]> 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 ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners