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{-# 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.Map.Syntax
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 Data.Foldable (Foldable)
import qualified Data.Foldable as Foldable
------------------------------------------------------------------------------
import Heist.Common
import Heist.Internal.Types.HeistState
------------------------------------------------------------------------------
------------------------------------------------------------------------------
-- | 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 DocumentFile with the appropriate template context set.
runDocumentFile :: Monad n
=> TPath
-> DocumentFile
-> Splice n
runDocumentFile tpath df = do
let markup = case dfDoc df of
X.XmlDocument _ _ _ -> Xml
X.HtmlDocument _ _ _ -> Html
modifyHS (\hs -> hs { _curMarkup = markup })
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
=> TPath
-> DocumentFile
-> HeistT n IO [Chunk n]
compileTemplate tpath df = do
!chunks <- runDocumentFile tpath df
return $! consolidate chunks
------------------------------------------------------------------------------
compileTemplates
:: Monad n
=> HeistState n
-> IO (Either [String] (HeistState n))
compileTemplates hs = do
(tmap, hs') <- runHeistT compileTemplates' (X.TextNode "") hs
return $ case _spliceErrors hs' of
[] -> Right $! hs { _compiledTemplateMap = tmap }
es -> Left $ map T.unpack es
------------------------------------------------------------------------------
compileTemplates'
:: Monad n
=> HeistT n IO (H.HashMap TPath ([Chunk n], MIMEType))
compileTemplates' = do
hs <- getHS
let tpathDocfiles :: [(TPath, DocumentFile)]
tpathDocfiles = map (\(a,b) -> (a, b))
(H.toList $ _templateMap hs)
foldM runOne H.empty tpathDocfiles
where
runOne tmap (tpath, df) = do
!mHtml <- compileTemplate 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 = do
pre <- getsHS _splicePrefix
res <- getsHS (H.lookup nm . _compiledSpliceMap)
canError <- getsHS _errorNotBound
if isNothing res && T.isPrefixOf pre nm && not (T.null pre)
then do
when canError $
tellSpliceError $ "No splice bound for " `mappend` nm
return Nothing
else return res
------------------------------------------------------------------------------
-- | 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) = do
msplice <- lookupSplice nm
fromMaybe compileStaticElement msplice
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 hs =
hs { _compiledSpliceMap = H.union (runMapNoErrors ss)
(_compiledSpliceMap hs) }
------------------------------------------------------------------------------
-- | 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 --
---------------
------------------------------------------------------------------------------
-- | This is the same as htmlNodeSplice.
nodeSplice :: (a -> [X.Node]) -> a -> Builder
nodeSplice f = X.renderHtmlFragment X.UTF8 . f
{-# DEPRECATED nodeSplice
"Use xmlNodeSplice or htmlNodeSplice, will be removed in Heist 1.1" #-}
------------------------------------------------------------------------------
-- | Converts a pure XML Node splice function to a pure Builder splice
-- function.
xmlNodeSplice :: (a -> [X.Node]) -> a -> Builder
xmlNodeSplice f = X.renderXmlFragment X.UTF8 . f
------------------------------------------------------------------------------
-- | Converts a pure HTML Node splice function to a pure Builder splice
-- function.
htmlNodeSplice :: (a -> [X.Node]) -> a -> Builder
htmlNodeSplice 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' mempty splice
where
splices' = mapV ($runtimeAction) splices
------------------------------------------------------------------------------
{-# INLINE foldMapM #-}
foldMapM :: (Monad f, Monoid m, Foldable list)
=> (a -> f m)
-> list a
-> f m
foldMapM f =
Foldable.foldlM (\xs x -> xs `seq` liftM (xs <>) (f x)) mempty
------------------------------------------------------------------------------
-- | Like withSplices, but evaluates the splice repeatedly for each element in
-- a list generated at runtime.
manyWithSplices :: (Foldable f, Monad n)
=> Splice n
-> Splices (RuntimeSplice n a -> Splice n)
-> RuntimeSplice n (f a)
-> Splice n
manyWithSplices splice splices runtimeAction = do
p <- newEmptyPromise
let splices' = mapV ($ getPromise p) splices
chunks <- withLocalSplices splices' mempty splice
return $ yieldRuntime $ do
items <- runtimeAction
foldMapM (\item -> putPromise p item >> codeGen chunks) items
------------------------------------------------------------------------------
-- | More powerful version of manyWithSplices that lets you also define
-- attribute splices.
manyWith :: (Foldable f, Monad n)
=> Splice n
-> Splices (RuntimeSplice n a -> Splice n)
-> Splices (RuntimeSplice n a -> AttrSplice n)
-> RuntimeSplice n (f a)
-> Splice n
manyWith splice splices attrSplices runtimeAction = do
p <- newEmptyPromise
let splices' = mapV ($ getPromise p) splices
let attrSplices' = mapV ($ getPromise p) attrSplices
chunks <- withLocalSplices splices' attrSplices' splice
return $ yieldRuntime $ do
items <- runtimeAction
foldMapM (\item -> putPromise p item >> codeGen chunks) items
------------------------------------------------------------------------------
-- | 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 :: (Foldable f, Monad n)
=> (RuntimeSplice n a -> Splice n)
-> RuntimeSplice n (f a)
-> Splice n
deferMany f getItems = do
promise <- newEmptyPromise
chunks <- f $ getPromise promise
return $ yieldRuntime $ do
items <- getItems
foldMapM (\item -> putPromise promise item >> codeGen chunks) items
------------------------------------------------------------------------------
-- | 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
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