:: a typeCtrl KGHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27
Moduleeffectful-core-2.3.0.1Haskell2010
Support for access to a write only value of a particular type.
The value is shared between multiple threads. If you want each thead 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.
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-2.3.0.1 · 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: 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!"