Why is my application executing top to bottom

Question

I thought I understood F# code execution but clearly I'm missing something. When I run the following

#!/usr/bin/env fsharpi

let a =
    System.Console.WriteLine("Function A")

let b =
    System.Console.WriteLine("Function B")

let c =
    System.Console.WriteLine("Function C")

c
b
a

I get the output:

Function A
Function B
Function C

So for some reason it's executing the functions as it reads them in instead of the function calls which are in reverse order.

Why is this?

Answer 1

I think you're misunderstanding what this line means:

let a =
    System.Console.WriteLine("Function A")

It assigns the result of System.Console.WriteLine("Function A") to a. If you run it you'll see a is typed as unit:

val a : unit = ()

And at that time "Function A" was already written to console.

What you probably want is a to be a function and not a value:

let a() =
    System.Console.WriteLine("Function A")

It can be called using a(). If you put that all together:

let a() =
    System.Console.WriteLine("Function A")

let b() =
    System.Console.WriteLine("Function B")

let c() =
    System.Console.WriteLine("Function C")

c()
b()
a()

you'll get what you expect:

Function C
Function B
Function A