[ Haskell ] Do not put %token into happy file if there are no tokens

Otherwise, we get a happy parse error.
BNFC · Jan 2, 2019 · f42310d · f42310d
1 parent 6bb327b
commit f42310d
Showing 1 changed file with 3 additions and 0 deletions.
diff --git a/source/src/BNFC/Backend/Haskell/CFtoHappy.hs b/source/src/BNFC/Backend/Haskell/CFtoHappy.hs
@@ -107,6 +107,9 @@ delimiter = "\n%%\n"
 
 -- Generate the list of tokens and their identifiers.
 tokens :: [(String,Int)] -> String
+tokens []   = "-- no tokens\n"
+  -- Andreas, 2019-01-02: "%token" followed by nothing is a Happy parse error.
+  -- Thus, if we have no tokens, do not output anything.
 tokens toks = "%token\n" ++ prTokens toks
  where prTokens []         = []
        prTokens ((t,k):tk) = "  " ++ render (convert t) ++