Control.Monad.Raise

Monadic raise semantics & helpers

Documentation

ensure :: (MonadRaise m, Raises m inner) => Either inner a -> m a

Lift a pure error (Either) into a MonadRaise context i.e. Turn Lefts into raises.

Examples

>>> :{
  mayFail :: Int -> Either FooErr Int
  mayFail n =
    if n > 50
      then Left FooErr
      else Right n
:}

>>> :{
  goesBoom :: (MonadRaise m, m `Raises` FooErr) => m Int
  goesBoom = do
    first  <- ensure $ mayFail 100
    second <- ensure $ mayFail 42
    return $ second * 10
:}

>>> goesBoom :: Result '[FooErr, BarErr] Int
Left (Identity FooErr)

ensureM :: (MonadRaise m, Raises m inner) => m (Either inner a) -> m a

A version of ensure that takes monadic actions

Examples

>>> :{
  mayFailM :: Monad m => Int -> m (Either (OpenUnion '[FooErr, BarErr]) Int)
  mayFailM n =
    return $ if n > 50
      then Left (openUnionLift FooErr)
      else Right n
:}

>>> :{
  foo :: (MonadRaise m, RaisesOnly m '[FooErr, BarErr]) => m Int
  foo = do
    first  <- ensureM $ mayFailM 100
    second <- ensureM $ mayFailM first
    return (second * 10)
:}

>>> runRescue (foo :: Rescue '[FooErr, BarErr] Int)
Left (Identity FooErr)

Class Reexports

module Control.Monad.Raise.Class

module Control.Monad.Raise.Constraint

Data Reexports

module Data.WorldPeace.Subset.Class