Fix escaping of interpolations after single quotes

Author: Lucus16

nixfmt.cabal

@@ -69,7 +69,16 @@ library
     Nixfmt.Pretty
     Nixfmt.Types
     Nixfmt.Util
-  other-extensions:    OverloadedStrings, LambdaCase, FlexibleInstances, DeriveFoldable, DeriveFunctor, StandaloneDeriving
+
+  other-extensions:
+    DeriveFoldable
+    DeriveFunctor
+    FlexibleInstances
+    LambdaCase
+    OverloadedStrings
+    StandaloneDeriving
+    TupleSections
+
   hs-source-dirs:      src
   build-depends:
       base             >= 4.12.0 && < 4.15

src/Nixfmt/Pretty.hs

@@ -23,7 +23,7 @@ import Nixfmt.Types
   (Ann(..), Binder(..), Expression(..), File(..), Leaf, ParamAttr(..),
   Parameter(..), Selector(..), SimpleSelector(..), StringPart(..), Term(..),
   Token(..), TrailingComment(..), Trivia, Trivium(..), tokenText)
-import Nixfmt.Util (commonIndentation, isSpaces)
+import Nixfmt.Util (commonIndentation, isSpaces, replaceMultiple)
 
 prettyCommentLine :: Text -> Doc
 prettyCommentLine l
@@ -395,10 +395,11 @@ prettyIndentedString parts = group $ base $
     text "''" <> line'
     <> nest 2 (sepBy newline (map (prettyLine escape unescapeInterpol) parts))
     <> text "''"
-    where escape
-              = Text.replace "$''${" "$${"
-              . Text.replace "${" "''${"
-              . Text.replace "''" "'''"
+    where escape = replaceMultiple
+              [ ("'${", "''\\'''${")
+              , ("${", "''${")
+              , ("''", "'''")
+              ]
 
           unescapeInterpol t
               | Text.null t        = t

src/Nixfmt/Util.hs

@@ -4,6 +4,8 @@
  - SPDX-License-Identifier: MPL-2.0
  -}
 
+{-# LANGUAGE TupleSections #-}
+
 module Nixfmt.Util
     ( manyP
     , someP
@@ -15,14 +17,18 @@ module Nixfmt.Util
     , identChar
     , isSpaces
     , pathChar
+    , replaceMultiple
     , schemeChar
     , uriChar
     ) where
 
+import Control.Applicative ((<|>))
 import Data.Char (isAlpha, isDigit, isSpace)
+import Data.Foldable (asum)
+import Data.List (unfoldr)
 import Data.Maybe (fromMaybe)
 import Data.Text as Text
-  (Text, all, commonPrefixes, concat, empty, stripEnd, stripPrefix, takeWhile)
+  (Text, all, commonPrefixes, concat, empty, null, splitAt, stripEnd, stripPrefix, takeWhile)
 import Text.Megaparsec
   (ParsecT, Stream, Token, Tokens, many, some, takeWhile1P, takeWhileP)
 
@@ -82,3 +88,20 @@ dropCommonIndentation unstrippedLines =
 
 isSpaces :: Text -> Bool
 isSpaces = Text.all (==' ')
+
+-- | Apply multiple independent replacements. This function passes over the text
+-- once and applies the first replacement it can find at each position. After a
+-- replacement is matched, the function continues after the replacement, not
+-- inside it.
+replaceMultiple :: [(Text, Text)] -> Text -> Text
+replaceMultiple replacements = mconcat . unfoldr replaceAny
+  where
+    -- | replaceAny assumes input is nonempty
+    replaceAny :: Text -> Maybe (Text, Text)
+    replaceAny t
+      | Text.null t = Nothing
+      | otherwise   = asum (map (replaceStart t) replacements)
+                      <|> Just (Text.splitAt 1 t)
+
+    replaceStart :: Text -> (Text, Text) -> Maybe (Text, Text)
+    replaceStart t (pat, rep) = (rep,) <$> Text.stripPrefix pat t