Why can't I use optional parameters in loose functions defined with "let"?
Why are they only allowed in member functions?
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?
It is allowed in OCaml. Therefore it should be possible to implement it in F#.
http://caml.inria.fr/pub/docs/manual-ocaml/lablexamples.html#sec43
I guess it has been omitted for simplicity in F#.
来源：https://stackoverflow.com/questions/946159/f-why-cant-i-use-optional-parameters-in-loose-functions