Add entities, and allow walking around

Add support for entities via a port of the EntityMap type, and implement
command support starting at basic hjkl.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/Xanthous/Orphans.hs b/src/Xanthous/Orphans.hs
new file mode 100644
index 000000000000..232eabf4efb1
--- /dev/null
+++ b/src/Xanthous/Orphans.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE UndecidableInstances, PatternSynonyms #-}
+{-# OPTIONS_GHC -Wno-orphans #-}
+-- |
+
+module Xanthous.Orphans () where
+
+import Xanthous.Prelude
+
+instance forall s a.
+  ( Cons s s a a
+  , MonoFoldable s
+  ) => Cons (NonNull s) (NonNull s) a a where
+  _Cons = prism hither yon
+    where
+      hither :: (a, NonNull s) -> NonNull s
+      hither (a, ns) =
+        let s = toNullable ns
+        in impureNonNull $ a <| s
+
+      yon :: NonNull s -> Either (NonNull s) (a, NonNull s)
+      yon ns = case ns ^? _Cons of
+        Nothing -> Left ns
+        Just (a, ns') -> Right (a, ns')