NonEmpty as a pure carrier for Choose. #250
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In keeping with #206, this offers a solo `Carrier` instance for `NonEmpty` lists over the `Choose` effects. Because `NonEmpty` is now in `base`, this doesn't add new dependencies, and nicely rounds out the carrier definition of `[]` for `NonDet` and `Maybe` for `Empty`.
patrickt
commented
Oct 6, 2019
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| -- | The class of carriers (results) for algebras (effect handlers) over signatures (effects), whose actions are given by the 'eff' method. | ||
| class (HFunctor sig, Monad m) => Carrier sig m | m -> sig where | ||
| -- | Construct a value in the carrier for an effect signature (typically a sum of a handled effect and any remaining effects). | ||
| eff :: sig m a -> m a | ||
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| instance Carrier Choose NonEmpty where | ||
| eff (Choose m) = m True S.<> m False |
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The qualified import is the nicest way to handle the fact that GHC 8.2 doesn’t have <> in scope, without mucking up an import with #ifdef or causing warnings on other compiler versions.
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Could also say mappend for that purpose without the extra import, but I’m not particularly fussed.
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NonEmpty isn’t a Monoid!
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Oh wait is mappend a synonym for <> now?
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Oh of course it isn’t! My bad.
robrix
approved these changes
Oct 7, 2019
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| -- | The class of carriers (results) for algebras (effect handlers) over signatures (effects), whose actions are given by the 'eff' method. | ||
| class (HFunctor sig, Monad m) => Carrier sig m | m -> sig where | ||
| -- | Construct a value in the carrier for an effect signature (typically a sum of a handled effect and any remaining effects). | ||
| eff :: sig m a -> m a | ||
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| instance Carrier Choose NonEmpty where | ||
| eff (Choose m) = m True S.<> m False |
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Could also say mappend for that purpose without the extra import, but I’m not particularly fussed.
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In keeping with #206, this offers a solo
Carrierinstance forNonEmptylists over theChooseeffects. BecauseNonEmptyis nowin
base, this doesn't add new dependencies, and nicely rounds outthe carrier definition of
[]forNonDetandMaybeforEmpty.