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diff --git a/users/grfn/xanthous/src/Xanthous/Messages/Template.hs b/users/grfn/xanthous/src/Xanthous/Messages/Template.hs
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+{-# LANGUAGE DeriveDataTypeable #-}
+--------------------------------------------------------------------------------
+module Xanthous.Messages.Template
+  ( -- * Template AST
+    Template(..)
+  , Substitution(..)
+  , Filter(..)
+
+    -- ** Template AST transformations
+  , reduceTemplate
+
+    -- * Template parser
+  , template
+  , runParser
+  , errorBundlePretty
+
+    -- * Template pretty-printer
+  , ppTemplate
+
+    -- * Rendering templates
+  , TemplateVar(..)
+  , nested
+  , TemplateVars(..)
+  , vars
+  , RenderError
+  , render
+  )
+where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude hiding
+                 (many, concat, try, elements, some, parts)
+--------------------------------------------------------------------------------
+import           Test.QuickCheck hiding (label)
+import           Test.QuickCheck.Instances.Text ()
+import           Test.QuickCheck.Instances.Semigroup ()
+import           Test.QuickCheck.Checkers (EqProp)
+import           Control.Monad.Combinators.NonEmpty
+import           Data.List.NonEmpty (NonEmpty(..))
+import           Data.Data
+import           Text.Megaparsec hiding (sepBy1, some)
+import           Text.Megaparsec.Char
+import qualified Text.Megaparsec.Char.Lexer as L
+import           Data.Function (fix)
+--------------------------------------------------------------------------------
+import Xanthous.Util (EqEqProp(..))
+--------------------------------------------------------------------------------
+
+genIdentifier :: Gen Text
+genIdentifier = pack <$> listOf1 (elements identifierChars)
+
+identifierChars :: String
+identifierChars = ['a'..'z'] <> ['A'..'Z'] <> ['-', '_']
+
+newtype Filter = FilterName Text
+  deriving stock (Show, Eq, Ord, Generic, Data)
+  deriving anyclass (NFData)
+  deriving (IsString) via Text
+
+instance Arbitrary Filter where
+  arbitrary = FilterName <$> genIdentifier
+  shrink (FilterName fn) = fmap FilterName . filter (not . null) $ shrink fn
+
+data Substitution
+  = SubstPath (NonEmpty Text)
+  | SubstFilter Substitution Filter
+  deriving stock (Show, Eq, Ord, Generic, Data)
+  deriving anyclass (NFData)
+
+instance Arbitrary Substitution where
+  arbitrary = sized . fix $ \gen n ->
+    let leaves =
+          [ SubstPath <$> ((:|) <$> genIdentifier <*> listOf genIdentifier)]
+        subtree = gen $ n `div` 2
+    in if n == 0
+       then oneof leaves
+       else oneof $ leaves <> [ SubstFilter <$> subtree <*> arbitrary ]
+  shrink (SubstPath pth) =
+    fmap SubstPath
+    . filter (not . any ((||) <$> null <*> any (`notElem` identifierChars)))
+    $ shrink pth
+  shrink (SubstFilter s f)
+    = shrink s
+    <> (uncurry SubstFilter <$> shrink (s, f))
+
+data Template
+  = Literal Text
+  | Subst Substitution
+  | Concat Template Template
+  deriving stock (Show, Generic, Data)
+  deriving anyclass (NFData)
+  deriving EqProp via EqEqProp Template
+
+instance Plated Template where
+  plate _ tpl@(Literal _) = pure tpl
+  plate _ tpl@(Subst _) = pure tpl
+  plate f (Concat tpl₁ tpl₂) = Concat <$> f tpl₁ <*> f tpl₂
+
+reduceTemplate :: Template -> Template
+reduceTemplate = transform $ \case
+  (Concat (Literal t₁) (Literal t₂)) -> Literal (t₁ <> t₂)
+  (Concat (Literal "") t) -> t
+  (Concat t (Literal "")) -> t
+  (Concat t₁ (Concat t₂ t₃)) -> Concat (Concat t₁ t₂) t₃
+  (Concat (Concat t₁ (Literal t₂)) (Literal t₃)) -> (Concat t₁ (Literal $ t₂ <> t₃))
+  t -> t
+
+instance Eq Template where
+  tpl₁ == tpl₂ = case (reduceTemplate tpl₁, reduceTemplate tpl₂) of
+    (Literal t₁, Literal t₂) -> t₁ == t₂
+    (Subst s₁, Subst s₂) -> s₁ == s₂
+    (Concat ta₁ ta₂, Concat tb₁ tb₂) -> ta₁ == tb₁ && ta₂ == tb₂
+    _ -> False
+
+instance Arbitrary Template where
+  arbitrary = sized . fix $ \gen n ->
+    let leaves = [ Literal . pack . filter (`notElem` ['\\', '{']) <$> arbitrary
+                 , Subst <$> arbitrary
+                 ]
+        subtree = gen $ n `div` 2
+        genConcat = Concat <$> subtree <*> subtree
+    in if n == 0
+       then oneof leaves
+       else oneof $ genConcat : leaves
+  shrink (Literal t) = Literal <$> shrink t
+  shrink (Subst s) = Subst <$> shrink s
+  shrink (Concat t₁ t₂)
+    = shrink t₁
+    <> shrink t₂
+    <> (Concat <$> shrink t₁ <*> shrink t₂)
+
+instance Semigroup Template where
+  (<>) = Concat
+
+instance Monoid Template where
+  mempty = Literal ""
+
+--------------------------------------------------------------------------------
+
+type Parser = Parsec Void Text
+
+sc :: Parser ()
+sc = L.space space1 empty empty
+
+lexeme :: Parser a -> Parser a
+lexeme = L.lexeme sc
+
+symbol :: Text -> Parser Text
+symbol = L.symbol sc
+
+identifier :: Parser Text
+identifier = lexeme . label "identifier" $ do
+  firstChar <- letterChar <|> oneOf ['-', '_']
+  restChars <- many $ alphaNumChar <|> oneOf ['-', '_']
+  pure $ firstChar <| pack restChars
+
+filterName :: Parser Filter
+filterName = FilterName <$> identifier
+
+substitutionPath :: Parser Substitution
+substitutionPath = SubstPath <$> sepBy1 identifier (char '.')
