List comprehension vs high-order functions in F#

Question

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

Answer 1

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.