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What is the `Cacheable` type class about? ```haskell class Eq a => Cacheable a where unchanged :: Accesses -> a -> a -> Bool default unchanged :: (Generic a, GCacheable (Rep a)) => Accesses -> a -> a -> Bool unchanged accesses a b = gunchanged (from a) (from b) accesses ``` Its only method is an alternative to `(==)`. The added value of `unchanged` (and the additional `Accesses` argument) arises _only_ for one type, namely `Dependency`. Indeed, the `Cacheable (Dependency a)` instance is non-trivial, whereas every other `Cacheable` instance is completely boilerplate (and indeed either generated from `Generic`, or simply `unchanged _ = (==)`). The `Cacheable (Dependency a)` instance is the only one where the `Accesses` argument is not just passed onwards. The only callsite of the `unchanged` method is in the `ArrowCache (Rule m)` method. That is to say that the `Cacheable` type class is used to decide when we can re-use parts of the schema cache between Metadata operations. So what is the `Cacheable (Dependency a)` instance about? Normally, the output of a `Rule m a b` is re-used when the new input (of type `a`) is equal to the old one. But sometimes, that's too coarse: it might be that a certain `Rule m a b` only depends on a small part of its input of type `a`. A `Dependency` allows us to spell out what parts of `a` are being depended on, and these parts are recorded as values of types `Access a` in the state `Accesses`. If the input `a` changes, but not in a way that touches the recorded `Accesses`, then the output `b` of that rule can be re-used without recomputing. So now you understand _why_ we're passing `Accesses` to the `unchanged` method: `unchanged` is an equality check in disguise that just needs some additional context. But we don't need to pass `Accesses` as a function argument. We can use the `reflection` package to pass it as type-level context. So the core of this PR is that we change the instance declaration from ```haskell instance (Cacheable a) => Cacheable (Dependency a) where ``` to ```haskell instance (Given Accesses, Eq a) => Eq (Dependency a) where ``` and use `(==)` instead of `unchanged`. If you haven't seen `reflection` before: it's like a `MonadReader`, but it doesn't require a `Monad`. In order to pass the current `Accesses` value, instead of simply passing the `Accesses` as a function argument, we need to instantiate the `Given Accesses` context. We use the `give` method from the `reflection` package for that. ```haskell give :: forall r. Accesses -> (Given Accesses => r) -> r unchanged :: (Given Accesses => Eq a) => Accesses -> a -> a -> Bool unchanged accesses a b = give accesses (a == b) ``` With these three components in place, we can delete the `Cacheable` type class entirely. The remainder of this PR is just to remove the `Cacheable` type class and its instances. PR-URL: https://github.com/hasura/graphql-engine-mono/pull/6877 GitOrigin-RevId: 7125f5e11d856e7672ab810a23d5bf5ad176e77f
47 lines
1.4 KiB
Haskell
47 lines
1.4 KiB
Haskell
-- | The 'WithOptional' Functor and associated operations.
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module Hasura.RQL.DDL.Webhook.Transform.WithOptional
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( WithOptional (..),
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withOptional,
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)
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where
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-------------------------------------------------------------------------------
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import Data.Aeson (FromJSON, ToJSON)
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import Data.Coerce (Coercible)
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import Hasura.Prelude
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-------------------------------------------------------------------------------
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-- | Enrich a 'Functor' @f@ with optionality; this is primarily useful when
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-- one wants to annotate fields as optional when using the Higher-Kinded Data
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-- pattern.
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--
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-- 'WithOptional'@ f@ is equivalent to @Compose Maybe f@.
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newtype WithOptional f result = WithOptional
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{ getOptional :: Maybe (f result)
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}
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deriving stock (Eq, Functor, Foldable, Generic, Show)
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deriving newtype (FromJSON, ToJSON)
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deriving newtype instance
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(NFData (f result)) =>
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NFData (WithOptional f result)
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-- | 'WithOptional' smart constructor for the special case of optional values
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-- that are representationally equivalent to some "wrapper" type.
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--
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-- For example:
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-- @
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-- withOptional \@HeaderTransformsAction headers == WithOptional $ fmap HeadersTransform headers
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-- @
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--
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-- In other words: this function observes the isomorphism between @'Maybe' a@
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-- and @'WithOptional' f b@ if an isomorphism exists between @a@ and @f b@.
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withOptional ::
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forall a b f.
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Coercible a (f b) =>
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Maybe a ->
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WithOptional f b
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withOptional = coerce
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