Implement messages

Implement messages almost the same as in the Rust version, only with
YAML instead of TOML this time, and a regular old mustache template
instead of something handrolled. Besides that, pretty much everything
here is the same.

src/Xanthous/Messages.hs

@@ -0,0 +1,87 @@
+{-# LANGUAGE TemplateHaskell #-}
+module Xanthous.Messages
+  ( Message(..)
+  , resolve
+  , MessageMap(..)
+  , lookupMessage
+
+    -- * Game messages
+  , messages
+  , message
+  ) where
+
+import Xanthous.Prelude
+import Data.List.NonEmpty
+import Test.QuickCheck hiding (choose)
+import Test.QuickCheck.Arbitrary.Generic
+import Test.QuickCheck.Instances.UnorderedContainers ()
+import Text.Mustache
+import Data.Aeson (FromJSON, ToJSON)
+import Data.Aeson.Generic.DerivingVia
+import Data.FileEmbed
+import qualified Data.Yaml as Yaml
+import Data.Aeson (toJSON)
+import Control.Monad.Random.Class (MonadRandom)
+
+import Xanthous.Random
+import Xanthous.Orphans ()
+
+data Message = Single Template | Choice (NonEmpty Template)
+  deriving stock (Show, Eq, Ord, Generic)
+  deriving anyclass (CoArbitrary, Function, NFData)
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ SumEnc UntaggedVal ]
+           Message
+
+instance Arbitrary Message where
+  arbitrary = genericArbitrary
+  shrink = genericShrink
+
+resolve :: MonadRandom m => Message -> m Template
+resolve (Single t) = pure t
+resolve (Choice ts) = choose ts
+
+data MessageMap = Direct Message | Nested (HashMap Text MessageMap)
+  deriving stock (Show, Eq, Ord, Generic)
+  deriving anyclass (CoArbitrary, Function, NFData)
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ SumEnc UntaggedVal ]
+           MessageMap
+
+instance Arbitrary MessageMap where
+  arbitrary = frequency [ (10, Direct <$> arbitrary)
+                        , (1, Nested <$> arbitrary)
+                        ]
+
+lookupMessage :: [Text] -> MessageMap -> Maybe Message
+lookupMessage [] (Direct msg) = Just msg
+lookupMessage (k:ks) (Nested m) = lookupMessage ks =<< m ^. at k
+lookupMessage _ _ = Nothing
+
+type instance Index MessageMap = [Text]
+type instance IxValue MessageMap = Message
+instance Ixed MessageMap where
+  ix [] f (Direct msg) = Direct <$> f msg
+  ix (k:ks) f (Nested m) = case m ^. at k of
+    Just m' -> ix ks f m' <&> \m'' ->
+      Nested $ m & at k ?~ m''
+    Nothing -> pure $ Nested m
+  ix _ _ m = pure m
+
+--------------------------------------------------------------------------------
+
+rawMessages :: ByteString
+rawMessages = $(embedFile "src/Xanthous/messages.yaml")
+
+messages :: MessageMap
+messages
+  = either (error . Yaml.prettyPrintParseException) id
+  $ Yaml.decodeEither' rawMessages
+
+message :: (MonadRandom m, ToJSON params) => [Text] -> params -> m Text
+message path params = maybe notFound renderMessage $ messages ^? ix path
+  where
+    renderMessage msg = do
+      tpl <- resolve msg
+      pure . toStrict . renderMustache tpl $ toJSON params
+    notFound = pure "Message not found"