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📝 the RunEffect instances.

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Rob Rix 2017-12-21 10:00:43 -05:00
parent 915d8706f0
commit f842f1423d
1 changed files with 5 additions and 0 deletions
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src/Control/Effect.hs
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@ -39,21 +39,26 @@ class RunEffect f a where
-- | Interpret the topmost effect in a computation with some sensible defaults (defined per-effect), and return the incremental 'Result'.
runEffect :: Eff (f ': fs) a -> Eff fs (Result f a)
-- | 'State' effects with 'Monoid'al states are interpreted starting from the 'mempty' state value into a pair of result value and final state.
instance Monoid b => RunEffect (State b) a where
type Result (State b) a = (a, b)
runEffect = flip runState mempty
-- | 'Reader' effects with 'Monoid'al environments are interpreted starting from the 'mempty' environment value.
instance Monoid b => RunEffect (Reader b) a where
runEffect = flip runReader mempty
-- | 'Fail' effects are interpreted into 'Either' s.t. failures are in 'Left' and successful results are in 'Right'.
instance RunEffect Fail a where
type Result Fail a = Either String a
runEffect = runFail
-- | 'Writer' effects are interpreted into a pair of result value and final log.
instance Monoid w => RunEffect (Writer w) a where
type Result (Writer w) a = (a, w)
runEffect = runWriter
-- | 'NonDetEff' effects are interpreted into a nondeterministic set of result values.
instance Ord a => RunEffect NonDetEff a where
type Result NonDetEff a = Set a
runEffect = relay (pure . point) (\ m k -> case m of