+
+substitutionFilter :: Parser Substitution
+substitutionFilter = do
+  path <- substitutionPath
+  fs <- some $ symbol "|" *> filterName
+  pure $ foldl' SubstFilter path fs
+  -- pure $ SubstFilter path f
+
+substitutionContents :: Parser Substitution
+substitutionContents
+  =   try substitutionFilter
+  <|> substitutionPath
+
+substitution :: Parser Substitution
+substitution = between (string "{{") (string "}}") substitutionContents
+
+literal :: Parser Template
+literal = Literal <$>
+  (   (string "\\{" $> "{")
+  <|> takeWhile1P Nothing (`notElem` ['\\', '{'])
+  )
+
+subst :: Parser Template
+subst = Subst <$> substitution
+
+template' :: Parser Template
+template' = do
+  parts <- many $ literal <|> subst
+  pure $ foldr Concat (Literal "") parts
+
+
+template :: Parser Template
+template = reduceTemplate <$> template' <* eof
+
+--------------------------------------------------------------------------------
+
+ppSubstitution :: Substitution -> Text
+ppSubstitution (SubstPath substParts) = intercalate "." substParts
+ppSubstitution (SubstFilter s (FilterName f)) = ppSubstitution s <> " | " <> f
+
+ppTemplate :: Template -> Text
+ppTemplate (Literal txt) = txt
+ppTemplate (Subst s) = "{{" <> ppSubstitution s <> "}}"
+ppTemplate (Concat tpl₁ tpl₂) = ppTemplate tpl₁ <> ppTemplate tpl₂
+
+--------------------------------------------------------------------------------
+
+data TemplateVar
+  = Val Text
+  | Nested (Map Text TemplateVar)
+  deriving stock (Show, Eq, Generic)
+  deriving anyclass (NFData)
+
+nested :: [(Text, TemplateVar)] -> TemplateVar
+nested = Nested . mapFromList
+
+instance Arbitrary TemplateVar where
+  arbitrary = sized . fix $ \gen n ->
+    let nst = fmap mapFromList . listOf $ (,) <$> arbitrary <*> gen (n `div` 2)
+    in if n == 0
+       then Val <$> arbitrary
+       else oneof [ Val <$> arbitrary
+                  , Nested <$> nst]
+
+newtype TemplateVars = Vars { getTemplateVars :: Map Text TemplateVar }
+  deriving stock (Show, Eq, Generic)
+  deriving anyclass (NFData)
+  deriving (Arbitrary) via (Map Text TemplateVar)
+
+type instance Index TemplateVars = Text
+type instance IxValue TemplateVars = TemplateVar
+instance Ixed TemplateVars where
+  ix k f (Vars vs) = Vars <$> ix k f vs
+instance At TemplateVars where
+  at k f (Vars vs) = Vars <$> at k f vs
+
+vars :: [(Text, TemplateVar)] -> TemplateVars
+vars = Vars . mapFromList
+
+lookupVar :: TemplateVars -> NonEmpty Text -> Maybe TemplateVar
+lookupVar vs (p :| []) = vs ^. at p
+lookupVar vs (p :| (p₁ : ps)) = vs ^. at p >>= \case
+  (Val _) -> Nothing
+  (Nested vs') -> lookupVar (Vars vs') $ p₁ :| ps
+
+data RenderError
+  = NoSuchVariable (NonEmpty Text)
+  | NestedFurther (NonEmpty Text)
+  | NoSuchFilter Filter
+  deriving stock (Show, Eq, Generic)
+  deriving anyclass (NFData)
+
+renderSubst
+  :: Map Filter (Text -> Text) -- ^ Filters
+  -> TemplateVars
+  -> Substitution
+  -> Either RenderError Text
+renderSubst _ vs (SubstPath pth) =
+  case lookupVar vs pth of
+    Just (Val v) -> Right v
+    Just (Nested _) -> Left $ NestedFurther pth
+    Nothing -> Left $ NoSuchVariable pth
+renderSubst fs vs (SubstFilter s fn) =
+  case fs ^. at fn of
+    Just filterFn -> filterFn <$> renderSubst fs vs s
+    Nothing -> Left $ NoSuchFilter fn
+
+render
+  :: Map Filter (Text -> Text) -- ^ Filters
+  -> TemplateVars             -- ^ Template variables
+  -> Template                 -- ^ Template
+  -> Either RenderError Text
+render _ _ (Literal s) = pure s
+render fs vs (Concat t₁ t₂) = (<>) <$> render fs vs t₁ <*> render fs vs t₂
+render fs vs (Subst s) = renderSubst fs vs s