Hello,

I recently encountered a situation where I had (effectively) the following polymorphic type:

type 'a record = { id : int; data : 'a }

and the following compare function

let compare {id = id1} {id = id2} = Pervasives.compare id1 id2

and wanted to put such records into a set. However, I could not figure out how to make the polymorphic 'a in the type definition "disappear" in the module argument to the Set.Make functor.

Interestingly, I had the same problem recently where I wanted to create a list of "tests" that could access some extra information when run (and would also return a list of tests to run later). I found that using a class type worked well:

class type test =

object

method name : string

method run : (test * float) list

end

To define a test, I simply do:

let test_download : Timed_events.test =

object (self)

val mutable cache = 0 (* anything I want *)

method name = "Download"

method run =

(* From here I can access the cache and plan to run the test again in 10 seconds *)

[self, 10.0]

end

Alan