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-- | This is the only module that uses unsafePerformIO; to make sure it's safe,
-- we disable all the known optimisations in GHC that can potentially mess
-- things up: <https://discourse.haskell.org/t/can-noinline-fail-to-prevent-inlining/14324>
-- Also, since there is nothing else in this module, there isn't really very
-- much for GHC /to/ mess up, which is the point.
{-# OPTIONS -fno-cse -fno-full-laziness -fno-state-hack #-}
module InitOnce.TheMap (onceInsert) where
import Control.Concurrent.MVar (modifyMVarMasked, newMVar, MVar)
import Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map (empty, lookup, insert)
import System.IO.Unsafe (unsafePerformIO)
import InitOnce.Types (Location, TypedAny)
{-# OPAQUE theMap #-}
theMap :: MVar (Map Location TypedAny)
theMap = unsafePerformIO (newMVar Map.empty)
-- | The IO action is executed with asynchronous exceptions masked (using
-- 'modifyMVarMasked').
{-# OPAQUE onceInsert #-}
onceInsert :: Location -> IO TypedAny -> TypedAny
onceInsert key mkVal =
unsafePerformIO $
modifyMVarMasked theMap $ \mp ->
case Map.lookup key mp of
Nothing -> do
v <- mkVal
return (Map.insert key v mp, v)
Just v -> return (mp, v)
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