In my OCaml project, I’d like to abstract away the details of running containers into specific modules based on the OS. Currently, I have working container setups for Windows and Linux, and I’ve haphazardly peppered if Sys.win32 then where I need differentiation, but this is OCaml, so let us use functors!
I started by fleshing out the bare bones in a new project. After dune init project functor, I created bin/s.ml containing the signature of the module CONTAINER.
module type CONTAINER = sig
val run : string -> unit
end
Then a trivial bin/linux.ml.
let run s = Printf.printf "Linux container '%s'\n" s
And bin/windows.ml.
let run s = Printf.printf "Windows container '%s'\n" s
Then in bin/main.ml, I can select the container system once and from then on use Container.foo to run the appropriate OS specific function.
let container = if Sys.win32 then (module Windows : S.CONTAINER) else (module Linux : S.CONTAINER)
module Container = (val container)
let () = Container.run "Hello, World!"
You can additionally create windows.mli and linux.mli containing simply include S.CONTAINER.
Now, let’s imagine that we needed to have some specific configuration options depending upon whether we are running on Windows or Linux. For demonstration purposes, let’s use the user account. On Windows, this is a string, typically ContainerAdministrator, whereas on Linux, it’s an integer UID of value 0.
We can update the module type in bin/s.ml to include the type t, and add an init function to return a t and add t as a parameter to run.
module type CONTAINER = sig
type t
val init : unit -> t
val run : t -> string -> unit
end
In bin/linux.ml, we can add the type and define uid as an integer, then add the init function to return the populated structure. run now accepts t as the first parameter.
type t = {
uid : int;
}
let init () = { uid = 0 }
let run t s = Printf.printf "Linux container user id %i says '%s'\n" t.uid s
In a similar vein, bin/windows.ml is updated like this
type t = {
username : string;
}
let init () = { username = "ContainerAdministrator" }
let run t s = Printf.printf "Windows container user name %s says '%s'\n" t.username s
And finally, in bin/main.ml we run Container.init () and use the returned type as a parameter to Container.run.
let container = if Sys.win32 then (module Windows : S.CONTAINER) else (module Linux : S.CONTAINER)
module Container = (val container)
let c = Container.init ()
let () = Container.run c "Hello, World!"