F# code organization: types & modules

Question

How do you decide between writing a function inside a module or as a static member of some type?

For example, in the source code of F#, there are lots of types that

Answer 1

In addition to the other answers there is one more case to use Modules:

For value types they can help to define static properties that do not get re-evaluated every time they are accessed. for example:

type [] Point =
    val x:float
    val y:float
    new (x,y) = {x=x;y=y}

    static member specialPoint1 = // sqrt is computed every time the property is accessed
        Point (sqrt 0.5 , sqrt 0.5 )

[]
module Point = 

    let specialPoint2 = // sqrt is computed only once when the Module is opened
        Point (sqrt 0.5 , sqrt 0.5 )