refactor pretty.hs to avoid using Text -> String

Using ViewPatterns, PatternSynonyms, OverloadedStrings to achieve a readable code
`escapeDoubleQuoteString` to escape nix double quoted string in both `prettyString` and `printNix`

Author: soulomoon

src/Nix/Pretty.hs

@@ -1,5 +1,7 @@
 {-# language CPP #-}
 {-# language AllowAmbiguousTypes #-}
+{-# language ViewPatterns, PatternSynonyms, OverloadedStrings #-}
+
 
 {-# options_ghc -fno-warn-name-shadowing #-}
 
@@ -99,21 +101,27 @@ wrapPath op sub =
     ("\"${" <> withoutParens sub <> "}\"")
     (wasPath sub)
 
+
+infixr 5 :<
+pattern (:<) :: Char -> Text -> Text
+pattern t :< ts <- (Text.uncons -> Just (t, ts))
+  where (:<) = Text.cons
+
+escapeDoubleQuoteString :: Text -> Text
+escapeDoubleQuoteString ('"':<xs) = "\\\"" <> escapeDoubleQuoteString xs
+escapeDoubleQuoteString ('$':<'{':<xs) = "\\${" <> escapeDoubleQuoteString xs
+escapeDoubleQuoteString ('$':<xs) = '$' :< escapeDoubleQuoteString xs
+escapeDoubleQuoteString (x:<xs) = maybe (one x) (('\\' :<) . one) (toEscapeCode x)
+  <> escapeDoubleQuoteString xs
+escapeDoubleQuoteString a = a
+
+
 prettyString :: NString (NixDoc ann) -> Doc ann
 prettyString (DoubleQuoted parts) = "\"" <> foldMap prettyPart parts <> "\""
  where
-  -- It serializes Text -> String, because the helper code is done for String,
-  -- please, can someone break that code.
-  prettyPart (Plain t)      = pretty . dollarEscape . toText . foldMap escape . toString $ t
+  prettyPart (Plain t)      = pretty $ escapeDoubleQuoteString t
   prettyPart EscapedNewline = "''\\n"
   prettyPart (Antiquoted r) = "${" <> withoutParens r <> "}"
-  escape '"' = "\\\""
-  escape '$' = "$" -- do not print $ as \$ if no { is following 
-  escape x   =
-    maybe
-      (one x)
-      (('\\' :) . one)
-      (toEscapeCode x)
 prettyString (Indented _ parts) = group $ nest 2 $ vcat
   ["''", content, "''"]
  where
@@ -383,9 +391,6 @@ prettyNThunk t =
           "(" <> fold (one "thunk from: " <> (prettyOriginExpr . _originExpr <$> ps)) <> ")"
         ]
 
--- | dollarEscape for double quoted string 
-dollarEscape :: Text -> Text
-dollarEscape = replace "${" "\\${"
 
 -- | This function is used only by the testing code.
 printNix :: forall t f m . MonadDataContext f m => NValue t f m -> Text
@@ -395,7 +400,7 @@ printNix = iterNValueByDiscardWith thk phi
 
   phi :: NValue' t f m Text -> Text
   phi (NVConstant' a ) = atomText a
-  phi (NVStr'      ns) = dollarEscape $ show $ ignoreContext ns
+  phi (NVStr'      ns) = "\"" <> escapeDoubleQuoteString (ignoreContext ns) <> "\""
   phi (NVList'     l ) = "[ " <> unwords l <> " ]"
   phi (NVSet' _ s) =
     "{ " <>