Implement saving+loading the game

Implement ToJSON and FromJSON for all of the various pieces of the game
state, and add a pair of functions saveGame/loadGame implementing a
prism to save the game as zlib-compressed JSON. To test this, there's
now Arbitrary, CoArbitrary, and Function instances for all the parts of
the game state - to get around circular imports with the concrete
entities this unfortunately is happening via orphan instances, plus an
hs-boot file to break a circular import that was just a little too hard
to remove by moving things around. Ugh.

1 files changed, 19 insertions, 0 deletions

diff --git a/src/Xanthous/Util/JSON.hs b/src/Xanthous/Util/JSON.hs
new file mode 100644
index 000000000000..91d1328e4a10
--- /dev/null
+++ b/src/Xanthous/Util/JSON.hs
@@ -0,0 +1,19 @@
+--------------------------------------------------------------------------------
+module Xanthous.Util.JSON
+  ( ReadShowJSON(..)
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+--------------------------------------------------------------------------------
+import Data.Aeson
+--------------------------------------------------------------------------------
+
+newtype ReadShowJSON a = ReadShowJSON a
+  deriving newtype (Read, Show)
+
+instance Show a => ToJSON (ReadShowJSON a) where
+  toJSON = toJSON . show
+
+instance Read a => FromJSON (ReadShowJSON a) where
+  parseJSON = withText "readable"
+    $ maybe (fail "Could not read") pure . readMay