Error in Mono Compiler: Files in libraries or multiple-file applications must begin with a namespace or module declaration

Question

I am trying to compile this example in mono on ubuntu.

However I get the error

wingsit@wingsit-laptop:~/MyFS/kitty$ fsc.exe -o kitty.exe  kittyAst.fs kittyParser.fs kittyLexer.fs main.fs 
Microsoft (R) F# 2.0 Compiler build 2.0.0.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

/home/wingsit/MyFS/kitty/kittyAst.fs(1,1): error FS0222: Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'

/home/wingsit/MyFS/kitty/kittyParser.fs(2,1): error FS0222: Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'

/home/wingsit/MyFS/kitty/kittyLexer.fsl(2,1): error FS0222: Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'
wingsit@wingsit-laptop:~/MyFS/kitty$ 

I am a newbie in F#. Is there something obvious I miss?

Answer 1

As Brian and Scott pointed out, you need to enclose the file in a namespace or module declaration. Just adding namespace SomeNamespace may not work if you have top-level let binding in the file (as these must be in some module). The following would be invalid:

namespace SomeNamespace
let foo a b = a + b   // Top-level functions not allowed in a namespace

That said, I prefer having namespace instead of module at the top-level and then wrapping all functions in a module explicitly (as I believe that makes code more readable):

namespace SomeNamespace
module FooFunctions =     
  let foo a b = a + b

But of course, you can add top-level module as Brian suggests (earlier versions of F# automatically used the name of the file in PascalCase as the name of top-level module used in the file):

// 'main.fs' would be compiled as:
module Main
let foo a b = a + b

Answer 2

You can probably fix it up by adding

module theCurrentFileName

to the top of each .fs file.

Indeed, this is a newer requirement, and the sample is old and needs updating.