From https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/parameters-and-arguments#named-arguments https://docs.microsoft.com/en-us/dotnet/fsharp/l

From https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/parameters-and-arguments#named-arguments https://docs.microsoft.com/en-us/dotnet/fsharp/language-refe... > Named arguments are allowed only for methods, not for let-bound functions, function values, or lambda expressions. > ... > Optional parameters are permitted only on members, not on functions created by using let bindings.