initial version generated by chatgpt, but cabal file does not work

app/Main.hs

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+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import Text.Pandoc
+import Text.Pandoc.Error
+import Text.Pandoc.Class (runIOorExplode)
+import Text.Pandoc.PDF (makePDF)
+import System.Environment (getArgs)
+import System.FilePath (replaceExtension)
+import System.Process (callProcess)
+import System.IO (writeFile)
+import System.Directory (doesFileExist)
+import Data.Text (Text)
+import qualified Data.Text as T
+import qualified Data.Text.IO as TIO
+import Data.Hashable (hash)
+import Control.Monad (when, void)
+
+-- Transform Mermaid code blocks into image embeds
+processMermaid :: Block -> IO Block
+processMermaid (CodeBlock (id', classes, _) contents)
+  | "mermaid" `elem` classes = do
+      let baseName = if null id' then "diagram-" ++ take 6 (show (abs (hash contents))) else id'
+          mmdFile = baseName ++ ".mmd"
+          svgFile = baseName ++ ".svg"
+
+      writeFile mmdFile contents
+      exists <- doesFileExist svgFile
+      when (not exists) $ void $ callProcess "mmdc" ["-i", mmdFile, "-o", svgFile]
+
+      return $ Para [Image nullAttr [] (T.pack svgFile, "Mermaid diagram")]
+processMermaid x = return x
+
+-- Walk the Pandoc AST and process blocks
+transformDoc :: Pandoc -> IO Pandoc
+transformDoc = walkM processMermaid
+
+main :: IO ()
+main = do
+  args <- getArgs
+  case args of
+    ["-pdf", path] -> compileToPDF path
+    ["-html", path] -> compileToHTML path
+    _ -> putStrLn "Usage: docster -pdf|-html <file.md>"
+
+compileToPDF :: FilePath -> IO ()
+compileToPDF path = do
+  content <- TIO.readFile path
+  pandoc <- runIOorExplode $ readMarkdown def content
+  transformed <- transformDoc pandoc
+
+  let outputPath = replaceExtension path "pdf"
+  result <- runIOorExplode $ makePDF "pdflatex" [] transformed
+  case result of
+    Left err -> error $ "PDF error: " ++ show err
+    Right bs -> writeFile outputPath bs >> putStrLn ("✅ PDF written to " ++ outputPath)
+
+compileToHTML :: FilePath -> IO ()
+compileToHTML path = do
+  content <- TIO.readFile path
+  pandoc <- runIOorExplode $ readMarkdown def content
+  transformed <- transformDoc pandoc
+
+  let outputPath = replaceExtension path "html"
+  html <- runIOorExplode $ writeHtml5String def transformed
+  TIO.writeFile outputPath html
+  putStrLn ("✅ HTML written to " ++ outputPath)