implement simple harmless "true check" to mustache

mustache/src/Text/Mustache/Compile.hs

@@ -118,6 +118,7 @@ getPartials = join . fmap getPartials'
 getPartials' :: Node Text -> [FilePath]
 getPartials' (Partial _ p) = return p
 getPartials' (Section _ n) = getPartials n
+getPartials' (ExistingSection _ n) = getPartials n
 getPartials' (InvertedSection _ n) = getPartials n
 getPartials' _                     = mempty
 

mustache/src/Text/Mustache/Internal/Types.hs

@@ -28,7 +28,6 @@ import qualified Data.Text.Lazy           as LT
 import qualified Data.Vector              as V
 import           Data.Word                (Word8, Word16, Word32, Word64)
 import           Language.Haskell.TH.Lift (deriveLift)
-import           Language.Haskell.TH.Syntax
 import           Numeric.Natural          (Natural)
 
 
@@ -101,6 +100,7 @@ type ASTree α = [Node α]
 data Node α
   = TextBlock α
   | Section DataIdentifier (ASTree α)
+  | ExistingSection DataIdentifier (ASTree α)
   | InvertedSection DataIdentifier (ASTree α)
   | Variable Bool DataIdentifier
   | Partial (Maybe α) FilePath

mustache/src/Text/Mustache/Parser.hs

@@ -38,7 +38,6 @@ module Text.Mustache.Parser
 import           Control.Monad
 import           Data.Char           (isAlphaNum, isSpace)
 import           Data.List           (nub)
-import           Data.Monoid         ((<>))
 import           Data.Text           as T (Text, null, pack)
 import           Prelude             as Prel
 import           Text.Mustache.Types
@@ -60,8 +59,10 @@ data MustacheState = MustacheState
   }
 
 
+data SectionStart = Normal | Existing | Inverted
+
 data ParseTagRes
-  = SectionBegin Bool DataIdentifier
+  = SectionBegin SectionStart DataIdentifier
   | SectionEnd DataIdentifier
   | Tag (Node Text)
   | HandledTag
@@ -79,6 +80,9 @@ partialBegin = '>'
 -- | @^@
 invertedSectionBegin :: Char
 invertedSectionBegin = '^'
+-- | @^@
+existingSectionBegin :: Char
+existingSectionBegin = '?'
 -- | @{@ and @}@
 unescape2 :: (Char, Char)
 unescape2 = ('{', '}')
@@ -223,16 +227,16 @@ parseLine = do
 
 
 continueFromTag :: ParseTagRes -> Parser STree
-continueFromTag (SectionBegin inverted name) = do
+continueFromTag (SectionBegin start name) = do
   textNodes <- flushText
   state@(MustacheState
     { currentSectionName = previousSection }) <- getState
   putState $ state { currentSectionName = return name }
   innerSectionContent <- parseText
-  let sectionTag =
-        if inverted
-          then InvertedSection
-          else Section
+  let sectionTag = case start of
+                     Normal -> Section
+                     Inverted -> InvertedSection
+                     Existing -> ExistingSection
   modifyState $ \s -> s { currentSectionName = previousSection }
   outerSectionContent <- parseText
   return (textNodes <> [sectionTag name innerSectionContent] <> outerSectionContent)
@@ -255,7 +259,7 @@ switchOnTag = do
   (MustacheState { sDelimiters = ( _, end )}) <- getState
 
   choice
-    [ SectionBegin False <$> (try (char sectionBegin) >> genParseTagEnd mempty)
+    [ SectionBegin Normal <$> (try (char sectionBegin) >> genParseTagEnd mempty)
     , SectionEnd
         <$> (try (char sectionEnd) >> genParseTagEnd mempty)
     , Tag . Variable False
@@ -266,7 +270,12 @@ switchOnTag = do
         <$> (try (char partialBegin) >> spaces >> (noneOf (nub end) `manyTill` try (spaces >> string end)))
     , return HandledTag
         << (try (char delimiterChange) >> parseDelimChange)
-    , SectionBegin True
+    , SectionBegin Existing
+        <$> (try (char existingSectionBegin) >> genParseTagEnd mempty >>= \case
+              n@(NamedData _) -> return n
+              _ -> parserFail "Existing Sections can not be implicit."
+            )
+    , SectionBegin Inverted
         <$> (try (char invertedSectionBegin) >> genParseTagEnd mempty >>= \case
               n@(NamedData _) -> return n
               _ -> parserFail "Inverted Sections can not be implicit."

mustache/src/Text/Mustache/Render.hs

@@ -148,6 +148,18 @@ substituteNode (Section (NamedData secName) secSTree) =
       shiftContext newContext $ substituteAST secSTree
     Nothing -> tellError $ SectionTargetNotFound secName
 
+-- substituting an existing section (exact invert of the inverted section)
+substituteNode (ExistingSection  Implicit _) = tellError InvertedImplicitSection
+substituteNode (ExistingSection (NamedData secName) invSecSTree) =
+  search secName >>= \case
+    Just (Bool False) -> return ()
+    Just (Array a)    | V.null a -> return ()
+    Just Null         -> return ()
+    Nothing           -> return ()
+    _                 -> contents
+  where
+    contents = mapM_ substituteNode invSecSTree
+
 -- substituting an inverted section
 substituteNode (InvertedSection  Implicit _) = tellError InvertedImplicitSection
 substituteNode (InvertedSection (NamedData secName) invSecSTree) =