Added beginnings of Pretty module

Data/SCargot/Pretty.hs

@@ -0,0 +1,58 @@
+module Data.SCargot.Pretty where
+
+
+-- | A 'LayoutOptions' value describes how to pretty-print a 'SExpr'.
+--   It describes how to print atoms, what horizontal space to fit
+--   it into, and other related options.
+--
+--   The 'swingIndent' value might require a big of explanation: in
+--   pretty-printing s-expressions, you have the option of whether
+--   to 'swing' expressions which get pushed to subsequent lines
+--   to the left, or to align them along the right. e.g. the
+--   s-expression @(foo a b)@ could use a non-swing indent as
+--
+--   > (foo arg-one
+--   >      arg-two)
+--
+--   or a swing indent as
+--
+--   > (foo arg-one
+--   >   arg-two)
+--
+--   often, in formatting Lisp code, control structures will
+--   swing subsequent expressions, as in
+--
+--   > (define (factorial n)
+--   >   (if (= n 0)
+--   >     1
+--   >     (* n (fact (- n 1)))))
+--
+--   but most functions will _not_ swing:
+--
+--   > (call-my-func arg-number-one
+--   >               arg-number-two
+--   >               arg-number-three)
+--
+--   The 'swingIndent' field lets you choose whether or not to
+--   swing subsequent s-expressions based on the atom in the car
+--   position of a list. You can default to always swinging subsequent
+--   expressions with @const True@ and never with @const False@, or
+--   choose based on some more advanced criteria. _If_ a swing happens,
+--   subsequent lines are indented based on the 'indentAmount' variable;
+--   otherwise, subsequent lines are indented based on the size of the
+--   @car@ of the list.
+data LayoutOptions a = LayoutOptions
+  { atomPrinter  :: a -> Text -- ^ How to serialize a given atom to 'Text'.
+  , swingIndent  :: a -> Bool -- ^ Whether or not to swing
+  , indentAmount :: Int       -- ^ How much to indent after a swing
+  , maxWidth     :: Maybe Int -- ^ The maximum width (if any)
+  }
+
+basicPrint :: (a -> Text) -> LayoutOptions a
+basicPrint f = LayoutOptions
+  { atomPrinter = f
+  , swingIndent = const False
+  , maxWidth    = Nothing
+  }
+
+prettyPrintSExpr :: LayoutOptions a -> SExpr a -> Text