Split Error into Throw & Catch effects #247
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| -- | @m@ is a carrier for @sig@ containing @eff@. | ||
| -- | ||
| -- Note that if @eff@ is a sum, it will be decomposed into multiple 'Member' constraints. While this technically allows one to combine multiple unrelated effects into a single 'Has' constraint, doing so has two significant drawbacks: | ||
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| -- 1. Due to [a problem with recursive type families](https://gitlab.haskell.org/ghc/ghc/issues/8095), this can lead to significantly slower compiles. | ||
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| -- 2. It defeats @ghc@’s warnings for redundant constraints, and thus can lead to a proliferation of redundant constraints as code is changed. | ||
| type Has eff sig m = (Members eff sig, Carrier sig m) |
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We’ve had a rather nasty gotcha lurking since #219 that wasn’t discovered until now: Using a Has constraint with an effect defined as a sum wouldn’t allow ghc to solve Member constraints for the members of that sum. Basically, given Member (A :+: B) sig, ghc wasn’t able to deduce Member A sig, which is necessary for Has A sig m to be solved.
This remained latent because NonDet, the only other effect defined as a sum of other effects, is used exclusively via the Alternative interface—we never use Has NonDet sig m constraints. It surfaced now because the common pattern is to use Error e even when you only need Throw e—in no small part because it wasn’t possible to say solely Throw e until this PR.
The solution is to decompose the sum into a series of individual Member constraints. Note that this technically makes Has capable of expressing multiple unrelated effects in a single constraint (synonym):
foo :: Has (Reader r :+: State i :+: Error e :+: Writer w) sig m => m ()However, as documented here and on Members, this can cause performance problems as well as defeating warnings, so it’s best to avoid it.
src/Control/Effect/Catch.hs
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| -- | Run a computation which can throw errors with a handler to run on error. | ||
| -- | ||
| -- Errors thrown by the handler will escape up to the nearest enclosing 'catchError' (if any). | ||
| -- Note that this effect does /not/ handle errors thrown from impure contexts such as IO, | ||
| -- nor will it handle exceptions thrown from pure code. If you need to handle IO-based errors, | ||
| -- consider if 'Control.Effect.Resource' fits your use case; if not, use 'liftIO' with | ||
| -- 'Control.Exception.try' or use 'Control.Exception.Catch' from outside the effect invocation. |
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I’ve removed the property tests for the time being; they’re being moved in #239 anyway.
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| instance HFunctor Fail | ||
| instance Effect Fail | ||
| type Fail = Throw String |
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There is no longer any reason for Fail to be distinct from Throw!
| newtype ThrowC e m a = ThrowC { runThrowC :: ErrorC e m a } | ||
| deriving (Alternative, Applicative, Functor, Monad, Fail.MonadFail, MonadFix, MonadIO, MonadPlus, MonadTrans) |
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You can, of course, also just use ErrorC directly, but maybe you want to guarantee the absence of Catch from the signature or something.
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I am really quite happy with how this one turned out ✨ |
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| instance HFunctor Fail | ||
| instance Effect Fail | ||
| type Fail = Throw String |
| -- | Decompose sums on the left into multiple 'Member' constraints. | ||
| -- | ||
| -- Note that while this, and by extension 'Control.Carrier.Has', can be used to group together multiple membership checks into a single (composite) constraint, large signatures on the left can slow compiles down due to [a problem with recursive type families](https://gitlab.haskell.org/ghc/ghc/issues/8095). | ||
| type family Members sub sup :: Constraint where |
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Thank you for this explicit kind signature 👍🏻
This PR explores splitting
Errorup into separateThrowandCatcheffects.Error eas a synonym forThrow e :+: Catch e.Failas a synonym forThrow String.Faileffect? #154.