tested and seems to be working

bs/TagEngine.bs

@@ -1,5 +1,6 @@
 package TagEngine(
     TagEngine(..),
+    Util.BasicResult(..),
     mkTagEngine) where
 
 import Vector
@@ -9,7 +10,7 @@ import Util
 
 interface (TagEngine :: # -> *) numTags =
     requestTag :: ActionValue UIntLog2N(numTags)
-    retireTag  :: UIntLog2N(numTags) -> Action
+    retireTag  :: UIntLog2N(numTags) -> ActionValue BasicResult
 
 -- The tag engine returns a tag that is unique for the duration of
 -- the lifetime of the tag. Useful when you need to tag transactions
@@ -56,7 +57,11 @@ mkTagEngine =
                 when
                     Just sampledStackPtr <- stackPtr
 
-            retireTag :: UIntLog2N(numTags) -> Action
+            -- retireTag isn't guarded so its up to external module to only attempt to
+            -- retire valid tags... At any rate, we can notify the requestor of failures
+            -- to retire tags - although the requestor can merely ignore this
+            -- notification.
+            retireTag :: UIntLog2N(numTags) -> ActionValue BasicResult
             retireTag tag =
                 do
                     let
@@ -71,4 +76,6 @@ mkTagEngine =
                             (select inUseVec tag) := False
                             (select freeStackVec nextStackPtrUint) := tag
                             stackPtr := Just nextStackPtrUint
-                        else action {}
+                            return Success
+                        else do
+                            return Failure

bs/Top.bs

@@ -116,22 +116,33 @@ mkSim = do
     |> do
          $display "got tag : " tagEngine.requestTag
     |> do
-         $display "retiring tag : 3"
-         tagEngine.retireTag 3
+         res <- tagEngine.retireTag 3
+         $display "retiring tag :   3 " (fshow res)
+         action {}
     |> do
          $display "got tag : " tagEngine.requestTag
     |> do
          $display "got tag : " tagEngine.requestTag
     |> do
-         $display "retiring tag : 4"
-         tagEngine.retireTag 4
+         res <- tagEngine.retireTag 4
+         $display "retiring tag :   4 " (fshow res)
+         action {}
+    |> do
+         res <- tagEngine.retireTag 4
+         $display "retiring tag :   4 " (fshow res)
+         action {}
+    |> do
+         res <- tagEngine.retireTag 0
+         $display "retiring tag :   0 " (fshow res)
+         action {}
     |> do
          $display "got tag : " tagEngine.requestTag
     |> do
          $display "got tag : " tagEngine.requestTag
     |> do
-         $display "retiring tag : 1"
-         tagEngine.retireTag 1
+         res <- tagEngine.retireTag 1
+         $display "retiring tag :   1 " (fshow res)
+         action {}
     |> do
          $display "got tag : " tagEngine.requestTag
 

bs/Util.bs

@@ -1,7 +1,14 @@
-package Util((|>), simulate_for) where
+package Util(
+  (|>),
+  BasicResult(..),
+  simulate_for) where
 
 infixr 0 |>
 
+data BasicResult = Success
+                 | Failure
+                 deriving (Bits, Eq, FShow)
+
 (|>) :: (a -> b) -> a -> b
 f |> x = f x;