Refactor Compiler.hs with higher-order functions to eliminate duplication

- Extract PDF and HTML compilation differences using strategy pattern
- Implement CompilationStrategy record with format-specific operations
- Create higher-order compileWithStrategy function for common pipeline
- Eliminate ~40 lines of duplicated code between PDF/HTML compilation
- Add LambdaCase extension for cleaner monadic error handling
- Maintain same external API while improving internal architecture
- Enable easy addition of new output formats through new strategies

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/Docster/Compiler.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE LambdaCase #-}
 
 -- | Document compilation functionality for PDF and HTML output
 module Docster.Compiler
@@ -25,81 +26,124 @@ import Control.Monad (void)
 successEmoji :: Text
 successEmoji = "✅"
 
--- | Compile markdown to PDF using XeLaTeX
+-- | Strategy pattern: Record of format-specific operations
-compileToPDF :: FilePath -> IO ()
+data CompilationStrategy = CompilationStrategy
-compileToPDF path = do
+  { -- | Format for diagram configuration
-  let sourceDir = SourceDir $ takeDirectory path
+    csOutputFormat :: OutputFormat
-      outputPath = OutputPath $ replaceExtension path "pdf"
+    -- | Pandoc writer function
+  , csWriter :: WriterOptions -> Pandoc -> PandocIO Text
+    -- | Post-processing function for the generated content
+  , csProcessOutput :: Text -> String -> IO (Either DocsterError ())
+    -- | Success message formatter
+  , csSuccessMessage :: String -> Text
+  }
 
-  result <- compileToPDFSafe sourceDir (OutputPath path) outputPath
+-- | PDF compilation strategy
-  case result of
+pdfStrategy :: CompilationStrategy
-    Left err -> throwIO err
+pdfStrategy = CompilationStrategy
-    Right _ -> return ()
+  { csOutputFormat = PDF
+  , csWriter = writeLaTeX
+  , csProcessOutput = processPDFOutput
+  , csSuccessMessage = \path -> successEmoji <> " PDF written to " <> T.pack path
+  }
 
--- | Safe PDF compilation with comprehensive error handling
+-- | HTML compilation strategy  
-compileToPDFSafe :: SourceDir -> OutputPath -> OutputPath -> IO (Either DocsterError ())
+htmlStrategy :: CompilationStrategy
-compileToPDFSafe sourceDir (OutputPath inputPath) (OutputPath outputPathStr) = do
+htmlStrategy = CompilationStrategy
+  { csOutputFormat = HTML
+  , csWriter = writeHtml5String
+  , csProcessOutput = processHTMLOutput
+  , csSuccessMessage = \path -> successEmoji <> " HTML written to " <> T.pack path
+  }
+
+-- | Process PDF output: LaTeX template application and PDF generation
+processPDFOutput :: Text -> String -> IO (Either DocsterError ())
+processPDFOutput latexOutput outputPath = do
+  let completeLatex = latexTemplate latexOutput
+  -- We need a Pandoc document for makePDF, but it's not used in the template function
+  -- Create a minimal document for the API
+  let dummyDoc = Pandoc nullMeta []
+  pdfResult <- runIO $ makePDF "xelatex" [] (\_ _ -> return completeLatex) def dummyDoc
+  case pdfResult of
+    Left err -> return $ Left $ PDFGenerationError $ T.pack $ show err
+    Right (Left err) -> return $ Left $ PDFGenerationError $ T.pack $ show err
+    Right (Right bs) -> do
+      BL.writeFile outputPath bs
+      return $ Right ()
+
+-- | Process HTML output: file writing and browser opening
+processHTMLOutput :: Text -> String -> IO (Either DocsterError ())
+processHTMLOutput html outputPath = do
+  TIO.writeFile outputPath html
+  
+  -- Open the generated HTML file in browser for verification
+  putStrLn $ "🌐 Opening " <> outputPath <> " in browser for error checking..."
+  void $ callProcess "open" [outputPath]
+  
+  return $ Right ()
+
+-- | Higher-order compilation function that takes a strategy and executes the pipeline
+compileWithStrategy :: CompilationStrategy -> SourceDir -> OutputPath -> OutputPath -> IO (Either DocsterError ())
+compileWithStrategy strategy sourceDir (OutputPath inputPath) (OutputPath outputPath) = do
+  -- Step 1: Read and parse markdown
   content <- TIO.readFile inputPath
   let readerOptions = def { readerExtensions = getDefaultExtensions "markdown" }
-      config = DiagramConfig sourceDir PDF
+      config = DiagramConfig sourceDir (csOutputFormat strategy)
   
