Dear Caml List:
        I think it is an old question. but it really bother me quite 
much .  Since i have a java background and touched ML not long, i found 
it is  hard not to think in a OOP way in some cases. (even though OCaml 
has an OO system, i feel  that only  the Module style  is the true way 
of programming in Ocaml, so i tried not to use classes as possible )
 Here is a situation i can not overcome:  for example
in a OOP style.:
  DNA and Protein can be a subclass of Seq. And in fact there are many 
 kind of sequences , so we can suppose the number of the kind of Seq is 
infinite. ie. the subclasses of Seq can be infinite.  and DNA and 
Protein are just two of them
  I want to get the length of the Seq.   so  in OOP Style . we can 
define a function :
  val  length : #seq -> int   to solve the problem .   or just use 
inheritance  seq#length
However if do like above , i must define say  class dna    or class 
protein  

Now i want not to use classes. How can i achieve the same effect?
Maybe i can use functor  like
module DefautSeq = Seq.Make ( DefaultSeqIntf : SeqIntf) struct end
module DNA = Seq.Make ( DNAIntf : SeqIntf )  struct    end
module Protein = Seq.Make (DNAIntf : SeqIntf) struct     end
now how to get the length of such seqs ?
(in fact length is usually included in the SeqIntf  .  here  i only to 
show a simple case.  
  for example Chromasome is a kind of DNA ,  some funtions only for  
Chromasome and DNA not suitable for Protein
  so those functions surely will not be included in SeqIntf .  here 
length () refers to such functions  )
if i do like this,
val length : DefaultSeq.t -> int.
DNA.t  will not be accepted by such function . only DefaultSeq .t accepted
I have tryied to use  
 module DefaultSeq  = DNA  before the  length function .  It is OK
But  First  it is not elegant .
 Secd , if module DefaultSeq  = DNA is after the length function.  It 
will not work.
Third ,  sometime you must restore the orignal module , it is tedious 
also not elegant :-(

And maybe can genrate a new functor  LENGTH . bu t i can not image to 
generate a module in  every case just a proper function will do !

Then,  How can i do ?  
Thanks
                               climb




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