[] let should_do_something() = Assert.AreEqual(2,2)

it would be more idiomatic to write it as

[] let should_do_something () = Assert.AreEqual(2,2)

The extra space implies that should_do_something is a function, which takes one argument which is a unit. This is more similar to the syntax for declaring a one argument function where the argument is actually a value, such as

let square_it x = x * x