GenServer

Client-Server Model

See: Erlang gen_server Behaviour

Client-server model

GenServer Typeclass

  1. class GenServer req rep st | req -> rep, rep -> st, st -> req where
  2.   handleCall :: HandleCall req rep st
  3.   handleCast :: HandleCast req rep st

Server Example

  1. module Demo.Server
  2.   ( start
  3.   , inc
  4.   , dec
  5.   , query
  6.   ) where
  8. import Prelude
  9. import Control.Behaviour.GenServer
  10.   ( class GenServer
  11.   , HandleCall
  12.   , HandleCast
  13.   , Init
  14.   , startLinkWith
  15.   , initOk
  16.   , call
  17.   , cast
  18.   , noReply
  19.   , reply
  20.   , shutdown
  21.   )
  22. import System.IO (println)
  24. data Request = Inc | Dec | Query
  25. data Reply = QueryResult Integer
  26. data State = State Integer
  28. name :: Atom
  29. name = :server
  31. start :: Process Pid
  32. start = startLinkWith name (init 20)
  34. -----------------------------------------------------------------------------
  35. -- | Server API
  36. -----------------------------------------------------------------------------
  38. inc :: Process ()
  39. inc = cast name Inc
  41. dec :: Process ()
  42. dec = cast name Dec
  44. query :: Process Integer
  45. query = do
  46.   QueryResult i <- call name Query
  47.   return i
  49. -----------------------------------------------------------------------------
  50. -- | Server callbacks
  51. -----------------------------------------------------------------------------
  53. instance GenServer Request Reply State where
  54.   handleCall = handleCall
  55.   handleCast = handleCast
  57. init :: Integer -> Init Request State
  58. init n = initOk (State n)
  60. handleCall :: HandleCall Request Reply State
  61. handleCall Query _from (State i) = do
  62.   println "Call: Query"
  63.   reply (QueryResult i) (State i)
  64. handleCall _req _from st =
  65.   shutdown :badRequest st
  67. handleCast :: HandleCast Request Reply State
  68. handleCast Inc (State n) = do
  69.   println "Cast: Inc"
  70.   noReply $ State (n+1)
  71. handleCast Dec (State n) = do
  72.   println "Cast: Dec"
  73.   noReply $ State (n-1)
  74. handleCast _ st = noReply st

Start a Server process

  1. -- | Start a standalone Server process.
  2. start :: forall req rep st. GenServer req rep st => (Init req st) -> Process Pid
  3. startWith :: forall req rep st. GenServer req rep st => Name -> (Init req st) -> Process Pid
  5. -- | Start a Server process as part of a supervision tree.
  6. startLink :: forall req rep st. GenServer req rep st => (Init req st) -> Process Pid
  7. startLinkWith :: forall req rep st. GenServer req rep st => Name -> (Init req st) -> Process Pid

Init callback

  1. -- | Init Result
  2. data InitResult req st
  3.   = InitOk st (Maybe (Action req))
  4.     -- ^ {ok, State}
  5.   | InitIgnore
  6.     -- ^ ignore
  7.   | InitStop ExitReason
  8.     -- ^ {stop, Reason}
  10. -- | Init callback
  11. type Init req st = Process (InitResult req st)

HandleCall and HandleCast

  1. -- | HandleCall callback
  2. type HandleCall req rep st
  3.   = req -> From -> st -> Process (Reply req rep st)
  5. -- | HandleCast callback
  6. type HandleCast req rep st
  7.   = req -> st -> Process (Reply req rep st)

Client APIs

  1. -- | Synchronous call to the server process.
  2. call :: forall req rep. Name -> req -> Process rep
  4. -- | Sends an asynchronous request to the server process.
  5. cast :: forall req. Name -> req -> Process ()