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authorGriffin Smith <root@gws.fyi>2020-01-05T04·48-0500
committerGriffin Smith <root@gws.fyi>2020-01-05T04·48-0500
commit6b0bab0e85266ce66836c4584f8cc83b307a3af5 (patch)
treecfb4dbe4c370c3f20277336e6be75171c572137a /src/Xanthous/Util/Comonad.hs
parente669b54f0c9be84dd1e4704ccae4b8169f7458a5 (diff)
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.
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diff --git a/src/Xanthous/Util/Comonad.hs b/src/Xanthous/Util/Comonad.hs
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+--------------------------------------------------------------------------------
+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 #-}