Base instances #206
Base instances #206
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robrix
changed the base branch from
separate-carrier-and-effect-modules
to
master
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| {-# LANGUAGE ConstraintKinds #-} | |||
| module Control.Carrier | |||
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We need .hs-boot files for Control.Carrier and Control.Carrier.Class to break the cycles in the module graph.
| import Control.Effect.NonDet (NonDet) | ||
| import Control.Effect.Reader (Reader(..)) | ||
| import Control.Effect.Sum ((:+:)(..)) | ||
| import Control.Effect.Writer (Writer(..)) |
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The payoff: we can import all of these effect modules right where we define the Carrier class.
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| instance Carrier Empty Maybe where | ||
| eff Empty = Nothing |
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And that in turn allows us to implement Carrier instances for these effects without them being orphaned.
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| instance Carrier (Error e) (Either e) where | ||
| eff (Throw e) = Left e | ||
| eff (Catch m h k) = either (k <=< h) k m |
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I called this branch base-instances both because they’re instances for types exported from base—Prelude, even!—and because, like PureC and LiftC, they’re usable as the base of an effectful context. The signature does not mention :+:, since they’re not monad transformers.
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Checking my understanding: I could have an effect stack State s :+: Error e and discharge it into an Either s a with just a runState?
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| instance Carrier (Reader r) ((->) r) where | ||
| eff (Ask k) r = k r r | ||
| eff (Local f m k) r = k (m (f r)) r |
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This allows you to use these instances anywhere expecting the type in question:
id' :: a -> a
id' = askWell, almost anywhere. I can’t figure out how to make this instance available to e.g. Control.Effect.Reader, and I suspect it’s due to some problem with the .hs-boot files. But this function compiles in this module, at least.
| ( Carrier(..) | ||
| ) where | ||
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| import Control.Effect.Class |
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In an effort to fix the aforementioned problem where Carrier (Reader r) ((->) r) isn’t available in e.g. Control.Effect.Reader, I attempted to duplicate the imports and instances here in the .hs-boot file, but it was not successful.
| @@ -11,7 +11,7 @@ module Control.Effect.Choose | |||
| , Choosing(..) | |||
| ) where | |||
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| import Control.Carrier | |||
| import {-# SOURCE #-} Control.Carrier | |||
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These directives indicate that it’s Control.Carrier that gets a .hs-boot file, and not, say, this module.
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| instance Carrier (Error e) (Either e) where | ||
| eff (Throw e) = Left e | ||
| eff (Catch m h k) = either (k <=< h) k m |
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Checking my understanding: I could have an effect stack State s :+: Error e and discharge it into an Either s a with just a runState?
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`.
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Oh, I wasn’t quite ready to merge this I don’t think! No worries, we’ll tidy things up in a follow-up PR. |
This PR builds on #204 to define
Carrierinstances for variousbasetypes, likeEither,(->), andMaybe.