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Subject: Lightweight way to extend existing module. Almost there?
From: Till Varoquaux <till@pps.jussieu.fr>
To: Caml Mailing List <caml-list@yquem.inria.fr>
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X-Spam: no; 0.00; ocaml:01 stdlabels:01 sig:01 stdlabels:01 val:01 bool:01 struct:01 stringlabels:01 bool:01 listlabels:01 arraylabels:01 needle:98 needle:98 pps:01 jussieu:01 

With the improvements to the module system in ocaml 3.12 extending a
signature just got much much lighter. Suppose I want the signature of
the module which in every respects is like StdLabels excepted that the
String module has an extra "is_prefix_of" I can write:

  module String : sig
    include module type of StdLabels.String
    val is_prefix_of : string ->  string ->  bool
  end

  include module type of StdLabels with module String :=  String

I know of no way to extend the implementation in a similarly concise
way (for which the length of the solution does not depend on the
number of top level items in StdLabels). Have I overlooked anything
obvious?

Till

P.S.: FYI existing solution for the implementation:

  module String = struct
    include StringLabels

    let is_prefix_of (needle:string) (haystack:string) : bool =
      let len = length needle in
      len <= length haystack && (sub haystack ~pos:0 ~len = needle)
  end

  (* Haven't quite found out how to include a module hiding another one... *)
  module List = ListLabels
  module Array = ArrayLabels