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cdac24c79f
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
100 lines
3.3 KiB
Haskell
100 lines
3.3 KiB
Haskell
{-# LANGUAGE DeriveAnyClass #-}
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module Hasura.RQL.DDL.Webhook.Transform.Method
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( -- * Method transformations
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Method (..),
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TransformFn (..),
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TransformCtx (..),
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MethodTransformFn (..),
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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.CaseInsensitive qualified as CI
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import Data.Text qualified as T
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import Data.Validation
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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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)
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import Hasura.RQL.DDL.Webhook.Transform.Request (RequestTransformCtx)
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-------------------------------------------------------------------------------
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-- | The actual request method 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 Method = Method (CI.CI T.Text)
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deriving stock (Generic)
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deriving newtype (Show, Eq)
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deriving anyclass (NFData)
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instance J.ToJSON Method where
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toJSON = J.String . CI.original . coerce
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instance J.FromJSON Method where
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parseJSON = J.withText "Method" (pure . coerce . CI.mk)
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instance Transform Method where
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-- NOTE: GHC does not let us attach Haddock documentation to data family
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-- instances, so 'MethodTransformFn' is defined separately from this
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-- wrapper.
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newtype TransformFn Method = MethodTransformFn_ MethodTransformFn
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deriving stock (Eq, Generic, Show)
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deriving newtype (NFData, FromJSON, ToJSON)
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newtype TransformCtx Method = TransformCtx RequestTransformCtx
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-- NOTE: GHC does not let us attach Haddock documentation to typeclass
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-- method implementations, so 'applyMethodTransformFn' is defined
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-- separately.
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transform (MethodTransformFn_ fn) (TransformCtx reqCtx) = applyMethodTransformFn fn reqCtx
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-- NOTE: GHC does not let us attach Haddock documentation to typeclass
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-- method implementations, so 'validateMethodTransformFn' is defined
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-- separately.
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validate engine (MethodTransformFn_ fn) = validateMethodTransformFn engine fn
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-- | The defunctionalized transformation on 'Method'.
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newtype MethodTransformFn
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= -- | Replace the HTTP existing 'Method' with a new one.
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Replace Method
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deriving stock (Eq, Generic, Show)
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deriving newtype (NFData, FromJSON, ToJSON)
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-- | Provide an implementation for the transformations defined by
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-- 'MethodTransformFn'.
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--
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-- If one views 'MethodTransformFn' 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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applyMethodTransformFn ::
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MonadError TransformErrorBundle m =>
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MethodTransformFn ->
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RequestTransformCtx ->
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Method ->
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m Method
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applyMethodTransformFn fn _context _oldMethod = case fn of
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Replace newMethod -> pure newMethod
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-- | Validate that the provided 'MethodTransformFn' is correct in the context
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-- 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 'MethodTransformFn'.
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--
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-- XXX: Do we want to validate the HTTP method verb?
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validateMethodTransformFn ::
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TemplatingEngine ->
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MethodTransformFn ->
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Validation TransformErrorBundle ()
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validateMethodTransformFn _engine = \case
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Replace _method -> pure ()
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