What is the equivalent of "import" in F#

Question

How can I import a function from another file in F#? Like you do import in Python. I tried open, #using, nothing works. I looked on the official docs and I still can't figure out how to use it.

Basically, I want something like this:

// Log.fs

module Log = 
    let log = 
        printfn "Hello, World"

// Program.fs
open Log

module main = 
    [<EntryPoint>]
    let main argv =
        log // Not working
        0

Answer 1

The equivalent to import is open.

If I take your pastebin code, it has a bunch of errors indeed, as shown on SharpLab. There are a few things to note here:

1. F# likes its code ordered. Each file is considered a separate compilation unit and anything that you need from it can only be referenced in a following file, not in a previous one.
2. Inside a single file, you can create circular references with and, but otherwise, the same applies: whatever type, value, module you want to use, it must already exist and be in scope.
3. You created a log-value, not a log-function.
4. You forgot the = sign after the module definitions.

Your original code is this:

// Program.fs
module main
open Log

[<EntryPoint>]
let main argv =
    printfn "Hello"
    log
    0 // return an integer exit code


// Log.fs
module Log

let log =
    printfn "Hello"

This gives the following errors:

error FS0039: The namespace or module 'Log' is not defined.

You get this one because you have a open Log, but the module Log doesn't exist yet.

error FS0010: Unexpected start of structured construct in definition. Expected '=' or other token.

This is about the last let, it must be indented.

error FS0010: Incomplete structured construct at or before this point in implementation file

Same thing, caused by the previous error.

After I change the order of your code, indent it appropriately, change let log to let log(), and add the necessary = signs, it just works, try it out:

// Log.fs
module Log =
    let log() =
        printfn "Hello"

// Program.fs
module main = 
    open Log

    [<EntryPoint>]
    let main argv =
        printfn "Hello"
        log()
        0 // return an integer exit code

Please note that inside a module you can remove the first level of indentation and the =-sign, but only if it is the only module in that file and it is the last file in your project (so generally I would advise against that, to keep it simple, just always indent and always have the =-sign there).

But just to show you an alternative that also works:

// Log.fs
module Log =
    let log() =
        printfn "Hello"

open Log

[<EntryPoint>]
let main argv =         // it is the last in the file or prj, this is allowed
    printfn "Hello"
    log()
    0 // return an integer exit code

Note also that if you place the code in different files, that you must add a namespace declaration at the very top of the file. Typically this will be the default namespace of your whole project.

Answer 2

This works.I get an error (in VS 2017) when using your syntax Files in libraries or multiple-file applications must begin with a namespace or module declaration. When using a module declaration at the start of a file the '=' sign is not allowed. If this is a top-level module, consider removing the = to resolve this error.

// Log.fs

module Log

let log = 
    printfn "Hello, World"