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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
42 lines
1.3 KiB
Haskell
42 lines
1.3 KiB
Haskell
{-# LANGUAGE TemplateHaskell #-}
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module Hasura.RemoteSchema.Metadata.Permission
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( RemoteSchemaPermissionDefinition (..),
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RemoteSchemaPermissionMetadata (..),
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)
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where
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import Data.Aeson qualified as J
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import Data.Aeson.TH qualified as J
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import Hasura.Prelude
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import Hasura.Session
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import Language.GraphQL.Draft.Printer qualified as G
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import Language.GraphQL.Draft.Syntax qualified as G
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import Text.Builder qualified as TB
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newtype RemoteSchemaPermissionDefinition = RemoteSchemaPermissionDefinition
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{ _rspdSchema :: G.SchemaDocument
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}
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deriving (Show, Eq, Generic)
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instance NFData RemoteSchemaPermissionDefinition
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instance Hashable RemoteSchemaPermissionDefinition
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instance J.FromJSON RemoteSchemaPermissionDefinition where
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parseJSON = J.withObject "RemoteSchemaPermissionDefinition" $ \obj -> do
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fmap RemoteSchemaPermissionDefinition $ obj J..: "schema"
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instance J.ToJSON RemoteSchemaPermissionDefinition where
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toJSON (RemoteSchemaPermissionDefinition schema) =
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J.object $ ["schema" J..= J.String (TB.run . G.schemaDocument $ schema)]
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data RemoteSchemaPermissionMetadata = RemoteSchemaPermissionMetadata
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{ _rspmRole :: RoleName,
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_rspmDefinition :: RemoteSchemaPermissionDefinition,
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_rspmComment :: Maybe Text
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}
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deriving (Show, Eq, Generic)
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$(J.deriveJSON hasuraJSON {J.omitNothingFields = True} ''RemoteSchemaPermissionMetadata)
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