New user of F#

Question

Hello All,

I don't know what is wrong here.

I'm trying to understand the link between 2 files.

I don't get a simple string to show.

Thanks in advance

Good.fs

Module Good

Let message =

    "Hello World of FSharp"

Programs.fs

namespace Good

let mes = message()

[<EntryPoint>]

let main () =

    printf mes

    0

---

Newbie

Answer 1

First: please make sure to insert the code with "Insert Code Block" - yours is misformated and as it is wrong ("Let", "Module", ...) - I guess Word or whatever took over and uppercased your stuff here.

Concerning the problem: I think the problem is that you try to invoke "message" with () while it is only a constant/value. And of course you do not give printf any formating to work upon - make sure to look at the documentation for printf

This one should not even compile - try this:

let mes = Good.message
printfn "%s" mes

Answer 2

module Good
let message =
   "Hello World of FSharp"
// Learn more about F# at http://fsharp.net
// See the 'F# Tutorial' project for more help.
namespace Good
[<EntryPoint>]
let main =
    let Test = Good.message
    sprintf "%s" Test
    0

Still not working. Thanks to reply.

When I will get this I think I will understand the basic and will be able to go further with that new programming language.

---

Newbie

Answer 3

look carefully: now you used sprintf instead of printf(n) - the s is there to give you a string back, this is similar to String.Format - nothing will be print to screen/console (this is missleading I guess) - the n at the end will indicate that you want a newline-character at the end of your output (string or console).

You should see a very good compiler-error for this, and I think you will only understand the basics if you learn to read those compiler-errors and the types intellisense provide.

Answer 4

I tried both of them,

The error I'm getting is

Namespaces cannot contain values. Consider using module to hold your value declarations.

The section underline is let main

I looked over the net and there is nothing explaining something close to what I achieve.

Thanks.

---

Newbie

Answer 5

According to MSDN doc your compiler is right "Namespaces cannot directly contain values and functions. Instead, values and functions must be included in modules, and modules are included in namespaces. Namespaces can contain types, modules." http://msdn.microsoft.com/en-us/library/dd233219.aspx

So working should look like this:

namespace Some
module Good =
    let message =
       "Hello World of FSharp"
    // Learn more about F# at http://fsharp.net
    // See the 'F# Tutorial' project for more help.
    [<EntryPoint>]
    let main args =
        let Test = message
        printfn "%s" Test
        0

Note that signature of main function should include parameter for command line arguments (args). And compiler will throw an error if both namespace and module will have the same name. So I gave name 'Some' to namespace above.

---

Petr

Answer 6

I tested your explanation.

I create a file Program.fs that is the main one.

// Learn more about F# at http://fsharp.net
// See the 'F# Tutorial' project for more help.
namespace Sharp
    // Learn more about F# at http://fsharp.net
    // See the 'F# Tutorial' project for more help.
    [<EntryPoint>]
    let main args =
        let Test = message
        printfn "%s" Test
        0

Then I create the other files that I link to it. Good.fs

module Sharp
let message =
   "Hello World of FSharp"

With that create I'm getting this error.

Namespaces cannot contain values. Consider using a module to hold your value declarations.

The part underline is "main args"

---

Newbie

Answer 7

In your Program.fs file you have namespace 'Sharp'

According to F# syntax, namespaces cannot directly contain values and functions, but you declared in your namespace function 'main'.

You should either not declare namespace at all or declare a module inside this namespace which would contain 'main' function declaration.

---

Petr

Answer 8

Sorry I'm not quite understanding.

When you create a windows application you always pass value to other screen and most of the time they are in different files.

Why I cannot pass the value to the main and print them.

Thanks. 

---

Newbie

Answer 9

You can use this for example:

1) File Good.fs

module Sharp
let message =
   "Hello World of FSharp"

2) File Program.fs



namespace Some
module Good =
    // Learn more about F# at http://fsharp.net
    // See the 'F# Tutorial' project for more help.
    [<EntryPoint>]
    let main args =
        let Test = Sharp.message
        printfn "%s" Test
        0

Note the using of qualified name Sharp.message from module Sharp (file Good.fs)

Or you can use 'open' directive to use 'message' value without module qualifier:

3) File Program.fs

namespace Some
open Sharp
module Good =
    // Learn more about F# at http://fsharp.net
    // See the 'F# Tutorial' project for more help.
    [<EntryPoint>]
    let main args =
        let Test = message
        printfn "%s" Test
        0

Make sure that file Good.fs is placed in the project tree in Visual Studio above the file Program.fs. If not - select the file in the Solution Explorer and use context menu items "Move Up" and "Move Down".

---

Petr

Answer 10

I have a related problem. In Visual Studio I create a project with 4 modules, listed in System Explorer as:

Third.fs

Second.fs

First.fs

Program.fs

In Program.fs, I type 'open First' and that compiles fine.

In First.fs, I type 'open Second' and get 'the namespace or module 'Second' is not defined.'

I must be overlooking something simple -- surely it is possible to call modules from somewhere other than 'Program.fs'?

EDIT: When I run it under FSI, it works ok, so it must be a bug of some kind I guess.