Back to Questions
1.2k
views1
comment
seq, laziness, and side-effects
f#

Hello,

I spent a lot f time debuggig some code because of seq has some intuitive interactions with side-effects. Consider this code:

1
2
3
4
5
6

let test() =
    let todo = Array.create 10 true
    todo.[0] <- false
    let queue = {0..9} |> Seq.filter (fun (i) -> todo.[i])
    Seq.iter (fun (i) -> todo.[i] <- false) queue
    queue

It produces ... the empty sequence! Isn't it strange that the Seq.iter accesses a queue, but by setting the array, actually changes the meaning of the queue?

.

Sequences are lazy, which is why you can create 

1

{for i in 0I .. 100000000000000000000000I -> "a"}


in a blink.

...and why you get an empty sequence when modifying the array it is based on.

Note that if you use 

1

let x = Seq.take 5 queue


before the array iteration, x will not be an empty list. If you want to keep the values of your seq right after the definition, you can use Seq.to_list or suchlike.

Hope this helps

By on 3/6/2008 5:54 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper