{-# LANGUAGE BangPatterns               #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE NoMonomorphismRestriction  #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TypeSynonymInstances       #-}

module Heist.Compiled.Internal where


------------------------------------------------------------------------------
import           Blaze.ByteString.Builder
import           Blaze.ByteString.Builder.Char.Utf8
import           Control.Arrow
import           Control.Monad
import           Control.Monad.RWS.Strict
import           Control.Monad.State.Strict
import qualified Data.Attoparsec.Text            as AP
import           Data.ByteString (ByteString)
import           Data.DList                      (DList)
import qualified Data.DList                      as DL
import qualified Data.HashMap.Strict             as H
import qualified Data.HashSet                    as S
import qualified Data.HeterogeneousEnvironment   as HE
import           Data.Maybe
import           Data.Text                       (Text)
import qualified Data.Text                       as T
import qualified Data.Text.Encoding              as T
import qualified Data.Vector                     as V
import qualified Text.XmlHtml                    as X
import qualified Text.XmlHtml.HTML.Meta          as X
------------------------------------------------------------------------------
import           Heist.Common
import           Heist.SpliceAPI
import           Heist.Types
------------------------------------------------------------------------------


------------------------------------------------------------------------------
-- | A compiled Splice is a HeistT computation that returns a @DList
-- (Chunk m)@.
--
-- The more interesting part of the type signature is what comes before the
-- return value.  The first type parameter in @'HeistT' n IO@ is the runtime
-- monad.  This reveals that the Chunks know about the runtime monad.  The
-- second type parameter in @HeistT n IO@ is @IO@.  This tells is that the
-- compiled splices themselves are run in the IO monad, which will usually
-- mean at load time.  Compiled splices run at load time, and they return
-- computations that run at runtime.
type Splice n = HeistT n IO (DList (Chunk n))


------------------------------------------------------------------------------
-- | Runs the parameter node's children and returns the resulting compiled
-- chunks.  By itself this function is a simple passthrough splice that makes
-- the spliced node disappear.  In combination with locally bound splices,
-- this function makes it easier to pass the desired view into your splices.
runChildren :: Monad n => Splice n
runChildren = runNodeList . X.childNodes =<< getParamNode
{-# INLINE runChildren #-}


renderFragment :: Markup -> [X.Node] -> Builder
renderFragment markup ns =
    case markup of
      Html -> X.renderHtmlFragment X.UTF8 ns
      Xml  -> X.renderXmlFragment X.UTF8 ns


------------------------------------------------------------------------------
-- | Yields pure text known at load time.
pureTextChunk :: Text -> Chunk n
pureTextChunk t = Pure $ T.encodeUtf8 t
{-# INLINE pureTextChunk #-}


------------------------------------------------------------------------------
-- | Yields a pure Builder known at load time.  You should use this and
-- 'yieldPureText' as much as possible to maximize the parts of your page that
-- can be compiled to static ByteStrings.
yieldPure :: Builder -> DList (Chunk n)
yieldPure = DL.singleton . Pure . toByteString
{-# INLINE yieldPure #-}


------------------------------------------------------------------------------
-- | Yields a runtime action that returns a builder.
yieldRuntime :: RuntimeSplice n Builder -> DList (Chunk n)
yieldRuntime = DL.singleton . RuntimeHtml
{-# INLINE yieldRuntime #-}


------------------------------------------------------------------------------
-- | Yields a runtime action that returns no value and is only needed for its
-- side effect.
yieldRuntimeEffect :: Monad n => RuntimeSplice n () -> DList (Chunk n)
yieldRuntimeEffect = DL.singleton . RuntimeAction
{-# INLINE yieldRuntimeEffect #-}


------------------------------------------------------------------------------
-- | A convenience wrapper around yieldPure for working with Text.  Roughly
-- equivalent to 'textSplice' from Heist.Interpreted.
yieldPureText :: Text -> DList (Chunk n)
yieldPureText = DL.singleton . pureTextChunk
{-# INLINE yieldPureText #-}


------------------------------------------------------------------------------
-- | Convenience wrapper around yieldRuntime allowing you to work with Text.
yieldRuntimeText :: Monad n => RuntimeSplice n Text -> DList (Chunk n)
yieldRuntimeText = yieldRuntime .  liftM fromText
{-# INLINE yieldRuntimeText #-}


------------------------------------------------------------------------------
-- | Returns a computation that performs load-time splice processing on the
-- supplied list of nodes.
runNodeList :: Monad n => [X.Node] -> Splice n
runNodeList = mapSplices runNode


------------------------------------------------------------------------------
-- | Runs a single splice and returns the builder computation.
runSplice :: (Monad n)
          => X.Node
          -> HeistState n
          -> Splice n
          -> IO [Chunk n]
runSplice node hs splice = do
    !a <- evalHeistT splice node hs
    return $! consolidate a


------------------------------------------------------------------------------
-- | Runs a DocumentFile with the appropriate template context set.
runDocumentFile :: Monad n
                => TPath
                -> DocumentFile
                -> Splice n
runDocumentFile tpath df = do
    addDoctype $ maybeToList $ X.docType $ dfDoc df
    modifyHS (setCurTemplateFile curPath .  setCurContext tpath)
    res <- runNodeList nodes
    dt <- getsHS (listToMaybe . _doctypes)
    let enc = X.docEncoding $ dfDoc df
    return $! (yieldPure (X.renderDocType enc dt) `mappend` res)
  where
    curPath     = dfFile df
    nodes       = X.docContent $! dfDoc df


------------------------------------------------------------------------------
compileTemplate :: Monad n
                => HeistState n
                -> TPath
                -> DocumentFile
                -> IO [Chunk n]
compileTemplate hs tpath df = do
    let markup = case dfDoc df of
                   X.XmlDocument _ _ _ -> Xml
                   X.HtmlDocument _ _ _ -> Html
        hs' = hs { _curMarkup = markup }
    !chunks <- runSplice nullNode hs' $! runDocumentFile tpath df
    return chunks
  where
    -- This gets overwritten in runDocumentFile
    nullNode = X.TextNode ""


------------------------------------------------------------------------------
compileTemplates :: Monad n => HeistState n -> IO (HeistState n)
compileTemplates hs = do
    ctm <- compileTemplates' hs
    return $! hs { _compiledTemplateMap = ctm }
--    let f = flip evalStateT HE.empty . unRT . codeGen
--    return $! hs { _compiledTemplateMap = H.map (first f) ctm }


------------------------------------------------------------------------------
compileTemplates' :: Monad n
                  => HeistState n
                  -> IO (H.HashMap TPath ([Chunk n], MIMEType))
compileTemplates' hs = do
    ctm <- foldM runOne H.empty tpathDocfiles
    return $! ctm
  where
    tpathDocfiles :: [(TPath, DocumentFile)]
    tpathDocfiles = map (\(a,b) -> (a, b))
                        (H.toList $ _templateMap hs)

    runOne tmap (tpath, df) = do
        !mHtml <- compileTemplate hs tpath df
        return $! H.insert tpath (mHtml, mimeType $! dfDoc df) tmap


------------------------------------------------------------------------------
-- | Consolidate consecutive Pure Chunks.
consolidate :: (Monad n) => DList (Chunk n) -> [Chunk n]
consolidate = consolidateL . DL.toList
  where
    consolidateL []     = []
    consolidateL (y:ys) = boilDown [] $! go [] y ys
      where
        ----------------------------------------------------------------------
        go soFar x [] = x : soFar
    
        go soFar (Pure a) ((Pure b) : xs) =
            go soFar (Pure $! a `mappend` b) xs
    
        go soFar (RuntimeHtml a) ((RuntimeHtml b) : xs) =
            go soFar (RuntimeHtml $! a `mappend` b) xs
    
        go soFar (RuntimeHtml a) ((RuntimeAction b) : xs) =
            go soFar (RuntimeHtml $! a >>= \x -> b >> return x) xs
    
        go soFar (RuntimeAction a) ((RuntimeHtml b) : xs) =
            go soFar (RuntimeHtml $! a >> b) xs
    
        go soFar (RuntimeAction a) ((RuntimeAction b) : xs) =
            go soFar (RuntimeAction $! a >> b) xs
    
        go soFar a (b : xs) = go (a : soFar) b xs
    
        ----------------------------------------------------------------------
        boilDown soFar []              = soFar
    
        boilDown soFar ((Pure h) : xs) = boilDown ((Pure $! h) : soFar) xs
    
        boilDown soFar (x : xs) = boilDown (x : soFar) xs


------------------------------------------------------------------------------
-- | Given a list of output chunks, consolidate turns consecutive runs of
-- @Pure Html@ values into maximally-efficient pre-rendered strict
-- 'ByteString' chunks.
codeGen :: Monad n => DList (Chunk n) -> RuntimeSplice n Builder
codeGen l = V.foldr mappend mempty $!
            V.map toAct $! V.fromList $! consolidate l
  where
    toAct !(RuntimeHtml !m)   = m
    toAct !(Pure !h)          = return $! fromByteString h
    toAct !(RuntimeAction !m) = m >> return mempty
{-# INLINE codeGen #-}


