:: a typeCtrl KGHC 9.14.1 · lts/ghc-9.14.x · 45dd537 · 2026-10-11
Moduleeffectful-core-2.7.1.2GHC2021
Support for access to a write only value of a particular type.
The value is shared between multiple threads. If you want each thread to manage its own version of the value, use Effectful.Writer.Static.Local.
Warning: Writer's state will be accumulated via left-associated uses of <>, which makes it unsuitable for use with types for which such pattern is inefficient. This applies, in particular, to the standard list type, which makes the Writer effect pretty niche.
If you just want to accumulate values, use Effectful.Output.Static.Shared.Array or Effectful.Output.Static.Shared.List.
Note: while the Writer from the
transformers package includes additional operations
pass and
censor, they don't cooperate with runtime
exceptions very well, so they're deliberately omitted here.
Provide access to a strict (WHNF), shared, write only value of type w.
type DispatchOf (Writer w) = 'Static 'NoSideEffectsDefined in effectful-core-2.7.1.2 · Effectful.Writer.Static.Shareddata StaticRep (Writer w)Writer (MVar w)Run a Writer effect and return the final value along with the final output.
Run a Writer effect and return the final output, discarding the final value.
Append the given output to the overall output of the Writer.
Execute an action and append its output to the overall output of the Writer.
Note: the output of tell executed from threads spawned within the nested action is accounted for only if it completes before listen merges the output, which happens as soon as the action finishes. In particular, the output of threads that outlive the scope of listen will be lost:
:{ runEff . execWriter @String $ do lock <- liftIO newEmptyMVar done <- liftIO newEmptyMVar tell "1" _ <- listen @String $ do tell "2" withEffToIO (ConcUnlift Ephemeral $ Limited 1) $ \unlift -> do _ <- forkIO $ do takeMVar lock unlift $ tell "3" putMVar done () pure () liftIO $ putMVar lock () liftIO $ takeMVar done tell "4":}"124"
Note: if an exception is received while the action is executed, the partial output of the action will still be appended to the overall output of the Writer:
:{ runEff . execWriter @String $ do tell "Hi" handle (\(_::ErrorCall) -> pure ((), "")) $ do tell " there" listen $ do tell "!" error "oops":}"Hi there!"