Improve error reporting: using EntryPoint attribute in an F# script

[anton-pt]

What

Running the code:

[<EntryPoint>]
let main argv =
    printfn "Hello, world!"   
    0

in FSI produces the output
val main : argv:string [] -> int
but doesn't actually invoke the function.

Why

Some beginners may be unsure about how F# scripts and FSI work and may run some code from an application which has an EntryPoint into FSI. This happened recently to a former colleague.

How

Using the EntryPoint attribute in FSI should produce a compiler warning indicating that it won't be invoked automatically.

[KevinRansom]

Agreed.

[isaacabraham]

@anton-pt can you suggest an error message that should be displayed - we should be able to come up with one easily, and then this can be marked as up for grabs.

[anton-pt]

How about something like this?

Warning: defined function [f] marked as EntryPoint was not invoked. Execute [f argv] in order to invoke [f] with the appropriate string array of command line arguments.

[bisen2]

It seems to me that FSI should implicitly run a method marked with the EntryPoint attribute. For example:

[<EntryPoint>]
let main argv =
    printfn "Hello, World!"
    0

should be equivalent to

let main argv =
    printfn "Hello, World!"
    0

main fsi.CommandLineArgs

[vzarytovskii]

We already have an implicit entry point created in scripts, adding some different behaviour, based on attribute would be confusing and, probably, a breaking change, for example consider the following code:

[<EntryPoint>]
let main argv =
    printfn "Hello, World!"
    0

#if INTERACTIVE
main fsi.CommandLineArgs
#endif

main will be executed twice, since second one is going to be generated by compiler.

I think, a warning should be produced instead (as it was suggested originally).

[dsyme]

I think, a warning should be produced instead (as it was suggested originally).

Yes, it makes sense

[KathleenDollard]

If we're just looking for text here, working from @anton-pt 's suggestion:

Function with [<EntryPoint>] are not invoked in FSI. [f] was not invoked. Execute [f <args>] in order to invoke [f] with the appropriate string array of command line arguments.

I assume the [f] will be replaced with the fuction name. I think "argv" is confusing if the user needs to type args there, so the will hopefully inspire the programmer to type real values there.