src/ZF/ex/CoUnit.thy

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+++ b/src/ZF/ex/CoUnit.thy	Fri Aug 12 12:28:46 1994 +0200
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+(*  Title: 	ZF/ex/CoUnit.ML
+    ID:         $Id$
+    Author: 	Lawrence C Paulson, Cambridge University Computer Laboratory
+    Copyright   1994  University of Cambridge
+
+Trivial codatatype definitions, one of which goes wrong!
+
+See discussion in 
+  L C Paulson.  A Concrete Final Coalgebra Theorem for ZF Set Theory.
+  Report 334,  Cambridge University Computer Laboratory.  1994.
+*)
+
+CoUnit = QUniv + "Datatype" +
+
+(*This degenerate definition does not work well because the one constructor's
+  definition is trivial!  The same thing occurs with Aczel's Special Final
+  Coalgebra Theorem
+*)
+consts
+  counit :: "i"
+codatatype
+  "counit" = Con ("x: counit")
+
+
+(*A similar example, but the constructor is non-degenerate and it works!
+  The resulting set is a singleton.
+*)
+
+consts
+  counit2 :: "i"
+codatatype
+  "counit2" = Con2 ("x: counit2", "y: counit2")
+
+end