------------------------------------------------------------------------------
-- | Looks up a splice in the compiled splice map.
lookupSplice :: Text -> HeistT n IO (Maybe (Splice n))
lookupSplice nm = getsHS (H.lookup nm . _compiledSpliceMap)


------------------------------------------------------------------------------
-- | Runs a single node.  If there is no splice referenced anywhere in the
-- subtree, then it is rendered as a pure chunk, otherwise it calls
-- compileNode to generate the appropriate runtime computation.
runNode :: Monad n => X.Node -> Splice n
runNode node = localParamNode (const node) $ do
    isStatic <- subtreeIsStatic node
    markup <- getsHS _curMarkup
    if isStatic
      then return $! yieldPure $! renderFragment markup [parseAttrs node]
      else compileNode node


parseAttrs :: X.Node -> X.Node
parseAttrs (X.Element nm attrs ch) = newAttrs `seq` X.Element nm newAttrs ch
  where
    newAttrs = map parseAttr attrs
parseAttrs !n = n

parseAttr :: (Text, Text) -> (Text, Text)
parseAttr (k,v) = (k, T.concat $! map cvt ast)
  where
    !ast = case AP.feed (AP.parse attParser v) "" of
            (AP.Done _ res) -> res
            (AP.Fail _ _ _) -> []
            (AP.Partial _ ) -> []
    cvt (Literal x) = x
    cvt (Ident i) = T.concat ["${", i, "}"]

------------------------------------------------------------------------------
-- | Checks whether a node's subtree is static and can be rendered up front at
-- load time.
subtreeIsStatic :: X.Node -> HeistT n IO Bool
subtreeIsStatic (X.Element nm attrs ch) = do
    isNodeDynamic <- liftM isJust $ lookupSplice nm
    attrSplices <- getsHS _attrSpliceMap
    let hasSubstitutions (k,v) = hasAttributeSubstitutions v ||
                                 H.member k attrSplices
    if isNodeDynamic
      then return False
      else do
          let hasDynamicAttrs = any hasSubstitutions attrs
          if hasDynamicAttrs
            then return False
            else do
                staticSubtrees <- mapM subtreeIsStatic ch
                return $ and staticSubtrees

subtreeIsStatic _ = return True


------------------------------------------------------------------------------
-- | Checks whether a string has any attribute substitutions.
hasAttributeSubstitutions :: Text -> Bool
hasAttributeSubstitutions txt = any isIdent ast
  where
    ast = case AP.feed (AP.parse attParser txt) "" of
            (AP.Done _ res) -> res
            (AP.Fail _ _ _) -> []
            (AP.Partial _ ) -> []


------------------------------------------------------------------------------
-- | Given a 'X.Node' in the DOM tree, produces a \"runtime splice\" that will
-- generate html at runtime.
compileNode :: Monad n => X.Node -> Splice n
compileNode (X.Element nm attrs ch) =
    -- Is this node a splice, or does it merely contain splices?
    lookupSplice nm >>= fromMaybe compileStaticElement
  where
    tag0 = T.append "<" nm
    end = T.concat [ "</" , nm , ">"]
    -- If the tag is not a splice, but it contains dynamic children
    compileStaticElement = do
        -- Parse the attributes: we have Left for static and Right for runtime
        compiledAttrs <- runAttributes attrs

        childHtml <- runNodeList ch

        return $! if null (DL.toList childHtml) && nm `S.member` X.voidTags
          then DL.concat [ DL.singleton $! pureTextChunk $! tag0
                         , DL.concat compiledAttrs
                         , DL.singleton $! pureTextChunk " />"
                         ]
          else DL.concat [ DL.singleton $! pureTextChunk $! tag0
                         , DL.concat compiledAttrs
                         , DL.singleton $! pureTextChunk ">"
                         , childHtml
                         , DL.singleton $! pureTextChunk $! end
                         ]
compileNode _ = error "impossible"


