add example showing why we cannot simply extract the witness from TerminatesWith...

[coinductive-monad.git] / Computation / Eval.v

diff --git a/Computation/Eval.v b/Computation/Eval.v
index 07d79e4..28a00a0 100644 (file)
--- a/Computation/Eval.v
+++ b/Computation/Eval.v
@@ -17,6 +17,25 @@ Fixpoint bounded_eval (A:Set) (n:nat) (c:#A) {struct n} : option A :=
   end.
 
 Inductive Unit : Set := unit : Unit.
+
+(*
+ * In theory we'd like to be able to do this (below), but it violates
+ * the Prop/Set separation:
+ *
+ * "Elimination of an inductive object of sort Prop is not allowed on a
+ *  predicate in sort Set because proofs can be eliminated only to
+ *  build proofs."
+ *)
+(*
+Definition eval (A:Set) (c:#A) (t:Terminates c) : A :=
+  match t with
+    | (Terminates_intro t') =>
+      match t' with
+        | (ex_intro x y) => x
+      end
+  end.
+*)
+
 Definition eval (A:Set) (c:#A) (t:Terminates c) : A :=
   match eval' c Unit (Return unit) (termination_is_safe A c t) with
     | exist x pf => x