seperate pure functions from IO

app/EpubParser.hs

@@ -10,6 +10,8 @@ module EpubParser
     , getChapter
     , getChapterTitle
     , Tag(..)
+    , prepareView
+    , ChapterView(..)
     ) where
 
 import Codec.Archive.Zip (Archive, findEntryByPath, fromEntry, toArchive)
@@ -21,7 +23,7 @@ import Control.Monad.Reader (ReaderT, MonadReader(ask), liftIO)
 import Control.Monad.Except (runExceptT)
 import System.FilePath (takeDirectory, (</>), normalise)
 import Data.List (find)
-import Text.HTML.TagSoup (parseTags, innerText, Tag(..))
+import Text.HTML.TagSoup (parseTags, innerText, Tag(..), renderTags)
 import Control.Concurrent.Async (Async)
 import Codec.Epub.Parse (getSpine, getMetadata, getManifest)
 import qualified Codec.Epub.Data.Metadata as DMeta
@@ -52,6 +54,12 @@ data EpubEnv = EpubEnv
   , eManifest :: DMan.Manifest
   }
 
+data ChapterView = ChapterView
+    { viewTitle :: T.Text
+    , viewHtml  :: T.Text
+    , viewIdx   :: Int
+    }
+
 openEpub :: FilePath -> IO (Either String EpubEnv)
 openEpub path = do
   rawZip <- B.readFile path
@@ -164,3 +172,13 @@ getChapterTitle tags =
     ishding _                = False
     isclose n  (TagClose n') = n == n'
     isclose _ _              = False
+
+prepareView :: AppState -> Int -> Maybe ChapterView
+prepareView st index
+  | index < 0 || index >= length (cAllChapters st) = Nothing
+  | otherwise = Just $ ChapterView
+    { viewTitle = bTitle st <> " - " <> getChapterTitle tags
+    , viewHtml  = renderTags tags
+    , viewIdx   = index
+    }
+  where tags = cAllChapters st !! index

app/UI.hs

@@ -15,10 +15,9 @@ import Data.List (find)
 import Data.GI.Base (AttrOp(..))
 import Control.Monad.Reader (runReaderT)
 import Control.Concurrent.STM (newTVarIO, readTVar, atomically, modifyTVar)
-import Text.HTML.TagSoup (renderTags)
 import Control.Concurrent.Async (async)
 import Codec.Epub.Data.Metadata (metaTitles, titleType, titleText)
-import EpubParser (EpubEnv(..), AppState(..), resolveSpine, getChapter, getChapterTitle)
+import EpubParser (EpubEnv(..), AppState(..), resolveSpine, getChapter, prepareView, ChapterView(..))
 import Persistence (saveLastRead, loadLastRead)
 
 runApp :: EpubEnv -> IO ()
@@ -48,17 +47,14 @@ runApp env' = do
         -- load all chapters on another thread
         let loadChapter index = do
               st <- atomically $ readTVar stateTVar
-              let chapters = cAllChapters st
-              if null chapters || index < 0 || index >= length chapters
-                then pure ()
-                else do
-                  let tags = chapters !! index
-                  let html = renderTags tags
-                  let fullTitle = bTitle st <> " - " <> getChapterTitle tags
-                  Gtk.set window [ #title := fullTitle ]
-                  WebKit.webViewLoadHtml webView html Nothing
-                  atomically $ modifyTVar stateTVar $ \s -> s { cIdx = index }
-                  _ <- async $ saveLastRead (bookPath env') index
+              case prepareView st index of
+                Nothing -> pure ()
+                Just view -> do
+                  Gtk.set window [ #title := viewTitle view ]
+                  WebKit.webViewLoadHtml webView (viewHtml view) Nothing
+
+                  atomically $ modifyTVar stateTVar $ \s -> s { cIdx = viewIdx view}
+                  _ <- async $ saveLastRead (bookPath env') (viewIdx view)
                   pure ()
 
         -- keyboard handling