return distance from path command (#1698)

Augments #836.

Should have access to the remaining distance, since it's available internally.  This will help in implementing #1696.

data/scenarios/Fun/horton.yaml

@@ -34,7 +34,8 @@ robots:
       - treads
       - wayfinder
 solution: |
-  def goDir = \f. \d.
+  def goDir = \f. \result.
+    let d = fst result in
     if (d == down) {grab; f;} {turn d; move; f;}
     end;
 
@@ -52,7 +53,7 @@ entities:
       - |
         Enables the `path` command:
       - |
-        `path : (unit + int) -> ((int * int) + text) -> cmd (unit + dir)`
+        `path : (unit + int) -> ((int * int) + text) -> cmd (unit + (dir * int))`
       - |
         Optionally supply a distance limit as the first argument, and
         supply either a location (`inL`) or an entity (`inR`) as the second argument.

data/scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml

@@ -15,7 +15,8 @@ objectives:
         return $ fCount >= 4;
       };
 solution: |
-  def goDir = \f. \d.
+  def goDir = \f. \result.
+    let d = fst result in
     if (d == down) {grab; f;} {turn d; move; f;}
     end;
 

data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/patrol.sw

@@ -1,4 +1,5 @@
-def goDir = \f. \d.
+def goDir = \f. \r.
+  let d = fst r in
   if (d == down) {
     eggHere <- ishere "egg";
     if eggHere {grab; return ()} {};

data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-solution.sw

@@ -1,4 +1,5 @@
-def goDir = \f. \d.
+def goDir = \f. \r.
+  let d = fst r in
   if (d == down) {
     grab; return ()
   } {

data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw

@@ -1,4 +1,5 @@
-def goDir = \goalItem. \f. \d.
+def goDir = \goalItem. \f. \r.
+  let d = fst r in
   if (d == down) {
     grab; return ()
   } {

data/scenarios/Testing/836-pathfinding/_836-path-exists/find-location-solution.sw

@@ -1,4 +1,5 @@
-def goDir = \f. \d.
+def goDir = \f. \r.
+  let d = fst r in
   if (d == down) {
     grab; return ()
   } {

src/Swarm/Game/Step/Path/Finding.hs

@@ -64,7 +64,7 @@ import Swarm.Util (hoistMaybe)
 pathCommand ::
   HasRobotStepState sig m =>
   PathfindingParameters (Cosmic Location) ->
-  m (Maybe Direction)
+  m (Maybe (Direction, Int))
 pathCommand parms = do
   currentWalkabilityContext <- use walkabilityContext
 
@@ -72,7 +72,7 @@ pathCommand parms = do
   eitherCachedPath <- retrieveCachedPath currentWalkabilityContext parms
 
   case eitherCachedPath of
-    Right foundCachedPath -> return $ Just $ nextDir foundCachedPath
+    Right foundCachedPath -> return $ Just $ mkResult foundCachedPath
     Left _ -> do
       -- This is a short-circuiting optimization; if the goal location itself
       -- is not a walkable cell, then no amount of searching will reach it.
@@ -86,8 +86,9 @@ pathCommand parms = do
         foundPath <- hoistMaybe maybeFoundPath
         -- NOTE: This will not cache the fact that a path was not found.
         lift $ recordCache (fmap (^. subworld) parms) currentWalkabilityContext $ robotLoc :| foundPath
-        return $ nextDir foundPath
+        return $ mkResult foundPath
  where
+  mkResult p = (nextDir p, length p)
   PathfindingParameters maybeDistanceLimit (Cosmic currentSubworld robotLoc) target = parms
 
   computePath =

src/Swarm/Language/Syntax.hs

@@ -545,7 +545,7 @@ constInfo c = case c of
     command 2 short . doc "Obtain shortest path to the destination." $
       [ "Optionally supply a distance limit as the first argument."
       , "Supply either a location (`inL`) or an entity (`inR`) as the second argument."
-      , "If a path exists, returns the direction to proceed along."
+      , "If a path exists, returns the direction to proceed along and the remaining distance."
       ]
   Push ->
     command 1 short . doc "Push an entity forward one step." $

src/Swarm/Language/Typecheck.hs

@@ -742,7 +742,7 @@ inferConst c = case c of
   Selfdestruct -> [tyQ| cmd unit |]
   Move -> [tyQ| cmd unit |]
   Backup -> [tyQ| cmd unit |]
-  Path -> [tyQ| (unit + int) -> ((int * int) + text) -> cmd (unit + dir) |]
+  Path -> [tyQ| (unit + int) -> ((int * int) + text) -> cmd (unit + (dir * int)) |]
   Push -> [tyQ| cmd unit |]
   Stride -> [tyQ| int -> cmd unit |]
   Turn -> [tyQ| dir -> cmd unit |]