Fix #388: check token defs for empty regular expression

source/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 Unreleased
 
+* LBNF: empty tokens types are now forbidden [#388]
+
 # 2.9.3
 
 Andreas Abel <[email protected]>  September 2021

source/src/BNFC/GetCF.hs

@@ -47,6 +47,7 @@ import System.IO   (hPutStrLn, stderr)
 -- Local imports:
 
 import qualified BNFC.Abs as Abs
+import BNFC.Abs (Reg(RAlts))
 import BNFC.Par
 
 import BNFC.CF
@@ -537,9 +538,16 @@ checkComments cf = concat
   where
   prags = cfgPragmas cf
 
--- | Check if any of the user-defined terminal categories is nullable.
+-- | Check if any of the user-defined terminal categories is nullable or empty.
 checkTokens :: CFG f -> Maybe String
-checkTokens cf
+checkTokens cf =
+  case catMaybes [ checkTokensEmpty cf, checkTokensNullable cf ] of
+    [] -> Nothing
+    ss -> Just $ concat ss
+
+-- | Check if any of the user-defined terminal categories is nullable.
+checkTokensNullable :: CFG f -> Maybe String
+checkTokensNullable cf
   | null pxs  = Nothing
   | otherwise = Just $ unlines $ concat
       [ [ "ERROR: The following tokens accept the empty string:" ]
@@ -548,6 +556,18 @@ checkTokens cf
   where
     pxs = [ px | TokenReg px _ regex <- cfgPragmas cf, nullable regex ]
 
+-- | Check if any of the user-defined terminal categories is nullable.
+checkTokensEmpty :: CFG f -> Maybe String
+checkTokensEmpty cf
+  | null pxs  = Nothing
+  | otherwise = Just $ unlines $ concat
+      [ [ "ERROR: The following tokens accept nothing:" ]
+      , printNames pxs
+      ]
+  where
+    -- The regular expression is already simplified, so we match against 0 directly.
+    pxs = [ px | TokenReg px _ (RAlts "") <- cfgPragmas cf ]
+
 
 -- we should actually check that
 -- (1) coercions are always between variants

testing/fail-lbnf/388-empty-regex.cf

@@ -0,0 +1,3 @@
+-- 2021-10-05 Issue #388, empty regular expression
+
+token Empty [""]+ | [""] | (char - char);

testing/fail-lbnf/388-empty-regex.err

@@ -0,0 +1,4 @@
+ERROR: The following tokens accept nothing:
+  fail-lbnf/388-empty-regex.cf:3:7: Empty
+
+Aborting.  (Use option --force to continue despite errors.)