I come from SML background and feel quite comfortable with high-order functions. But I don\'t really get the idea of list comprehension. Is there any situation where list co
Choosing between comprehensions and higher-order functions is mostly a matter of style. I think that comprehensions are sometimes more readable, but that's just a personal preference. Note that the cartesian
function could be written more elegantly like this:
let rec cartesian = function
| [] -> [[]]
| L::Ls ->
[ for C in cartesian Ls do for x in L do yield x::C ]
The interesting case is when writing recursive functions. If you use sequences (and sequence comprehensions), they remove some unnecessary allocation of temporary lists and if you use yield!
in a tail-call position, you can also avoid stack overflow exceptions:
let rec nums n =
if n = 100000 then []
else n::(nums (n+1))
// throws StackOverflowException
nums 0
let rec nums n = seq {
if n < 100000 then
yield n
yield! nums (n+1) }
// works just fine
nums 0 |> List.ofSeq
This is quite an interesting pattern, because it cannot be written in the same way using lists. When using lists, you cannot return some element and then make a recursive call, because it corresponds to n::(nums ...)
, which is not tail-recursive.
Looking at the generated code in ILSpy, you can see that list comprehensions are compiled to state machines (like methods using yield return
in C#), then passed to something like List.ofSeq
. Higher-order functions, on the other hand, are hand-coded, and frequently use mutable state or other imperative constructs to be as efficient as possible. As is often the case, the general-purpose mechanism is more expensive.
So, to answer your question, if performance is critical there is usually a higher-order function specific to your problem that should be preferred.
Adding to Tomas Petricek's answer. You can make the list version tail recursive.
let nums3 n =
let rec nums3internal acc n =
if n = 100000 then
acc
else
nums3internal (n::acc) (n+1) //Tail Call Optimization possible
nums3internal [] n |> List.rev
nums3 0
With the added benefit of a considerable speedup. At least when I measured with the stopwatch tool I get. (nums2 being the algorithm using Seq).
Nums2 takes 81.225500ms
Nums3 takes 4.948700ms
For higher numbers this advantage shrinks, because List.rev is inefficient. E.g. for 10000000 I get:
Nums2 takes 11054.023900ms
Nums3 takes 8256.693100ms