If you can't change a variable's value in Haskell, how do you create data structures?

Question

As per the title.

I have the following code which creates a binary search tree, but if I want it created and changed dynamically with user input, how would I do that if

Answer 1

You allocate a new tree node and the old one sticks around. This technique requires a really good allocator, but it enables all sorts of nifty devices because other parts of the program still have access to old nodes. This is a godsend for certain kinds of speculative algorithms or other tricks involving so-called "persistent data structures".

Eventually you allocate a new root for your tree and what then? As Dario says, you pass it as a parameter to a function (instead of storing it in a global variable).

So

• Mutation of a field in a struct allocated on the heap becomes allocation of a new struct on the heap.

• Mutation of a global variable becomes passing a parameter to a function

Sometimes it also makes sense to take what would have been a collection of global variables in C and put them all in an object allocated on heap.

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P.S. If you really want to, you can have mutable global variables in Haskell. It is, after all, the world's finest imperative programming language (according to Wadler or Peyton Jones, I forget whom). But if you are asking this question, you really don't want to. Yet.