use printable and erasable properties

data/entities.yaml

@@ -69,7 +69,7 @@
     char: '■'
   description:
     - A flat material made of pressed and dried wood fibers, used as a surface on which to inscribe symbols.
-  properties: [pickable, combustible]
+  properties: [pickable, combustible, printable]
   combustion:
     ignition: 0.5
     duration: [10, 20]

data/scenarios/Challenges/_telephone/solution.sw

@@ -69,7 +69,7 @@ def go =
       res <- scanAt (h-1) (v-1);
       case res
         (\_. return ())
-        (\t. atTerminal (p <- print (format ((h-1,v-1),t)); waitToPlace p))
+        (\t. atTerminal (p <- print "paper" (format ((h-1,v-1),t)); waitToPlace p))
     )
   )
 end

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

@@ -1215,27 +1215,30 @@ execConst runChildProg c vs s k = do
         Just v -> return (mkReturn v)
       _ -> badConst
     Print -> case vs of
-      [VText txt] -> do
-        paper <- ensureItem "paper" "print"
-        let newEntityName = "paper: " <> txt
-        robotInventory %= delete paper
-        robotInventory %= insert (paper & entityName .~ newEntityName)
+      [VText printableName, VText txt] -> do
+        printable <- ensureItem printableName "print"
+        (printable `hasProperty` Printable)
+          `holdsOrFail` ["You cannot print on", indefinite printableName <> "!"]
+        let newEntityName = printableName <> ": " <> txt
+        robotInventory %= delete printable
+        robotInventory %= insert (printable & entityName .~ newEntityName)
         return $ mkReturn newEntityName
       _ -> badConst
     Erase -> case vs of
-      [VText paperName] -> do
-        toErase <- ensureItem paperName "erase"
-        (paperName /= "paper")
-          `holdsOrFail` ["That is already blank!"]
-        ("paper: " `T.isPrefixOf` paperName)
-          `holdsOrFail` ["Can only erase paper, not", indefinite paperName <> "."]
+      [VText printableName] -> do
+        toErase <- ensureItem printableName "erase"
+        let (baseName, printedMatter) = T.break (==':') printableName
         em <- use $ landscape . terrainAndEntities . entityMap
-        paper <-
-          lookupEntityName "paper" em
-            `isJustOrFail` ["Can't erase, I don't know what paper is."]
+        erased <-
+          lookupEntityName baseName em
+            `isJustOrFail` ["I've never heard of", indefiniteQ baseName <> "."]
+        (erased `hasProperty` Printable)
+          `holdsOrFail` ["You cannot erase", indefinite baseName <> "!"]
+        (not $ T.null printedMatter)
+          `holdsOrFail` ["That is already blank!"]
 
         robotInventory %= delete toErase
-        robotInventory %= insert paper
+        robotInventory %= insert erased
         return $ mkReturn ()
       _ -> badConst
     Chars -> case vs of

src/swarm-lang/Swarm/Language/Syntax/Constants.hs

@@ -821,22 +821,22 @@ constInfo c = case c of
   Format -> function 1 $ shortDoc Set.empty "Turn an arbitrary value into a string."
   Read -> function 2 $ shortDoc Set.empty "Try to read a string into a value of the expected type."
   Print ->
-    command 1 short
+    command 2 short
       . doc
         (Set.singleton $ Mutation $ RobotChange InventoryChange)
-        "Print text onto a piece of paper."
-      $ [ "Consumes one `paper` entity from your inventory, and produces an entity"
-        , "whose name is \"paper: \" concatenated with the given text."
-        , "In conjunction with `format`, this can be used to print values onto paper"
+        "Print text onto an entity."
+      $ [ "`print p txt` Consumes one printable `p` entity from your inventory, and produces an entity"
+        , "whose name is concatenated with a colon and the given text."
+        , "In conjunction with `format`, this can be used to print values onto entities such as `paper`{=entity}"
         , "and give them to other robots, which can reconstitute the values with `read`."
         ]
   Erase ->
     command 1 short
       . doc
         (Set.singleton $ Mutation $ RobotChange InventoryChange)
-        "Erase a piece of paper."
-      $ [ "Consumes the named entity from your inventory, whose name must begin with"
-        , "\"paper: \", and produces a \"paper\" entity.  This can be used to undo"
+        "Erase an entity."
+      $ [ "Consumes the named printable entity from your inventory, which must have something"
+        , "printed on it, and produces an erased entity.  This can be used to undo"
         , "the effects of a `print` command."
         ]
   Concat -> binaryOp "++" 6 R $ shortDoc Set.empty "Concatenate the given strings."

src/swarm-lang/Swarm/Language/Typecheck.hs

@@ -1100,7 +1100,7 @@ inferConst c = run . runReader @TVCtx Ctx.empty . quantify $ case c of
   Exp -> arithBinT
   Format -> [tyQ| a -> Text |]
   Read -> [tyQ| Text -> a |]
-  Print -> [tyQ| Text -> Cmd Text |]
+  Print -> [tyQ| Text -> Text -> Cmd Text |]
   Erase -> [tyQ| Text -> Cmd Unit |]
   Concat -> [tyQ| Text -> Text -> Text |]
   Chars -> [tyQ| Text -> Int |]

src/swarm-scenario/Swarm/Game/Entity.hs

@@ -170,6 +170,10 @@ data EntityProperty
     Liquid
   | -- | Robots automatically know what this is without having to scan it.
     Known
+  | -- | Text can be printed on this entity with the
+    --   'Swarm.Language.Syntax.Print' command (and erased with
+    --   'Swarm.Language.Syntax.Erase')
+    Printable
   deriving (Eq, Ord, Show, Read, Enum, Bounded, Generic, Hashable)
 
 instance ToJSON EntityProperty where

src/swarm-tui/Swarm/TUI/View.hs

@@ -1122,6 +1122,7 @@ displayProperties = displayList . mapMaybe showProperty
   -- in challenge scenarios and not really something the player needs
   -- to know.
   showProperty Known = Nothing
+  showProperty Printable = Just "printable"
 
   displayList [] = emptyWidget
   displayList ps =