------------------------------------------------------------------------------
-- |
parseAtt :: Monad n => (Text, Text) -> HeistT n IO (DList (Chunk n))
parseAtt (k,v) = do
    mas <- getsHS (H.lookup k . _attrSpliceMap)
    maybe doInline (return . doAttrSplice) mas

  where
    cvt (Literal x) = return $ yieldPureText x
    cvt (Ident x) =
        localParamNode (const $ X.Element x [] []) $ getAttributeSplice x

    -- Handles inline parsing of $() splice syntax in attributes
    doInline = do
        let ast = case AP.feed (AP.parse attParser v) "" of
                    (AP.Done _ res) -> res
                    (AP.Fail _ _ _) -> []
                    (AP.Partial _ ) -> []
        chunks <- mapM cvt ast
        let value = DL.concat chunks
        return $ attrToChunk k value

    -- Handles attribute splices
    doAttrSplice splice = DL.singleton $ RuntimeHtml $ do
        res <- splice v
        return $ mconcat $ map attrToBuilder res


------------------------------------------------------------------------------
-- |
parseAtt2 :: Monad n
          => (Text, Text)
          -> HeistT n IO (RuntimeSplice n [(Text, Text)])
parseAtt2 (k,v) = do
    mas <- getsHS (H.lookup k . _attrSpliceMap)
    maybe doInline (return . doAttrSplice) mas

  where
    cvt (Literal x) = return $ return x
    cvt (Ident x) =
        localParamNode (const $ X.Element x [] []) $ getAttributeSplice2 x

    -- Handles inline parsing of $() splice syntax in attributes
    doInline = do
        let ast = case AP.feed (AP.parse attParser v) "" of
                    (AP.Done _ res) -> res
                    (AP.Fail _ _ _) -> []
                    (AP.Partial _ ) -> []
        chunks <- mapM cvt ast
        return $ do
            list <- sequence chunks
            return [(k, T.concat list)]

    -- Handles attribute splices
    doAttrSplice splice = splice v


------------------------------------------------------------------------------
-- | Performs splice processing on a list of attributes.  This is useful in
-- situations where you need to stop recursion, but still run splice
-- processing on the node's attributes.
runAttributes :: Monad n
              => [(Text, Text)] -- ^ List of attributes
              -> HeistT n IO [DList (Chunk n)]
runAttributes = mapM parseAtt


------------------------------------------------------------------------------
-- | Performs splice processing on a list of attributes.  This is useful in
-- situations where you need to stop recursion, but still run splice
-- processing on the node's attributes.
runAttributesRaw :: Monad n
                 -- Note that this parameter should not be changed to Splices
                 => [(Text, Text)] -- ^ List of attributes
                 -> HeistT n IO (RuntimeSplice n [(Text, Text)])
runAttributesRaw attrs = do
    arrs <- mapM parseAtt2 attrs
    return $ liftM concat $ sequence arrs


attrToChunk :: Text -> DList (Chunk n) -> DList (Chunk n)
attrToChunk !k !v = do
    DL.concat
        [ DL.singleton $! pureTextChunk $! T.concat [" ", k, "=\""]
        , v, DL.singleton $! pureTextChunk "\"" ]
    

attrToBuilder :: (Text, Text) -> Builder
attrToBuilder (k,v)
  | T.null v  = mconcat
    [ fromText " "
    , fromText k
    ]
  | otherwise = mconcat
    [ fromText " "
    , fromText k
    , fromText "=\""
    , fromText v
    , fromText "\""
    ]


------------------------------------------------------------------------------
getAttributeSplice :: Text -> HeistT n IO (DList (Chunk n))
getAttributeSplice name =
    lookupSplice name >>= fromMaybe
      (return $ DL.singleton $ Pure $ T.encodeUtf8 $
       T.concat ["${", name, "}"])
{-# INLINE getAttributeSplice #-}


getAttributeSplice2 :: Monad n => Text -> HeistT n IO (RuntimeSplice n Text)
getAttributeSplice2 name = do
    mSplice <- lookupSplice name
    case mSplice of
      Nothing -> return $ return $ T.concat ["${", name, "}"]
      Just splice -> do
        res <- splice
        return $ liftM (T.decodeUtf8 . toByteString) $ codeGen res
{-# INLINE getAttributeSplice2 #-}


