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(*
Copyright (c) 2000
Cambridge University Technical Services Limited
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
structure Universal :>
sig
type universal
type 'a tag
val tag : unit -> 'a tag
val tagIs : 'a tag -> universal -> bool
val tagInject : 'a tag -> 'a -> universal
val tagProject : 'a tag -> universal -> 'a
end =
struct
(* The universal type is based on exn which provides a tagged union.
We use opaque signature matching to create a different type. *)
type universal = exn
type 'a tag =
{
is: universal -> bool,
inject: 'a -> universal,
project: universal -> 'a
};
(* The Match exception is created in the General structure in the basis
library which hasn't been built yet. *)
fun tag () : 'a tag =
let
exception E of 'a;
in
{
inject = fn x => E x,
project = fn E x => x | _ => raise RunCall.Match,
is = fn E _ => true | _ => false
}
end ;
val tagIs : 'a tag -> universal -> bool = #is
val tagInject : 'a tag -> 'a -> universal = #inject
val tagProject : 'a tag -> universal -> 'a = #project
end;
(*
This code will test the above structure
datatype t = T of int ;
datatype x = X of string ;
val {is=ist,inject=injectT:t->universal,project=projectT} = tag();
val {is=isx,inject=injectX:x->universal,project=projectX} = tag();
val a = injectT (T 42) ;
val b = injectT (T 24) ;
val c = injectX (X "hello") ;
val d = injectX (X "mike") ;
map ist [a,b,c,d] ;
map isx [a,b,c,d] ;
projectT a ;
projectT b ;
projectT c ;
projectT d ;
projectX a ;
projectX b ;
projectX c ;
projectX d ;
*)
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