On Tue, Feb 04, 2025 at 08:25:58AM +0000, Richard W.M. Jones wrote: > unit is a bit like void in C
... > # let f () = > 2+5; > () ;; > Warning 10 [non-unit-statement]: this expression should have type unit. > val f : unit -> unit = <fun> So unit isn't exactly like void in C, although it has similarities. Functions in OCaml must have at least 1 parameter. If you want to simulate a function with no parameters, you give them one parameter of type 'unit'. The 'unit' type is a regular type (like 'int' or 'string'), but it only has a single possible value, written '()'. Thus 'f ()' means pass the parameter '()' (of type 'unit') to the function 'f'. It doesn't mean pass zero parameters. You can if you want (and this is even occasionally useful) have a function that takes two unit parameters, eg: # let f () () = () ;; val f : unit -> unit -> unit = <fun> or something else and a unit (also occasonally useful): # let g i () = string_of_int i ;; val g : int -> unit -> string = <fun> Similarly, OCaml functions can't return "nothing" (ie. void). They must return something, but if you don't want them to return anything you can have them return unit, which means they will return only the single value '()' (or could throw an exception). Functions can't return multiple values (unlike in LISP), but they can return an n-tuple, eg this is unnatural but possible: # let h () = (), () ;; val h : unit -> unit * unit = <fun> 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 _______________________________________________ Libguestfs mailing list -- guestfs@lists.libguestfs.org To unsubscribe send an email to guestfs-le...@lists.libguestfs.org
