F#: Why can't I use optional parameters in loose functions?

Question

Why can't I use optional parameters in loose functions defined with "let"?

Why are they only allowed in member functions?

Answer 1

I suspect they are provided only for compatibility with .NET functions. They aren't something you encounter in functional languages. The problem with an optional parameter is you can't curry it. If a function f's second parameter is optional, what is

let g = f x
?

Is it a function taking one argument, or a value obtained by evaluating f on x plus the default second parameter?