module Z.IO.RPC.MessagePack where
import Control.Monad
import Data.Bits
import Z.Data.PrimRef.PrimIORef
import qualified Z.Data.MessagePack.Builder as MB
import qualified Z.Data.MessagePack.Value as MV
import Z.Data.MessagePack (MessagePack)
import qualified Z.Data.MessagePack as MP
import qualified Z.Data.Parser as P
import qualified Z.Data.Text as T
import qualified Z.Data.Vector.FlatIntMap as FIM
import qualified Z.Data.Vector.FlatMap as FM
import qualified Z.Data.Vector as V
import Z.IO
import Z.IO.Network
data Client = Client
{ Client -> Counter
_clientSeqRef :: Counter
, Client -> Counter
_clientPipelineReqNum :: Counter
, Client -> BufferedInput
_clientBufferedInput :: BufferedInput
, Client -> BufferedOutput
_clientBufferedOutput :: BufferedOutput
}
rpcClient :: (Input dev, Output dev) => dev -> IO Client
rpcClient :: dev -> IO Client
rpcClient dev
uvs = dev -> dev -> Int -> Int -> IO Client
forall i o.
(Input i, Output o) =>
i -> o -> Int -> Int -> IO Client
rpcClient' dev
uvs dev
uvs Int
V.defaultChunkSize Int
V.defaultChunkSize
rpcClient' :: (Input i, Output o)
=> i
-> o
-> Int
-> Int
-> IO Client
rpcClient' :: i -> o -> Int -> Int -> IO Client
rpcClient' i
i o
o Int
recvBufSiz Int
sendBufSiz = do
Counter
seqRef <- Int -> IO Counter
newCounter Int
0
Counter
reqNum <- Int -> IO Counter
newCounter Int
0
BufferedInput
bi <- Int -> i -> IO BufferedInput
forall i. Input i => Int -> i -> IO BufferedInput
newBufferedInput' Int
recvBufSiz i
i
BufferedOutput
bo <- Int -> o -> IO BufferedOutput
forall o. Output o => Int -> o -> IO BufferedOutput
newBufferedOutput' Int
sendBufSiz o
o
Client -> IO Client
forall (m :: * -> *) a. Monad m => a -> m a
return (Counter -> Counter -> BufferedInput -> BufferedOutput -> Client
Client Counter
seqRef Counter
reqNum BufferedInput
bi BufferedOutput
bo)
call:: (MessagePack req, MessagePack res) => Client -> T.Text -> req -> IO res
call :: Client -> Text -> req -> IO res
call Client
cli Text
name req
req = do
Int
msgid <- Client -> Text -> req -> IO Int
forall req.
(HasCallStack, MessagePack req) =>
Client -> Text -> req -> IO Int
callPipeline Client
cli Text
name req
req
Int -> PipelineResult -> IO res
forall res.
(HasCallStack, MessagePack res) =>
Int -> PipelineResult -> IO res
fetchPipeline Int
msgid (PipelineResult -> IO res) -> IO PipelineResult -> IO res
forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
=<< HasCallStack => Client -> IO PipelineResult
Client -> IO PipelineResult
execPipeline Client
cli
notify :: MessagePack req => Client -> T.Text -> req -> IO ()
notify :: Client -> Text -> req -> IO ()
notify c :: Client
c@(Client Counter
_ Counter
_ BufferedInput
_ BufferedOutput
bo) Text
name req
req = Client -> Text -> req -> IO ()
forall req.
