Benefit of avoiding multiple list traversals

I've seen many examples in functional languages about processing a list and constructing a function to do something with its elements after receiving some additional value (usually not present at the time the function was generated), such as:

In all these examples, the authors generally remark the benefit of traversing the original list only once. But I can't keep myself from thinking "sure, instead of traversing a list of N elements, you are traversing a chain of N evaluations, so what?". I know there must be some benefit to it, could someone explain it please?

Edit: Thanks to both for the answers. Unfortunately, that's not what I wanted to know. I'll try to clarify my question, so it's not confused with the (more common) one about creating intermediate lists (which I already read about in various places). Also thanks for correcting my post formatting.

I'm interested in the cases where you construct a function to be applied to a list, where you don't yet have the necessary value to evaluate the result (be it a list or not). Then you can't avoid generating references to each list element (even if the list structure is not referenced anymore). And you have the same memory accesses as before, but you don't have to deconstruct the list (pattern matching).

For example, see the "staging" chapter in the mentioned ML book. I've tried it in ML and Racket, more specifically the staged version of "append" which traverses the first list and returns a function to insert the second list at the tail, without traversing the first list many times. Surprisingly for me, it was much faster even considering it still had to copy the list structure as the last pointer was different on each case.

The following is a variant of map which after applied to a list, it should be faster when changing the function. As Haskell is not strict, I would have to force the evaluation of listMap [1..100000] in cachedList (or maybe not, as after the first application it should still be in memory).

listMap = foldr comb (const [])
  where comb x rest = \f -> f x : rest f

cachedList = listMap [1..100000]
doubles = cachedList (2*)
squares = cachedList (\x -> x*x)

-- print doubles and squares
-- ...

I know in Haskell it doesn't make a difference (please correct me if I'm wrong) using comb x rest f = ... vs comb x rest = \f -> ..., but I chose this version to emphasize the idea.

Update: after some simple tests, I couldn't find any difference in execution times in Haskell. The question then is only about strict languages such as Scheme (at least the Racket implementation, where I tested it) and ML.


So the answer to your question is, partial compilation. Done ahead of time, it makes it so that there's no need to traverse the list to get to the individual elements - all the references are found in advance and stored inside the pre-compiled function.

As to your concern about the need for that function to be traversed too, it would be true in interpreted languages. But compilation eliminates this problem.

In the presence of laziness this coding trick may lead to the opposite results. Having full equations, e.g. Haskell GHC compiler is able to perform all kinds of optimizations, which essentially eliminate the lists completely and turn the code into an equivalent of loops. This happens when we compile the code with e.g. -O2 switch.

Writing out the partial equations may prevent this compiler optimization and force the actual creation of functions - with drastic slowdown of the resulting code. I tried your cachedList code and saw a 0.01s execution time turn into 0.20s (don't remember right now the exact test I did).

Executing a few extra arithmetic instructions in your loop body is cheaper than executing a few extra memory fetches, basically.

Traversals mean doing lots of memory access, so the less you do, the better. Fusion of traversals reduces memory traffic, and increases the straight line compute load, so you get better performance.

Concretely, consider this program to compute some math on a list:

go :: [Int] -> [Int]
go = map (+2) . map (^3)

Clearly, we design it with two traversals of the list. Between the first and the second traversal, a result is stored in an intermediate data structure. However, it is a lazy structure, so only costs O(1) memory.

Now, the Haskell compiler immediately fuses the two loops into:

go = map ((+2) . (^3))

Why is that? After all, both are O(n) complexity, right? The difference is in the constant factors.

Considering this abstraction: for each step of the first pipeline we do:

  i <- read memory          -- cost M
  j = i ^ 3                 -- cost A
  write memory j            -- cost M
  k <- read memory          -- cost M
  l = k + 2                 -- cost A
  write memory l            -- cost M

so we pay 4 memory accesses, and 2 arithmetic operations.

For the fused result we have:

  i <- read memory          -- cost M
  j = (i ^ 3) + 2           -- cost 2A
  write memory j            -- cost M

where A and M are the constant factors for doing math on the ALU and memory access.

There are other constant factors as well (two loop branches) instead of one.

So unless memory access is free (it is not, by a long shot) then the second version is always faster.

Note that compilers that operate on immutable sequences can implement array fusion, the transformation that does this for you. GHC is such a compiler.

There is another very important reason. If you traverse a list only once, and you have no other reference to it, the GC can release the memory claimed by the list elements as you traverse them. Moreover, if the list is generated lazily, you always have only a constant memory consumption. For example

import Data.List

main = do
    let xs = [1..10000000]
        sum = foldl' (+) 0 xs
        len = foldl' (\_ -> (+ 1)) 0 xs
    print (sum / len)

computes sum, but needs to keep the reference to xs and the memory it occupies cannot be released, because it is needed to compute len later. (Or vice versa.) So the program consumes a considerable amount of memory, the larger xs the more memory it needs.

However, if we traverse the list only once, it is created lazily and the elements can be GC immediately, so no matter how big the list is, the program takes only O(1) memory.

{-# LANGUAGE BangPatterns #-}
import Data.List

main = do
    let xs = [1..10000000]
        (sum, len) = foldl' (\(!s,!l) x -> (s + x, l + 1)) (0, 0) xs
    print (sum / len)

Sorry in advance for a chatty-style answer.

That's probably obvious, but if we're talking about the performance, you should always verify hypotheses by measuring.

A couple of years ago I was thinking about the operational semantics of GHC, the STG machine. And I asked myself the same question — surely the famous "one-traversal" algorithms are not that great? It only looks like one traversal on the surface, but under the hood you also have this chain-of-thunks structure which is usually quite similar to the original list.

I wrote a few versions (varying in strictness) of the famous RepMin problem — given a tree filled with numbers, generate the tree of the same shape, but replace every number with the minimum of all the numbers. If my memory is right (remember — always verify stuff yourself!), the naive two-traversal algorithm performed much faster than various clever one-traversal algorithms.

I also shared my observations with Simon Marlow (we were both at an FP summer school during that time), and he said that they use this approach in GHC. But not to improve performance, as you might have thought. Instead, he said, for a big AST (such as Haskell's one) writing down all the constructors takes much space (in terms of lines of code), and so they just reduce the amount of code by writing down just one (syntactic) traversal.

Personally I avoid this trick because if you make a mistake, you get a loop which is a very unpleasant thing to debug.

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