Safe Haskell | Safe |
---|---|

Language | Haskell2010 |

Strict write-only state

- data Writer w v = Writer !w v
- tell :: Member (Writer w) r => w -> Eff r ()
- censor :: forall w a r. Member (Writer w) r => (w -> w) -> Eff r a -> Eff r a
- runWriter :: (w -> b -> b) -> b -> Eff (Writer w ': r) a -> Eff r (a, b)
- runFirstWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w)
- runLastWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w)
- runListWriter :: Eff (Writer w ': r) a -> Eff r (a, [w])
- runMonoidWriter :: Monoid w => Eff (Writer w ': r) a -> Eff r (a, w)

# Documentation

The Writer monad

In MTL's Writer monad, the told value must have a |Monoid| type. Our writer has no such constraints. If we write a |Writer|-like interpreter to accumulate the told values in a monoid, it will have the |Monoid w| constraint then

Writer !w v |

censor :: forall w a r. Member (Writer w) r => (w -> w) -> Eff r a -> Eff r a Source #

Transform the state being produced.

runWriter :: (w -> b -> b) -> b -> Eff (Writer w ': r) a -> Eff r (a, b) Source #

Handle Writer requests, using a user-provided function to accumulate values, hence no Monoid constraints.

runFirstWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w) Source #

Handle Writer requests by taking the first value provided.

runLastWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w) Source #

Handle Writer requests by overwriting previous values.