------------------------------------------------------------------------------
-- | Promises are used for referencing the results of future runtime
-- computations during load time splice processing.
newtype Promise a = Promise (HE.Key a)


------------------------------------------------------------------------------
-- | Gets the result of a promised runtime computation.
getPromise :: (Monad n) => Promise a -> RuntimeSplice n a
getPromise (Promise k) = do
    mb <- gets (HE.lookup k)
    return $ fromMaybe e mb

  where
    e = error $ "getPromise: dereferenced empty key (id "
                ++ show (HE.getKeyId k) ++ ")"
{-# INLINE getPromise #-}


------------------------------------------------------------------------------
-- | Adds a promise to the runtime splice context.
putPromise :: (Monad n) => Promise a -> a -> RuntimeSplice n ()
putPromise (Promise k) x = modify (HE.insert k x)
{-# INLINE putPromise #-}


------------------------------------------------------------------------------
-- | Modifies a promise.
adjustPromise :: Monad n => Promise a -> (a -> a) -> RuntimeSplice n ()
adjustPromise (Promise k) f = modify (HE.adjust f k)
{-# INLINE adjustPromise #-}


------------------------------------------------------------------------------
-- | Creates an empty promise.
newEmptyPromise :: HeistT n IO (Promise a)
newEmptyPromise = do
    keygen <- getsHS _keygen
    key    <- liftIO $ HE.makeKey keygen
    return $! Promise key
{-# INLINE newEmptyPromise #-}


-- ------------------------------------------------------------------------------
-- -- | Creates an empty promise with some error checking to help with debugging.
-- newEmptyPromiseWithError :: (Monad n)
--                          => String -> HeistT n IO (Promise a)
-- newEmptyPromiseWithError from = do
--     keygen <- getsHS _keygen
--     prom   <- liftM Promise $ liftIO $ HE.makeKey keygen
--     yieldRuntimeEffect $ putPromise prom
--                        $ error
--                        $ "deferenced empty promise created at" ++ from
--     return prom
-- {-# INLINE newEmptyPromiseWithError #-}


------------------------------------------------------------------------------
-- | Binds a compiled splice.  This function should not be exported.
bindSplice :: Text             -- ^ tag name
           -> Splice n         -- ^ splice action
           -> HeistState n     -- ^ source state
           -> HeistState n
bindSplice n v ts =
    ts { _compiledSpliceMap = H.insert n v (_compiledSpliceMap ts) }


------------------------------------------------------------------------------
-- | Binds a list of compiled splices.  This function should not be exported.
bindSplices :: Splices (Splice n)  -- ^ splices to bind
            -> HeistState n        -- ^ source state
            -> HeistState n
bindSplices ss ts = foldr (uncurry bindSplice) ts $ splicesToList ss


------------------------------------------------------------------------------
-- | Adds a list of compiled splices to the splice map.  This function is
-- useful because it allows compiled splices to bind other compiled splices
-- during load-time splice processing.
withLocalSplices :: Splices (Splice n)
                 -> Splices (AttrSplice n)
                 -> HeistT n IO a
                 -> HeistT n IO a
withLocalSplices ss as = localHS (bindSplices ss . bindAttributeSplices as)


------------------------------------------------------------------------------
-- | Looks up a compiled template and returns a runtime monad computation that
-- constructs a builder.
renderTemplate :: Monad n
               => HeistState n
               -> ByteString
               -> Maybe (n Builder, MIMEType)
renderTemplate hs nm =
    fmap (first (interpret . DL.fromList) . fst) $!
      lookupTemplate nm hs _compiledTemplateMap


------------------------------------------------------------------------------
-- | Looks up a compiled template and returns a compiled splice.
callTemplate :: Monad n
             => ByteString
             -> HeistT n IO (DList (Chunk n))
callTemplate nm = do
    hs <- getHS
    runNodeList $ maybe (error err) (X.docContent . dfDoc . fst) $
      lookupTemplate nm hs _templateMap
  where
    err = "callTemplate: "++(T.unpack $ T.decodeUtf8 nm)++(" does not exist")


interpret :: Monad n => DList (Chunk n) -> n Builder
interpret = flip evalStateT HE.empty . unRT . codeGen


------------------------------------------------------------------------------
-- | Converts a pure text splice function to a pure Builder splice function.
textSplice :: (a -> Text) -> a -> Builder
textSplice f = fromText . f