(HasCallStack, MessagePack req) =>
Client -> Text -> req -> IO ()
notifyPipeline Client
c Text
name req
req IO () -> IO () -> IO ()
forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
>> HasCallStack => BufferedOutput -> IO ()
BufferedOutput -> IO ()
flushBuffer BufferedOutput
bo
type PipelineId = Int
type PipelineResult = FIM.FlatIntMap MV.Value
callPipeline :: HasCallStack => MessagePack req => Client -> T.Text -> req -> IO PipelineId
callPipeline :: Client -> Text -> req -> IO Int
callPipeline (Client Counter
seqRef Counter
reqNum BufferedInput
_ BufferedOutput
bo) Text
name req
req = do
Int
msgid <- Counter -> IO Int
forall a. Prim a => PrimIORef a -> IO a
readPrimIORef Counter
seqRef
Counter -> Int -> IO ()
forall a. Prim a => PrimIORef a -> a -> IO ()
writePrimIORef Counter
seqRef (Int
msgidInt -> Int -> Int
forall a. Num a => a -> a -> a
+Int
1)
Counter -> (Int -> Int) -> IO ()
forall a. Prim a => PrimIORef a -> (a -> a) -> IO ()
modifyPrimIORef Counter
reqNum (Int -> Int -> Int
forall a. Num a => a -> a -> a
+Int
1)
let !msgid' :: Int
msgid' = Int
msgid Int -> Int -> Int
forall a. Bits a => a -> a -> a
.&. Int
0xFFFFFFFF
BufferedOutput -> Builder () -> IO ()
forall a. HasCallStack => BufferedOutput -> Builder a -> IO ()
writeBuilder BufferedOutput
bo (Builder () -> IO ()) -> Builder () -> IO ()
forall a b. (a -> b) -> a -> b
$ do
Int -> Builder ()
MB.arrayHeader Int
4
Int64 -> Builder ()
MB.int Int64
0
Int64 -> Builder ()
MB.int (Int -> Int64
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int
msgid')
Text -> Builder ()
MB.str Text
name
req -> Builder ()
forall a. MessagePack a => a -> Builder ()
MP.encodeMessagePack req
req
Int -> IO Int
forall (m :: * -> *) a. Monad m => a -> m a
return Int
msgid'
notifyPipeline :: HasCallStack => MessagePack req => Client -> T.Text -> req -> IO ()
notifyPipeline :: Client -> Text -> req -> IO ()
notifyPipeline (Client Counter
_ Counter
_ BufferedInput
_ BufferedOutput
bo) Text
name req
req = do
BufferedOutput -> Builder () -> IO ()
forall a. HasCallStack => BufferedOutput -> Builder a -> IO ()
writeBuilder BufferedOutput
bo (Builder () -> IO ()) -> Builder () -> IO ()
forall a b. (a -> b) -> a -> b
$ do
Int -> Builder ()
MB.arrayHeader Int
3
Int64 -> Builder ()
MB.int Int64
2
Text -> Builder ()
MB.str Text
name
req -> Builder ()
forall a. MessagePack a => a -> Builder ()
MP.encodeMessagePack req
req
data RPCException = RPCException MV.Value CallStack deriving Int -> RPCException -> ShowS
[RPCException] -> ShowS
RPCException -> String
(Int -> RPCException -> ShowS)
-> (RPCException -> String)
-> ([RPCException] -> ShowS)
-> Show RPCException
forall a.
(Int -> a -> ShowS) -> (a -> String) -> ([a] -> ShowS) -> Show a
showList :: [RPCException] -> ShowS
$cshowList :: [RPCException] -> ShowS
show :: RPCException -> String
$cshow :: RPCException -> String
showsPrec :: Int -> RPCException -> ShowS
$cshowsPrec :: Int -> RPCException -> ShowS
Show
instance Exception RPCException
execPipeline :: HasCallStack => Client -> IO PipelineResult
execPipeline :: Client -> IO PipelineResult
execPipeline (Client Counter
_ Counter
reqNum BufferedInput
bi BufferedOutput
bo) = do
HasCallStack => BufferedOutput -> IO ()
BufferedOutput -> IO ()
flushBuffer BufferedOutput
bo
Int
n <- Counter -> IO Int
forall a. Prim a => PrimIORef a -> IO a
readPrimIORef Counter
reqNum
Counter -> Int -> IO ()
forall a. Prim a => PrimIORef a -> a -> IO ()
writePrimIORef Counter
reqNum Int
0
Int -> [IPair Value] -> PipelineResult
forall v. Int -> [IPair v] -> FlatIntMap v
FIM.packN Int
n ([IPair Value] -> PipelineResult)
-> IO [IPair Value] -> IO PipelineResult
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Int -> IO (IPair Value) -> IO [IPair Value]
forall (m :: * -> *) a. Applicative m => Int -> m a -> m [a]
replicateM Int
n (do
(Int64
msgid, Value
err, Value
v) <- Parser (Int64, Value, Value)
-> BufferedInput -> IO (Int64, Value, Value)
forall a. HasCallStack => Parser a -> BufferedInput -> IO a
readParser (do
Word8
tag <- Parser Word8
P.anyWord8
Bool -> Parser () -> Parser ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when (Word8
tag Word8 -> Word8 -> Bool
forall a. Eq a => a -> a -> Bool
/= Word8
0x94) (Text -> Parser ()
forall a. Text -> Parser a
P.fail' (Text -> Parser ()) -> Text -> Parser ()
forall a b. (a -> b) -> a -> b
$ Text
"wrong response tag: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Word8 -> Text
forall a. Print a => a -> Text
T.toText Word8
tag)
!Value
typ <- Parser Value
MV.value
!Value
seq <- Parser Value
MV.value
!Value
err <- Parser Value
MV.value
!Value
v <- Parser Value
MV.value
case Value
typ of
MV.Int Int64
1 -> case Value
seq of
MV.Int Int64
msgid | Int64
msgid Int64 -> Int64 -> Bool
forall a. Ord a => a -> a -> Bool
>= Int64
0 Bool -> Bool -> Bool
&& Int64
msgid Int64 -> Int64 -> Bool
forall a. Ord a => a -> a -> Bool
<= Int64
0xFFFFFFFF ->
(Int64, Value, Value) -> Parser (Int64, Value, Value)
forall (m :: * -> *) a. Monad m => a -> m a
return (Int64
msgid, Value
err, Value
v)
Value
_ -> Text -> Parser (Int64, Value, Value)
forall a. Text -> Parser a
P.fail' (Text -> Parser (Int64, Value, Value))
-> Text -> Parser (Int64, Value, Value)
forall a b. (a -> b) -> a -> b
$ Text
"wrong msgid: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
seq
Value
_ -> Text -> Parser (Int64, Value, Value)
forall a. Text -> Parser a
P.fail' (Text -> Parser (Int64, Value, Value))
-> Text -> Parser (Int64, Value, Value)
forall a b. (a -> b) -> a -> b
$ Text
"wrong response type: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
typ
) BufferedInput
bi
Bool -> IO () -> IO ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when (Value
err Value -> Value -> Bool
forall a. Eq a => a -> a -> Bool
/= Value
MV.Nil) (IO () -> IO ()) -> IO () -> IO ()
forall a b. (a -> b) -> a -> b
$ RPCException -> IO ()
forall e a. Exception e => e -> IO a
throwIO (Value -> CallStack -> RPCException
RPCException Value
err CallStack
HasCallStack => CallStack
callStack)
IPair Value -> IO (IPair Value)
forall (m :: * -> *) a. Monad m => a -> m a
return (Int -> Value -> IPair Value
forall a. Int -> a -> IPair a
V.IPair (Int64 -> Int
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int64
msgid) Value
v))
fetchPipeline :: HasCallStack => MessagePack res => PipelineId -> PipelineResult -> IO res
fetchPipeline :: Int -> PipelineResult -> IO res
fetchPipeline Int
msgid PipelineResult
r = do
Text -> Either ConvertError res -> IO res
forall e a. (HasCallStack, Print e) => Text -> Either e a -> IO a
unwrap Text
"EPARSE" (Either ConvertError res -> IO res)
-> (Value -> Either ConvertError res) -> Value -> IO res
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Value -> Either ConvertError res
forall a. MessagePack a => Value -> Either ConvertError a
MP.convertValue (Value -> IO res) -> IO Value -> IO res
forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
=<<
Text -> Text -> Maybe Value -> IO Value
forall a. HasCallStack => Text -> Text -> Maybe a -> IO a
unwrap' Text
"ENOMSG" (Text
"missing message in response: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Int -> Text
forall a. Print a => a -> Text
T.toText Int
msgid)
(Int -> PipelineResult -> Maybe Value
forall v. Int -> FlatIntMap v -> Maybe v
FIM.lookup Int
msgid PipelineResult
r)
type ServerLoop = (UVStream -> IO ()) -> IO ()
type ServerService = T.Text -> Maybe ServerHandler
data ServerHandler where
CallHandler :: (MessagePack req, MessagePack res) => (req -> IO res) -> ServerHandler
NotifyHandler :: MessagePack req => (req -> IO ()) -> ServerHandler
simpleRouter :: [(T.Text, ServerHandler)] -> ServerService
