GHC 9.14.1 · lts/ghc-9.14.x · 7a6c33e · 2026-10-12
Moduleeffectful-core-2.7.1.2GHC2021
Support for access to a write only value of a particular type.
The value is thread local. If you want it to be shared between threads, use Effectful.Writer.Static.Shared.
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.Local.Array or Effectful.Output.Static.Local.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), thread local, write only value of type
w.
type DispatchOf (Writer w) = 'Static 'NoSideEffectsDefined in effectful-core-2.7.1.2 · Effectful.Writer.Static.Localdata StaticRep (Writer w)Writer wRun 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!"