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Message-ID: <4B0405CA.80704@simula.no>
Date: Wed, 18 Nov 2009 15:33:46 +0100
From: Hans Ole Rafaelsen <hans@simula.no>
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Subject: Polymorphic function in reference cell
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X-Spam: no; 0.00; rafaelsen:01 foo:01 foo:01 struct:01 1.0:98 1.0:98 polymorphic:01 polymorphic:01 functions:01 functions:01 undefined:01 undefined:01 defined:02 module:03 module:03 

Hi,

I have a module that have several functions that take a polymorphic 
function as part of their arguments (Foo.f1 and Foo.f2 in the example).

module Foo  = struct

  let f1 f =
    f 1

  let f2 f =
    f 1.0

  let f1_ref = (ref (fun _ -> raise (Failure "undefined") : ('a -> 'a) ))
  let set_f1_ref f =
    f1_ref := f

  let f2_ref = (ref (fun _ -> raise (Failure "undefined") : ('a -> 'a) ))
  let set_f2_ref f =
    f2_ref := f

  let use_f1 () =
    !f1_ref 1

  let use_f2 () =
    !f2_ref 1.0

  let internal_f v =
    v

  let internal_f1 () =
    internal_f 1

  let internal_f2 () =
    internal_f 1.0

end

let f v =
  v

let a = Foo.f1 f
let b = Foo.f2 f
let () = Foo.set_f1_ref f
let () = Foo.set_f2_ref f
let c = Foo.use_f1 ()
let d = Foo.use_f2 ()


In my code I don't want to pass this function around to to all functions 
where Foo.f1 is called. For this reason I would like to store this 
function that Foo.f1 needs within Foo. (The function is created outside 
the module and bound with values generated outside the module.) However 
you can not store polymorphic functions in references. So I have to make 
one reference for each concrete type it is used for. What I really would 
like is use it as if it was defined within Foo, like internal_f and used 
like the internal_f1 and internal_f2.

Is it possible to only store one reference to this kind of function 
within a module, or do I really have to write a reference for each usage 
of the function? If I have to create a reference for each case, do 
anyone have some trick to avoid code repetition?

Regards,

Hans Ole