simpleRouter :: [(Text, ServerHandler)] -> ServerService
simpleRouter [(Text, ServerHandler)]
handles Text
name = Text -> FlatMap Text ServerHandler -> Maybe ServerHandler
forall k v. Ord k => k -> FlatMap k v -> Maybe v
FM.lookup Text
name FlatMap Text ServerHandler
handleMap
where
handleMap :: FlatMap Text ServerHandler
handleMap = [(Text, ServerHandler)] -> FlatMap Text ServerHandler
forall k v. Ord k => [(k, v)] -> FlatMap k v
FM.packR [(Text, ServerHandler)]
handles
serveRPC :: ServerLoop -> ServerService -> IO ()
serveRPC :: ServerLoop -> ServerService -> IO ()
serveRPC ServerLoop
serve = ServerLoop -> Int -> Int -> ServerService -> IO ()
serveRPC' ServerLoop
serve Int
V.defaultChunkSize Int
V.defaultChunkSize
serveRPC' :: ServerLoop
-> Int
-> Int
-> ServerService -> IO ()
serveRPC' :: ServerLoop -> Int -> Int -> ServerService -> IO ()
serveRPC' ServerLoop
serve Int
recvBufSiz Int
sendBufSiz ServerService
handle = ServerLoop
serve ServerLoop -> ServerLoop
forall a b. (a -> b) -> a -> b
$ \ UVStream
uvs -> do
BufferedInput
bi <- Int -> UVStream -> IO BufferedInput
forall i. Input i => Int -> i -> IO BufferedInput
newBufferedInput' Int
recvBufSiz UVStream
uvs
BufferedOutput
bo <- Int -> UVStream -> IO BufferedOutput
forall o. Output o => Int -> o -> IO BufferedOutput
newBufferedOutput' Int
sendBufSiz UVStream
uvs
BufferedInput -> BufferedOutput -> IO ()
loop BufferedInput
bi BufferedOutput
bo
where
loop :: BufferedInput -> BufferedOutput -> IO ()
loop BufferedInput
bi BufferedOutput
bo = do
Maybe (Either (Text, Value) (Int64, Text, Value))
req <- BIO Void (Either (Text, Value) (Int64, Text, Value))
-> IO (Maybe (Either (Text, Value) (Int64, Text, Value)))
forall inp out. BIO inp out -> IO (Maybe out)
pull (Parser (Either (Text, Value) (Int64, Text, Value))
-> BufferedInput
-> BIO Void (Either (Text, Value) (Int64, Text, Value))
forall a. HasCallStack => Parser a -> BufferedInput -> Source a
sourceParserFromBuffered (do
Word8
tag <- Parser Word8
P.anyWord8
case Word8
tag of
Word8
0x93 -> do
!Value
typ <- Parser Value
MV.value
!Value
name <- Parser Value
MV.value
!Value
v <- Parser Value
MV.value
case Value
typ of
MV.Int Int64
2 -> case Value
name of
MV.Str Text
name' -> Either (Text, Value) (Int64, Text, Value)
-> Parser (Either (Text, Value) (Int64, Text, Value))
forall (m :: * -> *) a. Monad m => a -> m a
return ((Text, Value) -> Either (Text, Value) (Int64, Text, Value)
forall a b. a -> Either a b
Left (Text
name', Value
v))
Value
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong RPC name: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
name
Value
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong notification type: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
typ
Word8
0x94 -> do
!Value
typ <- Parser Value
MV.value
!Value
seq <- Parser Value
MV.value
!Value
name <- Parser Value
MV.value
!Value
v <- Parser Value
MV.value
case Value
typ of
MV.Int Int64
0 -> case Value
seq of
MV.Int Int64
msgid | Int64
msgid Int64 -> Int64 -> Bool
forall a. Ord a => a -> a -> Bool
>= Int64
0 Bool -> Bool -> Bool
&& Int64
msgid Int64 -> Int64 -> Bool
forall a. Ord a => a -> a -> Bool
<= Int64
0xFFFFFFFF -> case Value
name of
MV.Str Text
name' -> Either (Text, Value) (Int64, Text, Value)
-> Parser (Either (Text, Value) (Int64, Text, Value))
forall (m :: * -> *) a. Monad m => a -> m a
return ((Int64, Text, Value) -> Either (Text, Value) (Int64, Text, Value)
forall a b. b -> Either a b
Right (Int64
msgid, Text
name', Value
v))
Value
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong RPC name: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
name
Value
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong msgid: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
seq
Value
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong request type: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Value -> Text
