cons operator (::) performance in F#

Question

In official documentation it is stated that :: is faster than @ .

Once all lists are immutable in F#, why is there a difference? In any case the original list should be copied. but in case with :: we will prepend, while in case with @ we will append. it should be the same complexity.

What is the difference?

Answer 1

Your assumption is incorrect.

When you prepend with :: , the original list is not , in fact, copied. The original list remains in place, in memory, exactly where it was, and the new list now consists of the new element and a pointer to the original list, which is its tail.

Consider this:

let x = [1;2;3]
let y = 4 :: x

This program will produce the following memory layout:

 y -> 4
       \
        v
        1 -> 2 -> 3 -> (END)
        ^
       /
      x

This is only possible because lists are immutable: since we know that the original list will never change, we can just conscript it to be the tail of our new list.

This sort of arrangement is generally called " persistent data structure ". Singly-linked list is only the simplest one of such structures.

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When appending with @ , on the other hand, you do indeed have to copy the whole original list. You cannot reuse any part of it.

This is why prepending is faster.