Private modifier for F# types

Question

If we have

module M

type private A = B | C

(...)

The private modifier means that A will only be usable from within the module M. But in this case:

namespace N

type private A = B | C

(...)

What will be the effect of that private modifier? It doesn't make sense for a CLR namespace to have private members, as the namespace can be extended at will, so will that private modifier have any effect at all?

Answer 1

If you use the private modifier like that within a namespace, it makes the type or module private to the file -- that is, the type can be used by anything below it within the same file.

When the type is compiled, the F# compiler will treat it as internal within the .NET metadata. (Which makes sense, since the type is really internal -- the compiler just enforces the constraint that it can only be used by types/functions within the same file.)

I use this feature quite often BTW -- it's nice for small helper types that are only used internally in a few spots since it keeps them from cluttering up the IntelliSense.

EDIT: Another cool trick you can use this for is to create file-scoped, "built-in" helper functions.

If you create a module like this, the functions in it will be automatically available to anything below it (thanks to [<AutoOpen>]) but since the module is marked private the functions won't clog up your IntelliSense (when working on your other source files) or hide existing functions.

[<AutoOpen>]
module private Helpers =
    let [<Literal>] blah = "blah"

    let lessThanTwo value =
        value < 2.0