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+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+-- | This module provides the 'FormattedWriter' monad, which extends a basic
+-- "Writer" over 'Text' by allowing formatting features like indentation, line 
+-- breaks, and ensuring the output is well-structured for readability. 
+-- 
+-- The 'FormattedWriter' is useful for generating pretty-printed output, such as
+-- source code generation, where indentation and newline management are essential.
+module Language.Fiddle.Compiler.Backend.Internal.FormattedWriter
+  ( -- * Types
+    FormattedWriter,       -- | The main writer monad with formatting features.
+    -- * Core Operations
+    ensureNL,              -- | Ensures that there is a newline at the end of the current line.
+    flush,                 -- | Flushes any pending text (writes the pending line).
+    incIndent,             -- | Increases the indentation level.
+    decIndent,             -- | Decreases the indentation level.
+    indented,              -- | Performs an action with increased indentation.
+    execFormattedWriter,   -- | Runs the 'FormattedWriter' and produces the final formatted 'Text'.
+  )
+where
+
+import Control.Monad (forM_)
+import Control.Monad.RWS.Strict
+import Data.Text (Text)
+import qualified Data.Text as Text
+
+-- | Internal state for the 'FormattedWriter' monad.
+-- 
+-- * 'indentLevel': The current level of indentation.
+-- * 'pendingLine': The current line that is being built but not yet written.
+data Fmt = Fmt
+  { indentLevel :: Int,    -- ^ Current indentation level.
+    pendingLine :: Text    -- ^ Text that has been added but not yet written to the output.
+  }
+
+-- | The 'FormattedWriter' is a monad that provides functionality for writing
+-- formatted text, with control over indentation and newlines.
+newtype FormattedWriter a = FormattedWriter (RWS () Text Fmt a)
+  deriving newtype (Functor, Applicative, Monad, MonadState Fmt)
+
+-- | 'MonadWriter' instance for 'FormattedWriter', which allows writing text
+-- while maintaining proper indentation and ensuring that newlines are handled
+-- correctly. Each line is prefixed with the correct indentation level.
+instance MonadWriter Text FormattedWriter where
+  tell txt = FormattedWriter $ do
+    -- Get the current indentation level and prepare the indentation prefix.
+    indent <- (`Text.replicate` "  ") <$> gets indentLevel
+    -- Split the text into lines.
+    let lines = Text.splitOn "\n" txt
+    -- For each line, write the indent, the pending line, and then the line itself.
+    forM_ (Prelude.init lines) $ \line -> do
+      pending <- gets pendingLine
+      modify $ \s -> s {pendingLine = ""}  -- Reset pending line
+      tell indent  -- Add indentation
+      tell pending -- Write any pending text
+      tell line    -- Write the actual line
+      tell "\n"    -- Add a newline after the line
+    -- The last fragment is kept as the new pending line, to be written later.
+    modify $ \s -> s {pendingLine = pendingLine s <> Prelude.last lines}
+
+  -- Allow listening to the written text within the 'FormattedWriter' context.
+  listen (FormattedWriter fn) = FormattedWriter $ listen fn
+
+  -- Allow transformation of the written text via a pass operation.
+  pass (FormattedWriter fn) = FormattedWriter $ pass fn
+
+-- | Ensures that a newline is written if there is any pending text. This is
+-- useful for ensuring that the output ends on a clean line.
+ensureNL :: FormattedWriter ()
+ensureNL = do
+  p <- gets pendingLine
+  if Text.null p
+    then return ()
+    else do
+      modify $ \s -> s {pendingLine = ""}
+      tell p
+      tell "\n"
+
+-- | Flushes the pending line by writing it to the output and clearing the
+-- pending state. This is useful when you want to force the current line to
+-- be written immediately.
+flush :: FormattedWriter ()
+flush = do
+  p <- gets pendingLine
+  modify $ \s -> s {pendingLine = ""}
+  tell p
+
+-- | Increases the indentation level by one. This will affect all subsequent
+-- lines written within the 'FormattedWriter' monad.
+incIndent :: FormattedWriter ()
+incIndent = modify (\(Fmt id p) -> Fmt (id + 1) p)
+
+-- | Decreases the indentation level by one. This will affect all subsequent
+-- lines written within the 'FormattedWriter' monad.
+decIndent :: FormattedWriter ()
+decIndent = modify (\(Fmt id p) -> Fmt (id - 1) p)
+
+-- | Runs the given 'FormattedWriter' action with an additional indentation level.
+-- Once the action completes, the indentation level is decreased.
+indented :: FormattedWriter () -> FormattedWriter ()
+indented fn = do
+  incIndent  -- Increase indentation level
+  fn         -- Run the action with the increased indentation
+  decIndent  -- Restore the original indentation level
+
+-- | Runs a 'FormattedWriter' and returns the final formatted 'Text' output.
+-- It ensures that any pending lines are flushed before returning the result.
+execFormattedWriter :: FormattedWriter a -> Text
+execFormattedWriter ((>> flush) -> FormattedWriter rws) =
+  snd $ execRWS rws () (Fmt 0 "")  -- Execute the RWS monad, starting with no indentation and no pending line.
+