compiler: reuse derived item properties from last compilation

A benchmark on an already bulit gallery with ~600 pictures shows a ~90% speedup:

Before:

  Time (mean ± σ):      2.879 s ±  0.125 s    [User: 14.686 s, System: 5.511 s]
  Range (min … max):    2.774 s …  3.203 s    10 runs

After:

  Time (mean ± σ):     289.5 ms ±  15.1 ms    [User: 596.1 ms, System: 359.3 ms]
  Range (min … max):   272.8 ms … 323.0 ms    10 runs

GitHub: closes #97

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/src/Input.hs b/compiler/src/Input.hs
index 1316cdd..2480f5b 100644
--- a/compiler/src/Input.hs
+++ b/compiler/src/Input.hs
@@ -28,7 +28,7 @@ import Control.Exception (Exception, AssertionFailed(..), throw, throwIO)
 import Control.Monad.IO.Class (MonadIO, liftIO)
 import Data.Function ((&))
 import Data.Functor ((<&>))
-import Data.Maybe (catMaybes)
+import Data.Maybe (catMaybes, fromMaybe)
 import Data.Bool (bool)
 import Data.List (find)
 import Data.Time.Clock (UTCTime)
@@ -91,7 +91,7 @@ readSidecarFile :: FilePath -> IO Sidecar
 readSidecarFile filepath =
   doesFileExist filepath
   >>= bool (return Nothing) (decodeYamlFile filepath)
-  <&> maybe emptySidecar id
+  <&> fromMaybe emptySidecar
 
 
 readInputTree :: AnchoredFSNode -> IO InputTree