Inconsistent (apparently) behaviour with static members in F#

Solution 1:

I think your question is a good one. The definition of a static property is syntactically very similar to definition of a value - so I can see why you think the two should behave the same.

This is slightly misleading, because behind the senes, properties have a getter method that is evaluated each time the property is accessed. As a minimal example, in the following, accessing A1.B twice prints "hi" twice:

type A1 = A1 with
  static member B = 
    printfn "hi"

A1.B; A1.B

There is actually a more verbose syntax for properties in F#, which reveals what is going on - the fact that B is actually backed by a get() method that is invoked by the compiled code behind the scenes:

type A2 = A2 with
  static member B 
    with get() = printfn "hi"

A2.B; A2.B

Now, you would never actually do this in practice, but you can even invoke the method (which is hidden from IntelliSense) directly:

A2.get_B(); A2.get_B()

Solution 2:

The following first 2 F# snippets get different results, but I find this a little inconsistent, even though I understand that members and values have different semantics. I would expect both of them getting the same results (the second one, as they have a similar value syntax).

You're expecting that evaluating _b caused it to re-evaluate itself, but that's not how values evaluate in F#. If I define let x = 12 it doesn't re-evaluate what x is each time I access it.

That's why it always prints 1. When _b is first accessed, it is evaluated once and that's it.