                               ---------------
                               -- New Stuff --
                               ---------------


------------------------------------------------------------------------------
-- | Converts a pure Node splice function to a pure Builder splice function.
nodeSplice :: (a -> [X.Node]) -> a -> Builder
nodeSplice f = X.renderHtmlFragment X.UTF8 . f


------------------------------------------------------------------------------
-- | Converts a pure Builder splice function into a monadic splice function
-- of a RuntimeSplice.
pureSplice :: Monad n => (a -> Builder) -> RuntimeSplice n a -> Splice n
pureSplice f n = return $ yieldRuntime (return . f =<< n)


------------------------------------------------------------------------------
-- | Runs a splice, but first binds splices given by splice functions that
-- need some runtime data.
withSplices :: Monad n
            => Splice n
            -- ^ Splice to be run
            -> Splices (RuntimeSplice n a -> Splice n)
            -- ^ Splices to be bound first
            -> RuntimeSplice n a
            -- ^ Runtime data needed by the above splices
            -> Splice n
withSplices splice splices runtimeAction =
    withLocalSplices splices' noSplices splice
  where
    splices' = mapS ($runtimeAction) splices


------------------------------------------------------------------------------
-- | Like withSplices, but evaluates the splice repeatedly for each element in
-- a list generated at runtime.
manyWithSplices :: Monad n
                => Splice n
                -> Splices (RuntimeSplice n a -> Splice n)
                -> RuntimeSplice n [a]
                -> Splice n
manyWithSplices splice splices runtimeAction = do
    p <- newEmptyPromise
    let splices' = mapS ($ getPromise p) splices
    chunks <- withLocalSplices splices' noSplices splice
    return $ yieldRuntime $ do
        items <- runtimeAction
        res <- forM items $ \item -> putPromise p item >> codeGen chunks
        return $ mconcat res


------------------------------------------------------------------------------
-- | Similar to 'mapSplices' in interpreted mode.  Gets a runtime list of
-- items and applies a compiled runtime splice function to each element of the
-- list.
deferMany :: Monad n
          => (RuntimeSplice n a -> Splice n)
          -> RuntimeSplice n [a]
          -> Splice n
deferMany f getItems = do
    promise <- newEmptyPromise
    chunks <- f $ getPromise promise
    return $ yieldRuntime $ do
        items <- getItems
        res <- forM items $ \item -> do
            putPromise promise item
            codeGen chunks
        return $ mconcat res


------------------------------------------------------------------------------
-- | Saves the results of a runtme computation in a 'Promise' so they don't
-- get recalculated if used more than once.
deferMap :: Monad n
         => (a -> RuntimeSplice n b)
         -> (RuntimeSplice n b -> Splice n)
         -> RuntimeSplice n a -> Splice n
deferMap f pf n = do
    p2 <- newEmptyPromise
    let action = yieldRuntimeEffect $ putPromise p2 =<< f =<< n
    res <- pf $ getPromise p2
    return $ action `mappend` res


------------------------------------------------------------------------------
-- | Like deferMap, but only runs the result if a Maybe function of the
-- runtime value returns Just.  If it returns Nothing, then no output is
-- generated.
-- 
-- This is a good example of how to do more complex flow control with
-- promises.  The generalization of this abstraction is too complex to be
-- distilled to elegant high-level combinators.  If you need to implement your
-- own special flow control, then you should use functions from the
-- `Heist.Compiled.LowLevel` module similarly to how it is done in the
-- implementation of this function.
mayDeferMap :: Monad n
            => (a -> RuntimeSplice n (Maybe b))
            -> (RuntimeSplice n b -> Splice n)
            -> RuntimeSplice n a -> Splice n
mayDeferMap f pf n = do
    p2 <- newEmptyPromise
    action <- pf $ getPromise p2
    return $ yieldRuntime $ do
        mb <- f =<< n
        case mb of
          Nothing -> return mempty
          Just b -> do
            putPromise p2 b
            codeGen action


------------------------------------------------------------------------------
-- | Converts an RuntimeSplice into a Splice, given a helper function that
-- generates a Builder.
bindLater :: (Monad n)
          => (a -> RuntimeSplice n Builder)
          -> RuntimeSplice n a
          -> Splice n
bindLater f p = return $ yieldRuntime $ f =<< p