refactor the main scalar expression parser into nicer components

Language/SQL/SimpleSQL/Parser.lhs

@@ -166,9 +166,6 @@ parser names means that they have been left factored.
 >         commaSep1 scalarExpr'
 > windowSuffix _ = fail ""
 
-TODO: review all the suffix functions, use the optionSuffix combinator
-to simplify the suffix parsers
-
 > app :: P ScalarExpr
 > app = aggOrApp >>= optionSuffix windowSuffix
 
@@ -365,46 +362,13 @@ The parsers:
 >     makeOp (o,ws) = try $ PostfixOp o e <$ keywords_ ws
 >     keywords_ = try . mapM_ keyword_
 
-Wrapper for non 'bExpr' parsing. See the between parser for
-explanation.
+All the binary operators are parsed as same precedence and left
+associativity.
 
-> scalarExpr' :: P ScalarExpr
-> scalarExpr' = scalarExpr'' False
-
-The main scalar expression parser which includes the binary operator
-parsing. All the binary operators are parsed as same precedence and
-left associativity. This will be fixed in a pass over the abstract
-syntax, which isn't working at the moment.
-
-TODO:
-left factor: stuff which starts with identifier
-
-move app>>=windowSuffix to a separate parser above next to app and
-windowSuffix themselves
-
-split out the binary op parsing from this function to above
-
-> scalarExpr'' :: Bool -> P ScalarExpr
-> scalarExpr'' bExpr = factor >>= trysuffix
+> binaryOperatorSuffix :: Bool -> ScalarExpr -> P ScalarExpr
+> binaryOperatorSuffix bExpr e0 =
+>     BinOp <$> opSymbol <*> return e0 <*> factor
 >   where
->     factor = choice [literal
->                     ,scase
->                     ,cast
->                     ,extract
->                     ,substring
->                     ,subquery
->                     ,prefixUnaryOp
->                     ,try app
->                     ,try dottedIden
->                     ,identifier
->                     ,sparens]
->     trysuffix e = try (suffix e) <|> return e
->     suffix e0 = choice
->                 [BinOp <$> opSymbol <*> return e0 <*> factor
->                 ,inSuffix e0
->                 ,betweenSuffix e0
->                 ,postfixOpSuffix e0
->                 ] >>= trysuffix
 >     opSymbol = choice
 >         (map (try . symbol) binOpSymbolNames
 >         ++ map (try . keywords) binOpMultiKeywordNames
@@ -439,6 +403,48 @@ which should be a parse error.
 > fixFixities :: ScalarExpr -> ScalarExpr
 > fixFixities = id
 
+== scalar expressions
+
+TODO:
+left factor stuff which starts with identifier
+
+This parses most of the scalar exprs. I'm not sure if factor is the
+correct terminology here. The order of the parsers and use of try is
+carefully done to make everything work. It is a little fragile and
+could at least do with some heavy explanation.
+
+> factor :: P ScalarExpr
+> factor = choice [literal
+>                 ,scase
+>                 ,cast
+>                 ,extract
+>                 ,substring
+>                 ,subquery
+>                 ,prefixUnaryOp
+>                 ,try app
+>                 ,try dottedIden
+>                 ,identifier
+>                 ,sparens]
+
+putting the factor together with the extra bits
+
+> scalarExpr'' :: Bool -> P ScalarExpr
+> scalarExpr'' bExpr = factor >>= trysuffix
+>   where
+>     trysuffix e = try (suffix e) <|> return e
+>     suffix e0 = choice
+>                 [binaryOperatorSuffix bExpr e0
+>                 ,inSuffix e0
+>                 ,betweenSuffix e0
+>                 ,postfixOpSuffix e0
+>                 ] >>= trysuffix
+
+Wrapper for non 'bExpr' parsing. See the between parser for
+explanation.
+
+> scalarExpr' :: P ScalarExpr
+> scalarExpr' = scalarExpr'' False
+
 The scalarExpr wrapper. The idea is that directly nested scalar
 expressions use the scalarExpr' parser, then other code uses the
 scalarExpr parser and then everyone gets the fixity fixes and it's