Don't attempt to constant-fold empty arrays

fco2/EvalConstants.hs

@@ -58,6 +58,8 @@ simplifyExpression ps e
 
 --{{{  expression evaluator
 evalLiteral :: A.Expression -> EvalM OccValue
+evalLiteral (A.Literal _ _ (A.ArrayLiteral _ []))
+    = throwError "empty array"
 evalLiteral (A.Literal _ _ (A.ArrayLiteral _ aes))
     = liftM OccArray (mapM evalLiteralArray aes)
 evalLiteral l = evalSimpleLiteral l

fco2/Types.hs

@@ -316,6 +316,7 @@ data BytesInResult =
   BIJust Int            -- ^ Just that many bytes.
   | BIOneFree Int Int   -- ^ An array type; A bytes, times unknown dimension B.
   | BIUnknown           -- ^ No idea.
+  deriving (Show, Eq)
 
 -- | Return the size in bytes of a data type.
 bytesInType :: (PSM m, Die m) => A.Type -> m BytesInResult