use explicit data type for sign in interval literals

Language/SQL/SimpleSQL/Parse.lhs

@@ -686,8 +686,8 @@ this. also fix the monad -> applicative
 
 > intervalLit :: Parser ScalarExpr
 > intervalLit = try (keyword_ "interval" >> do
->     s <- optionMaybe $ choice [True <$ symbol_ "+"
->                               ,False <$ symbol_ "-"]
+>     s <- optionMaybe $ choice [Plus <$ symbol_ "+"
+>                               ,Minus <$ symbol_ "-"]
 >     lit <- singleQuotesOnlyStringTok
 >     q <- optionMaybe intervalQualifier
 >     mkIt s lit q)

Language/SQL/SimpleSQL/Pretty.lhs

@@ -45,7 +45,9 @@ which have been changed to try to improve the layout of the output.
 > scalarExpr _ (NumLit s) = text s
 > scalarExpr _ (IntervalLit s v f t) =
 >     text "interval"
->     <+> me (\x -> if x then text "+" else text "-") s
+>     <+> me (\x -> text $ case x of
+>                              Plus -> "+"
+>                              Minus -> "-") s
 >     <+> quotes (text v)
 >     <+> intervalTypeField f
 >     <+> me (\x -> text "to" <+> intervalTypeField x) t

Language/SQL/SimpleSQL/Syntax.lhs

@@ -7,6 +7,7 @@
 >     ,Name(..)
 >     ,TypeName(..)
 >     ,IntervalTypeField(..)
+>     ,Sign(..)
 >     ,PrecMultiplier(..)
 >     ,PrecUnits(..)
 >     ,SetQuantifier(..)
@@ -95,7 +96,7 @@
 >       -- | text of interval literal, units of interval precision,
 >       -- e.g. interval 3 days (3)
 >     | IntervalLit
->       {ilSign :: Maybe Bool -- ^ true if + used, false if - used
+>       {ilSign :: Maybe Sign -- ^ if + or - used
 >       ,ilLiteral :: String -- ^ literal text
 >       ,ilFrom :: IntervalTypeField
 >       ,ilTo :: Maybe IntervalTypeField
@@ -250,6 +251,9 @@ in other places
 > data IntervalTypeField = Itf String (Maybe (Integer, Maybe Integer))
 >                          deriving (Eq,Show,Read,Data,Typeable)
 
+> data Sign = Plus | Minus
+>             deriving (Eq,Show,Read,Data,Typeable)
+
 > data PrecMultiplier = PrecK | PrecM | PrecG | PrecT | PrecP
 >                       deriving (Eq,Show,Read,Data,Typeable)
 > data PrecUnits = PrecCharacters

tools/Language/SQL/SimpleSQL/SQL2011Queries.lhs

@@ -711,9 +711,9 @@ TODO: unicode escape
 >     ,("interval '1' day(3)"
 >      ,IntervalLit Nothing "1" (Itf "day" $ Just (3,Nothing)) Nothing)
 >     ,("interval + '1' day(3)"
->      ,IntervalLit (Just True) "1" (Itf "day" $ Just (3,Nothing)) Nothing)
+>      ,IntervalLit (Just Plus) "1" (Itf "day" $ Just (3,Nothing)) Nothing)
 >     ,("interval - '1' second(2,2)"
->      ,IntervalLit (Just False) "1" (Itf "second" $ Just (2,Just 2)) Nothing)
+>      ,IntervalLit (Just Minus) "1" (Itf "second" $ Just (2,Just 2)) Nothing)
 >     ,("interval '1' year to month"
 >      ,IntervalLit Nothing "1" (Itf "year" Nothing)
 >                                   (Just $ Itf "month" Nothing))