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Date: Fri, 11 Apr 1997 10:05:21 +0200
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From: Christian Boos <boos@arthur.u-strasbg.fr>
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To: caml-list@inria.fr
CC: Basile.Starynkevitch@cea.fr, Mark Hayden <hayden@cs.cornell.edu>
Subject: Re: Q: hashtables of parametrized types in Ocaml? 
In-Reply-To: <199704041903.OAA17096@gulag.cs.cornell.edu>
References: <199704041144.NAA11665@soleil.serma.cea.fr>
	<199704041903.OAA17096@gulag.cs.cornell.edu>
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Mark Hayden writes:
 > 
 > Here is a similar example that works for me.
 > This is for hash tables that use MD5 digest 
 > strings for keys.
 > 
 > --Mark
 > <<.mli and .ml snipped>>

	Hello, 

  I don't think this was an example similar to the original post. 
What Basile needed was a parametrized type for the key type:

He wrote:
 > >I tried 
 > >
 > >    module SymbolHashtbl =
 > >      Hashtbl.Make(struct
 > >	type t = 'a asymbol_t
 > >	let equal = (==)
 > >	let hash = symhash
 > >      end)
 > >
 > >but it doesn't work! How can I achieve an equivalent result? Of course
 > > <<...>>
 > >Perhaps a double functor is the answer, but I can't figure it out!

I think his idea of a "double functor" is the good one.


Try:

    module MakeHashtbl (S : sig type v end) =
      Hashtbl.Make 
        (struct
          type t = S.v asymbol_t
          let equal = (==)
          let hash = symhash
        end)


and then later:

    module NodeSymbolHashtbl = MakeHashtbl (sig type v = node_t end)


-- Christian