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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
122 lines
4.6 KiB
Haskell
122 lines
4.6 KiB
Haskell
{-# LANGUAGE ApplicativeDo #-}
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module Hasura.RQL.DDL.Webhook.Transform.QueryParams
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( -- * Query transformations
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QueryParams (..),
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TransformFn (..),
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TransformCtx (..),
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QueryParamsTransformFn (..),
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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.Aeson qualified as J
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import Data.HashMap.Strict qualified as M
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import Data.Validation (Validation)
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import Data.Validation qualified as V
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import Hasura.Prelude
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import Hasura.RQL.DDL.Webhook.Transform.Class
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( TemplatingEngine,
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Transform (..),
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TransformErrorBundle (..),
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UnescapedTemplate (..),
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)
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import Hasura.RQL.DDL.Webhook.Transform.Request
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( RequestTransformCtx,
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runUnescapedRequestTemplateTransform',
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validateRequestUnescapedTemplateTransform',
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)
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import Network.HTTP.Client.Transformable qualified as HTTP
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-------------------------------------------------------------------------------
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-- | The actual query params we are transforming.
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--
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-- This newtype is necessary because otherwise we end up with an
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-- orphan instance.
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newtype QueryParams = QueryParams {unQueryParams :: HTTP.Query}
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instance Transform QueryParams where
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-- NOTE: GHC does not let us attach Haddock documentation to data family
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-- instances, so 'QueryParamsTransformFn' is defined separately from this
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-- wrapper.
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newtype TransformFn QueryParams
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= QueryParamsTransformFn_ QueryParamsTransformFn
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deriving stock (Show, Eq, Generic)
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deriving newtype (NFData, FromJSON, ToJSON)
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newtype TransformCtx QueryParams = TransformCtx RequestTransformCtx
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-- NOTE: GHC does not let us attach Haddock documentation to typeclass
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-- method implementations, so 'applyQueryParamsTransformFn' is defined
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-- separately.
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transform (QueryParamsTransformFn_ fn) (TransformCtx reqCtx) = applyQueryParamsTransformFn fn reqCtx
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-- NOTE: GHC does not let us attach Haddock documentation to typeclass
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-- method implementations, so 'validateQueryParamsTransformFn' is defined
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-- separately.
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validate engine (QueryParamsTransformFn_ fn) =
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validateQueryParamsTransformFn engine fn
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-- | The defunctionalized transformation 'QueryParams'
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newtype QueryParamsTransformFn
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= AddOrReplace [(UnescapedTemplate, Maybe UnescapedTemplate)]
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deriving stock (Eq, Generic, Show)
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deriving newtype (NFData)
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-- | Provide an implementation for the transformations defined by
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-- 'QueryParamsTransformFn'.
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--
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-- If one views 'QueryParamsTransformFn' as an interface describing HTTP method
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-- transformations, this can be seen as an implementation of these
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-- transformations as normal Haskell functions.
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applyQueryParamsTransformFn ::
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MonadError TransformErrorBundle m =>
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QueryParamsTransformFn ->
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RequestTransformCtx ->
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QueryParams ->
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m QueryParams
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applyQueryParamsTransformFn fn context _oldQueryParams = case fn of
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AddOrReplace addOrReplaceParams -> do
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-- NOTE: We use `ApplicativeDo` here to take advantage of Validation's
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-- applicative sequencing
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queryParams <- liftEither . V.toEither $
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for addOrReplaceParams \(rawKey, rawValue) -> do
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key <- runUnescapedRequestTemplateTransform' context rawKey
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value <- traverse (runUnescapedRequestTemplateTransform' context) rawValue
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pure $
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if key == "null" || value == Just "null"
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then Nothing
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else Just (key, value)
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pure $ QueryParams (catMaybes queryParams)
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-- | Validate that the provided 'QueryParamsTransformFn' is correct in the
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-- context of a particular 'TemplatingEngine'.
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--
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-- This is a product of the fact that the correctness of a given transformation
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-- may be dependent on zero, one, or more of the templated transformations
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-- encoded within the given 'QueryParamsTransformFn'.
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validateQueryParamsTransformFn ::
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TemplatingEngine ->
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QueryParamsTransformFn ->
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Validation TransformErrorBundle ()
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validateQueryParamsTransformFn engine = \case
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AddOrReplace addOrReplaceParams ->
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-- NOTE: We use `ApplicativeDo` here to take advantage of
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-- Validation's applicative sequencing
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for_ addOrReplaceParams \(key, val) -> do
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validateRequestUnescapedTemplateTransform' engine key
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traverse_ (validateRequestUnescapedTemplateTransform' engine) val
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-- NOTE: There's a bug in `ApplicativeDo` which infers a `Monad`
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-- constraint on this block if it doens't end with `pure ()`
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pure ()
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{-# ANN validateQueryParamsTransformFn ("HLint: ignore Redundant pure" :: String) #-}
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instance J.ToJSON QueryParamsTransformFn where
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toJSON (AddOrReplace addOrReplace) = J.toJSON $ M.fromList addOrReplace
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instance J.FromJSON QueryParamsTransformFn where
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parseJSON v = AddOrReplace . M.toList <$> J.parseJSON v
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