forall a. Print a => a -> Text
T.toText Value
typ
Word8
_ -> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a. Text -> Parser a
P.fail' (Text -> Parser (Either (Text, Value) (Int64, Text, Value)))
-> Text -> Parser (Either (Text, Value) (Int64, Text, Value))
forall a b. (a -> b) -> a -> b
$ Text
"wrong request tag: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Word8 -> Text
forall a. Print a => a -> Text
T.toText Word8
tag
) BufferedInput
bi)
case Maybe (Either (Text, Value) (Int64, Text, Value))
req of
Just (Left (Text
name, Value
v)) -> do
case ServerService
handle Text
name of
Just (NotifyHandler req -> IO ()
f) -> do
req -> IO ()
f (req -> IO ()) -> IO req -> IO ()
forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
=<< Text -> Either ConvertError req -> IO req
forall e a. (HasCallStack, Print e) => Text -> Either e a -> IO a
unwrap Text
"EPARSE" (Value -> Either ConvertError req
forall a. MessagePack a => Value -> Either ConvertError a
MP.convertValue Value
v)
Maybe ServerHandler
_ -> Text -> Text -> IO ()
forall a. HasCallStack => Text -> Text -> IO a
throwOtherError Text
"ENOTFOUND" Text
"notification method not found"
BufferedInput -> BufferedOutput -> IO ()
loop BufferedInput
bi BufferedOutput
bo
Just (Right (Int64
msgid, Text
name, Value
v)) -> do
case ServerService
handle Text
name of
Just (CallHandler req -> IO res
f) -> do
Either SomeException res
res <- IO res -> IO (Either SomeException res)
forall e a. Exception e => IO a -> IO (Either e a)
try (req -> IO res
f (req -> IO res) -> IO req -> IO res
forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
=<< Text -> Either ConvertError req -> IO req
forall e a. (HasCallStack, Print e) => Text -> Either e a -> IO a
unwrap Text
"EPARSE" (Value -> Either ConvertError req
forall a. MessagePack a => Value -> Either ConvertError a
MP.convertValue Value
v))
BufferedOutput -> Builder () -> IO ()
forall a. HasCallStack => BufferedOutput -> Builder a -> IO ()
writeBuilder BufferedOutput
bo (Builder () -> IO ()) -> Builder () -> IO ()
forall a b. (a -> b) -> a -> b
$ do
Int -> Builder ()
MB.arrayHeader Int
4
Int64 -> Builder ()
MB.int Int64
1
Int64 -> Builder ()
MB.int (Int64 -> Int64
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int64
msgid)
case Either SomeException res
res of
Left SomeException
e -> do
Text -> Builder ()
MB.str (String -> Text
T.pack (String -> Text) -> String -> Text
forall a b. (a -> b) -> a -> b
$ SomeException -> String
forall a. Show a => a -> String
show (SomeException
e :: SomeException))
Builder ()
MB.nil
Right res
res -> do
Builder ()
MB.nil
res -> Builder ()
forall a. MessagePack a => a -> Builder ()
MP.encodeMessagePack res
res
HasCallStack => BufferedOutput -> IO ()
BufferedOutput -> IO ()
flushBuffer BufferedOutput
bo
Maybe ServerHandler
_ -> do
BufferedOutput -> Builder () -> IO ()
forall a. HasCallStack => BufferedOutput -> Builder a -> IO ()
writeBuilder BufferedOutput
bo (Builder () -> IO ()) -> Builder () -> IO ()
forall a b. (a -> b) -> a -> b
$ do
Int -> Builder ()
MB.arrayHeader Int
4
Int64 -> Builder ()
MB.int Int64
1
Int64 -> Builder ()
MB.int (Int64 -> Int64
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int64
msgid)
Text -> Builder ()
MB.str (Text -> Builder ()) -> Text -> Builder ()
forall a b. (a -> b) -> a -> b
$ Text
"request method: " Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Text
name Text -> Text -> Text
forall a. Semigroup a => a -> a -> a
<> Text
" not found"
Builder ()
MB.nil
HasCallStack => BufferedOutput -> IO ()
BufferedOutput -> IO ()
flushBuffer BufferedOutput
bo
BufferedInput -> BufferedOutput -> IO ()
loop BufferedInput
bi BufferedOutput
bo
Maybe (Either (Text, Value) (Int64, Text, Value))
_ -> () -> IO ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()