Re: [GHC] #12622: Unboxed static pointers lead to missing SPT entries

17 Nov 2016

      #12622: Unboxed static pointers lead to missing SPT entries
-------------------------------------+-------------------------------------
        Reporter:  mboes             |                Owner:
                                     |  facundo.dominguez
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2709
       Wiki Page:                    |  Phab:D2720
-------------------------------------+-------------------------------------

Comment (by facundo.dominguez):

 Hello Simon,

 #11656 is needed for programming in the large, similarly to how local
 definitions avoid polluting the top-level namespace of a module with many
 functions that relate to its very internals, which would make it harder to
 maintain it.

 Of the points you raise, the first three are what we designed it to be
 from the start. But I agree that the mechanism for the linter to discern
 static forms is more involved than expected. We might review this
 mechanism, but I feel it would be excessive to disregard support for local
 definitions because of linting.

 As an example of how the namespace pollution prevented using static
 pointers, I offer a use case where StaticPtrs were used to persist and
 share the states of a state machine.
 If static forms did not support local definitions, then a module with many
 state machines with many states and events each would have to bear a lot
 of "internal" top-level functions.

 {{{
 -- | This module defines state machines whose state can be serialized
 -- and execution can be resumed later. It uses the package distributed-
 closure
 -- to create the closures.

 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE StaticPointers #-}
 {-# LANGUAGE TypeSynonymInstances #-}
 module Main where

 import Control.Distributed.Closure
 import Control.Monad
 import GHC.StaticPtr

 -- | A state can be final, in which case it yields a result, or it
 provides
 -- a computation which yields the next state.
 --
 -- The Closure type is used to keep track of the current state, and it
 -- can be persisted or sent to other nodes for subsequent execution.
 --
 -- In this implementation machines have a single push-button. Transitions
 are
 -- done at the user request. It would be possible to extend it to react to
 -- other events.
 newtype MState m a = MState (Closure (m (NextState m a)))
 type NextState m a = Either (MState m a) a

 -- | @anbncn@ is a state machine which recognizes words in the set
 -- @{ a^n b^n c^n | n <- [1..] }@.
 --
 -- It yields the amount of repetitions @n@ found in the input.
 anbncn :: MState IO Int
 anbncn = MState $ mkClosure (static as) 0
   where
     as :: Int -> IO (NextState IO Int)
     as an = do
       x <- getChar
       return $ case x of
         'a'          -> Left $ MState $ mkClosure (static as) (an + 1)
         'b' | an > 0 -> Left $ MState $ mkClosure (static bs) (an, 1)
         _            -> Right 0

     bs :: (Int, Int) -> IO (NextState IO Int)
     bs (an, bn) = do
       x <- getChar
       return $ case x of
         'b'            -> Left $ MState $ mkClosure (static bs) (an, bn +
 1)
         'c' | an == bn -> Left $ MState $ mkClosure (static cs) (an,
 1)
         _              -> Right 0

     cs :: (Int, Int) -> IO (NextState IO Int)
     cs (an, cn) = do
       x <- getChar
       return $ case x of
         'c' | an == cn + 1 -> Right $ cn + 1
             | otherwise    -> Left $ MState $ mkClosure (static cs) (an,
 cn + 1)
         _   | an == cn     -> Right cn
             | otherwise    -> Right 0

 -- Make one transition.
 pushButton :: MState m a -> m (NextState m a)
 pushButton (MState c) = unclosure c

 -- Make all transitions of the state machine.
 runStateMachine :: Monad m => MState m a -> m a
 runStateMachine = pushButton >=> either runStateMachine return

 main :: IO ()
 main = do
     putStrLn "start typing a, b or c"
     n <- runStateMachine anbncn
     putStrLn $ "The amount of repetitions is " ++ show n

 -- | Produces a closure from a static pointer and a serializable value.
 mkClosure :: Static (Serializable a) => StaticPtr (a -> b) -> a -> Closure
 b
 mkClosure sp a = closure sp `cap` cpure closureDict a

 instance Static (Serializable Int) where
   closureDict = closure $ static Dict

 instance Static (Serializable (Int, Int)) where
   closureDict = closure $ static Dict
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12622#comment:9
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