From 6b0bab0e85266ce66836c4584f8cc83b307a3af5 Mon Sep 17 00:00:00 2001
From: Griffin Smith <root@gws.fyi>
Date: Sat, 4 Jan 2020 23:48:51 -0500
Subject: Add support for multiple levels

Add a data structure, based on the zipper comonad, which provides
support for multiple levels, each of which is its own entity map. The
current level is provided by coreturn, which the `entities` lens has
been updated to use. Nothing currently supports going up or down levels
yet - that's coming next.
---
 src/Xanthous/Util/Comonad.hs | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 src/Xanthous/Util/Comonad.hs

(limited to 'src/Xanthous/Util')

diff --git a/src/Xanthous/Util/Comonad.hs b/src/Xanthous/Util/Comonad.hs
new file mode 100644
index 000000000000..9e158cc8e2d4
--- /dev/null
+++ b/src/Xanthous/Util/Comonad.hs
@@ -0,0 +1,24 @@
+--------------------------------------------------------------------------------
+module Xanthous.Util.Comonad
+  ( -- * Store comonad utils
+    replace
+  , current
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+--------------------------------------------------------------------------------
+import Control.Comonad.Store.Class
+--------------------------------------------------------------------------------
+
+-- | Replace the current position of a store comonad with a new value by
+-- comparing positions
+replace :: (Eq i, ComonadStore i w) => w a -> a -> w a
+replace w x = w =>> \w' -> if pos w' == pos w then x else extract w'
+{-# INLINE replace #-}
+
+-- | Lens into the current position of a store comonad.
+--
+--     current = lens extract replace
+current :: (Eq i, ComonadStore i w) => Lens' (w a) a
+current = lens extract replace
+{-# INLINE current #-}
-- 
cgit 1.4.1