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Message-ID: <3B3C7F49.D058A72D@htec.demon.co.uk>
Date: Fri, 29 Jun 2001 14:14:49 +0100
From: Chris Quinn <cq@htec.demon.co.uk>
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CC: caml-list@inria.fr
Subject: Re: [Caml-list] checking for same ctor type?
References: <200106280240.TAA14693@smtp2-cm.mail.eni.net>
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Pervasives.compare is fine for nullary constructors or where you don't mind the performance hit of the function recursing into the structure of the arguments.

A more efficient equality predicate that is really just a cut down version of [compare]:

module Kludge =
  struct
    let cmp (a: 't) (b: 't) =
      let a' = Obj.repr a and b' = Obj.repr b in
      match Obj.is_block a', Obj.is_block b' with
      | true,true ->
          Obj.tag a' = Obj.tag b'
      | false,false ->
          a' = b'
      | _ -> false
  end

It of course relies on the physical representation of sum types.
It has the merit of working on a type of any number of constructors, but the mis-merit of also operating over functions for instance!
To constrain such a comparison to sum types would require it to be built into the compiler.

Chris Q.
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