Question

another noob question regarding F#.

If I have the following code…

let ExeC =
    printfn "c"
    3

let ExeB b = 
    printfn "b"
    2

let ExeA = 
    printfn "a"
    1

printfn "Example %d " ExeA
printfn "Example %d " (ExeB 1)
printfn "Example %d " ExeC

The output is as follows…

c
a
Example 1
b
Example 2
Example 3

What seems unusual here is the order that the code is executing in. In a previous question Brian mentioned something about expressions, I was hoping someone could explain this a bit more. It almost seems like the compiler is intelligently pre-executing things to calculate values… but I don’t know?

Answer 1

ExeA and ExeC aren’t functions, but single values. The compiler ensures that values initialise in the order in which they’re declared in the source file, so what’s happening here is:

1. ExeC initialises
2. ExeA initialises
3. Example 1 is printed, using ExeA‘s initialised value
4. The ExeB function is called as normal
5. Example 3 is printed, using ExeC‘s initialised value

If you want ExeA and ExeC to be truly lazy — that is, to control when their side effects run — you could turn them into functions that accept unit:

let ExeC () =
    printfn "c"
    3

let ExeB b = 
    printfn "b"
    2

let ExeA () = 
    printfn "a"
    1

printfn "Example %d " (ExeA ())
printfn "Example %d " (ExeB 1)
printfn "Example %d " (ExeC ())