[Libguestfs] IRC question about ?foo etc

14:50 < cbosdonnat> pino|work, do you know how I could get ?tar passed from one function to the other in the checksum code? 14:51 < pino|work> cbosdonnat: pass it as it is, ie ?foo 14:52 < cbosdonnat> pino|work, indeed, works better... I didn't know I could use the ? in the calling function and I used the ~ instead 14:53 < pino|work> ? is handled like an option type This is a bit complicated, but I think it may be more obvious if you know there are two distinct issues: Issue 1: When calling a function, ?foo is short for ?foo:foo and ~bar is short for ~bar:bar If you're calling a function that takes a labelled parameter ~bar and you want to pass (eg) a string you can do: func ~bar:"hello" If you want to pass a variable [really: a binding] to the function, then: let v = "hello" in func ~bar:v But as a special case, if the variable is called bar already, then this works: let bar = "hello" in func ~bar because ~bar is short for ~bar:bar (and the same for optional parameters like ?foo). This leads naturally to code that looks like: let other_func ~foo ~bar ?baz = func ~foo ~bar ?baz Issue 2: Optional arguments are implemented as option types (eg. None, Some foo), except when defaults are used If a function is defined as: let func ?foo ~bar () = ... then bar is passed as an ordinary parameter, it's just that at compile time it is given a label (the labels disappear completely at runtime). So there is no overhead or difference for the ~bar parameter. However ?foo is implemented as an option. If the function is called this way: func ~bar () then what actually happens is the first parameter is passed as None, and foo is assigned to None inside the function. If it is called this way: func ~foo:"hello" ~bar () then Some "hello" is passed to the function and assigned to foo inside the function. [And as you can see, the syntax is confusingly inconsistent as well, welcome to OCaml.] This works slightly differently if the optional parameter has a default, eg: let func ?(foo = "hello") ~bar () = ... In this case, the implementation still passes an option (None or Some ..) to the function (because the function still needs to have a way to know if it was called with or without the ?foo parameter), but the implementation also implicitly adds the following code to the beginning of the function: let func ?(foo = "hello") ~bar () = let foo = match [real ?foo param] with None -> "hello" | Some x -> x in ... so foo inside the function is not an option type in this case. HTH, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top