the creation of the structure to hold the left/right trees on the Integer) still references the exact same values from the old branch, no copying needed!

Since the data structures are immutable, there's no harm in doing this.