fix Ctrl-C so it doesn't lose all definitions in scope

src/swarm-engine/Swarm/Game/CESK.hs

@@ -394,7 +394,7 @@ prepareTerm e t = case whnfType (e ^. envTydefs) (ptBody (t ^. sType)) of
 
 -- | Cancel the currently running computation.
 cancel :: CESK -> CESK
-cancel cesk = Out VUnit s' []
+cancel cesk = Up Cancel s' (cesk ^. cont)
  where
   s' = resetBlackholes $ cesk ^. store
 

src/swarm-engine/Swarm/Game/Exception.hs

@@ -9,6 +9,7 @@ module Swarm.Game.Exception (
   Exn (..),
   IncapableFix (..),
   formatExn,
+  exnSeverity,
   IncapableFixWords (..),
 
   -- * Helper functions
@@ -32,6 +33,7 @@ import Swarm.Language.JSON ()
 import Swarm.Language.Pretty (prettyText)
 import Swarm.Language.Requirements.Type (Requirements (..))
 import Swarm.Language.Syntax (Const, Term)
+import Swarm.Log (Severity (..))
 import Swarm.Util
 import Witch (from)
 
@@ -67,6 +69,11 @@ data Exn
     --   be caught by a @try@ block (but at least it will not crash
     --   the entire UI).
     Fatal Text
+  | -- | The user manually cancelled the computation (e.g. by hitting
+    --   Ctrl-C). This cannot be caught by a @try@ block, and results
+    --   in the CESK machine unwinding the stack all the way back to
+    --   the top level.
+    Cancel
   | -- | An infinite loop was detected via a blackhole.  This cannot
     --   be caught by a @try@ block.
     InfiniteLoop
@@ -94,11 +101,21 @@ formatExn em = \case
       , "Please report this as a bug at"
       , "<" <> swarmRepoUrl <> "issues/new>."
       ]
+  Cancel -> "Computation cancelled."
   InfiniteLoop -> "Infinite loop detected!"
   (CmdFailed c t _) -> T.concat [prettyText c, ": ", t]
   (User t) -> "Player exception: " <> t
   (Incapable f caps tm) -> formatIncapable em f caps tm
 
+exnSeverity :: Exn -> Severity
+exnSeverity = \case
+  Fatal {} -> Critical
+  Cancel {} -> Info
+  InfiniteLoop {} -> Error
+  Incapable {} -> Error
+  CmdFailed {} -> Error
+  User {} -> Error
+
 -- ------------------------------------------------------------------
 -- INCAPABLE HELPERS
 -- ------------------------------------------------------------------

src/swarm-engine/Swarm/Game/Step.hs

@@ -848,7 +848,7 @@ stepCESK cesk = case cesk of
     let s' = resetBlackholes s
     h <- hasCapability CLog
     em <- use $ landscape . terrainAndEntities . entityMap
-    when h $ void $ traceLog RobotError Error (formatExn em exn)
+    when h $ void $ traceLog RobotError (exnSeverity exn) (formatExn em exn)
     return $ case menv of
       Nothing -> Out VExc s' []
       Just env -> Suspended VExc env s' []