Back to Questions
713
views1
comment
Newbie question
f#

I have the following sample code:

1
2
3
4
5
6
7
8
9
10

printfn "%s" "a"

(fun s -> printfn "%s" s) "b"

Seq.map (fun s -> printfn "%s" s) [|"c"|]

Seq.map (fun s -> printfn "%s" s) [|"d";"e";"f"|]

System.Console.ReadLine()

The expected output is:

1
2
3
4
5
6
7
8
9
10
11
12

a

b

c

d

e

f

but the actual output is:

1
2
3
4

a

b

Is this correct? From what I can tell (by opening similar code in Reflector), the printfn function isn't actually compiled in when it is part of the function used by Seq.map. This is confusing, since it seems like other constructs (I've tried Seq.fold) work as expected.

This has been tested using the Sept. CTP and VS 2008 SP1.

.

Seq.map creates a new sequence, which is lazy i.e. not evaluated till it is enumerated. To make this sample work what you can do is:

1
2

let mySeq = Seq.map (fun s -> printfn "%s" s) [|"c"|]
printfn "%A" mySeq

Here you bind the sequence you create to a identifier mySeq and enumerate it to cause the side effect (printing to the console). But this probably is what you wanted to do, if you just want to enumerate a sequence apply a side effect function to each element use Seq.iter:

1

Seq.iter (fun s -> printfn "%s" s) [|"c"|]

Cheers,
Rob

By on 11/5/2008 1:28 PM ()

    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