How to execute module do block? - f#

Question

I need to do some setup in a module that I wanted to accomplish by leveraging a do block. Strangely though, my do block never seems to get hit.
Even stranger, If I load the module code into fsi, it does get hit. Here is my example:
Main.fs
[<EntryPoint>]
let main args =
printfn "%b" TestNamespace.M.x
0
TestModule.fs
namespace TestNamespace
module M =
do
printfn "In do"
failwith "Error" // this is line 6
let x = true
When I run the compiled executable I get
>test.exe
true
Why didn't the exception get thrown? If I run the module in FSI by itself I get
In do
System.Exception: Error
at <StartupCode$FSI_0006>.$FSI_0006.main#() in C:\Projects\Personal2\Playground\fsscripts\fsscripts\TestModule.fs:line 6
Stopped due to error
So it got the exception.
I see that in the decompliation that the do initializers get rolled into a seperate class
namespace \u003CStartupCode\u0024fsscripts\u003E
{
internal static class \u0024Library1
{
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
[CompilerGenerated]
[DebuggerNonUserCode]
internal static int init\u0040;
static \u0024Library1()
{
ExtraTopLevelOperators.PrintFormatLine<Unit>((PrintfFormat<Unit, TextWriter, Unit, Unit>) new PrintfFormat<Unit, TextWriter, Unit, Unit, Unit>("In do"));
Operators.FailWith<Unit>("Error");
bool x = M.x;
}
}
}
VS the actual module code:
namespace TestNamespace
{
[CompilationMapping(SourceConstructFlags.Module)]
public static class M
{
public static bool x
{
[DebuggerNonUserCode] get
{
return true;
}
}
}
}
So how do I make sure the do block actually gets executed?
--
Edit, given the above example counts as a simple constant expression so won't produce an observable initialization, why does the following also not work?
[<EntryPoint>]
let main args =
printfn "%d" (TestNamespace.M.x id 1)
0
namespace TestNamespace
module M =
do
printfn "In do"
failwith "Error"
let x f a = f a
This prints out 1 no problem.
Edit, after having re-read Tomas's comments its because a function is considered a constant expression.

Answer 1

For a good explanation of the problem, see the answer to this previous SO question. The important bit says:
the static initializer for the file is executed on first access of a value that has observable initialization
Now, "observable initialization" is somewhat tricky idea, but simple constant initialization definitely does not have observable initialization - that's why the do block is not executed. You can trick the compiler into thinking that there is some imperative action, for example by adding do ():
module M =
do
printfn "In do"
failwith "Error" // this is line 6
let x = (do ()); true

Answer 2

You can get the behavior you want, while maintaining the same public interface, with a class:
type M private () =
static do
printfn "In do"
failwith "Error"
static member val x = true

Answer 3

Here is a clean way to do it.
First, note that if you wanted the initialization code to run every time the function is called, you would do this:
module M =
let init () =
printfn "In init"
let x f a =
init ()
printfn "In x"
f a
So, if you want it to be called just once (static initialization), you can simply remove the () from both places:
module M =
let init =
printfn "In init"
let x f a =
init
printfn "In x"
f a
The nice thing is that you have documented your design that the init code will be called first. If you have several separate blocks of initialization code, it is clear which dependency you are relying on (although the first initialization will execute all such blocks of course).
UPDATE
Here is a version, which works in a Release build too. Not quite so clean, but almost:
module Init =
open System.Runtime.CompilerServices
[<MethodImpl(MethodImplOptions.NoInlining)>]
let call init = init
module M =
let init =
printfn "In init"
let x f a =
Init.call init
printfn "In x"
f a
Note that init is still a unit value, so Init.call is a non-inlined function that does nothing at all. So there is the overhead of a function call to nothing.
An alternative, which also works but seems a little strange is this:
module M =
let mutable init =
printfn "In init"
let x f a =
init <- init
printfn "In x"
f a
Can anyone improve on this?