-  pandocResult <- runIO $ readMarkdown readerOptions content
+  parseResult <- parseMarkdown readerOptions content
-  case pandocResult of
+  case parseResult of
-    Left err -> return $ Left $ FileError $ "Failed to parse markdown: " <> T.pack (show err)
+    Left err -> return $ Left err
-    Right pandoc -> do
+    Right pandoc -> 
-      transformResult <- transformDocument config pandoc
+      -- Step 2: Transform document (process Mermaid diagrams)
-      case transformResult of
+      transformDocument config pandoc >>= \case
         Left err -> return $ Left err
-        Right transformed -> do
+        Right transformed -> 
-          latexResult <- runIO $ writeLaTeX def transformed
+          -- Step 3: Generate output using format-specific writer
-          case latexResult of
+          generateOutput strategy transformed >>= \case
-            Left err -> return $ Left $ PDFGenerationError $ "LaTeX generation failed: " <> T.pack (show err)
+            Left err -> return $ Left err
-            Right latexOutput -> do
+            Right output -> do
-              let completeLatex = latexTemplate latexOutput
+              -- Step 4: Process output and write to file
-              pdfResult <- runIO $ makePDF "xelatex" [] (\_ _ -> return completeLatex) def transformed
+              processResult <- csProcessOutput strategy output outputPath
-              case pdfResult of
+              case processResult of
-                Left err -> return $ Left $ PDFGenerationError $ T.pack $ show err
+                Left err -> return $ Left err
-                Right (Left err) -> return $ Left $ PDFGenerationError $ T.pack $ show err
+                Right _ -> do
-                Right (Right bs) -> do
+                  -- Step 5: Print success message
-                  BL.writeFile outputPathStr bs
+                  putStrLn $ T.unpack $ csSuccessMessage strategy outputPath
-                  putStrLn $ T.unpack $ successEmoji <> " PDF written to " <> T.pack outputPathStr
                   return $ Right ()
 
+-- | Parse markdown with error handling
+parseMarkdown :: ReaderOptions -> Text -> IO (Either DocsterError Pandoc)
+parseMarkdown readerOptions content = do
+  pandocResult <- runIO $ readMarkdown readerOptions content
+  return $ case pandocResult of
+    Left err -> Left $ FileError $ "Failed to parse markdown: " <> T.pack (show err)
+    Right pandoc -> Right pandoc
+
+-- | Generate output using the strategy's writer with error handling
+generateOutput :: CompilationStrategy -> Pandoc -> IO (Either DocsterError Text)
+generateOutput strategy transformed = do
+  result <- runIO $ csWriter strategy def transformed
+  return $ case result of
+    Left err -> Left $ case csOutputFormat strategy of
+      PDF -> PDFGenerationError $ "LaTeX generation failed: " <> T.pack (show err)
+      HTML -> FileError $ "HTML generation failed: " <> T.pack (show err)
+    Right output -> Right output
+
+-- | Compile markdown to PDF using XeLaTeX
+compileToPDF :: FilePath -> IO ()
+compileToPDF = compileWithFormat pdfStrategy "pdf"
+
 -- | Compile markdown to HTML
 compileToHTML :: FilePath -> IO ()
-compileToHTML path = do
+compileToHTML = compileWithFormat htmlStrategy "html"
-  let sourceDir = SourceDir $ takeDirectory path
-      outputPath = OutputPath $ replaceExtension path "html"
 
-  result <- compileToHTMLSafe sourceDir (OutputPath path) outputPath
+-- | Higher-order function to compile with any format strategy
+compileWithFormat :: CompilationStrategy -> String -> FilePath -> IO ()
+compileWithFormat strategy extension path = do
+  let sourceDir = SourceDir $ takeDirectory path
+      outputPath = OutputPath $ replaceExtension path extension
+  
+  result <- compileWithStrategy strategy sourceDir (OutputPath path) outputPath
   case result of
     Left err -> throwIO err
     Right _ -> return ()
-
--- | Safe HTML compilation with comprehensive error handling
-compileToHTMLSafe :: SourceDir -> OutputPath -> OutputPath -> IO (Either DocsterError ())
-compileToHTMLSafe sourceDir (OutputPath inputPath) (OutputPath outputPathStr) = do
-  content <- TIO.readFile inputPath
-  let readerOptions = def { readerExtensions = getDefaultExtensions "markdown" }
-      config = DiagramConfig sourceDir HTML
-  
-  pandocResult <- runIO $ readMarkdown readerOptions content
-  case pandocResult of
-    Left err -> return $ Left $ FileError $ "Failed to parse markdown: " <> T.pack (show err)
-    Right pandoc -> do
-      transformResult <- transformDocument config pandoc
-      case transformResult of
-        Left err -> return $ Left err
-        Right transformed -> do
-          htmlResult <- runIO $ writeHtml5String def transformed
-          case htmlResult of
-            Left err -> return $ Left $ FileError $ "HTML generation failed: " <> T.pack (show err)
-            Right html -> do
-              TIO.writeFile outputPathStr html
-              putStrLn $ T.unpack $ successEmoji <> " HTML written to " <> T.pack outputPathStr
-              
-              -- Open the generated HTML file in browser for verification
-              putStrLn $ "🌐 Opening " <> outputPathStr <> " in browser for error checking..."
-              void $ callProcess "open" [outputPathStr]
-              
-              return $ Right ()