make less inefficient

- do all filters in one go
- do all post lists in one run of the program
- only write files if they are changed
  (so make repeats less work)
- simplify pandoc command for meta pages
  (this might not actually make a difference)

blog-meta/lib/Misc.hs

@@ -1,12 +1,16 @@
 module Misc where
 
-import qualified System.Console.GetOpt as GetOpt
+import Control.Monad
+import Data.ByteString.Lazy qualified as LazyBS
+import Data.Char (isAlphaNum, isAscii, toLower)
+import Data.Text (Text)
+import Data.Text qualified as Text
+import Data.Text.IO qualified as Text
+import Data.Time
+import System.Console.GetOpt qualified as GetOpt
+import System.Directory qualified as Dir
 import System.Environment
 import System.Exit
-import Data.Text (Text)
-import Data.Time
-import qualified Data.Text as Text
-import Data.Char (isAlphaNum, isAscii, toLower)
 import Text.Pandoc.Definition
 
 -- | exception on 'Left'
@@ -97,3 +101,15 @@ rssDate (ID d) =
 showDate :: IsoDate -> Text
 showDate (ID d) =
   Text.pack $ toLower <$> formatTime defaultTimeLocale "%a %-d %B %Y" d
+
+writeIfDifferentBS :: FilePath -> LazyBS.ByteString -> IO ()
+writeIfDifferentBS fn content = do
+  exists <- Dir.doesFileExist fn
+  old <- if exists then Just <$> LazyBS.readFile fn else pure Nothing
+  unless (old == Just content) $ LazyBS.writeFile fn content
+
+writeIfDifferentT :: FilePath -> Text -> IO ()
+writeIfDifferentT fn content = do
+  exists <- Dir.doesFileExist fn
+  old <- if exists then Just <$> Text.readFile fn else pure Nothing
+  unless (old == Just content) $ Text.writeFile fn content