What
This code leads to the following error:
error FS0003: This value is not a function and cannot be applied.
Why
Beginners to the language try to read this error and 9/10 times don't understand what it means.
How
Consider replacing the error message with something like:
"x" is a simple value of type "int", and not a function. It does not take in any arguments, but you are trying to pass the value "10" as an argument to it.