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This commit applies ormolu to the whole Haskell code base by running `make format`. For in-flight branches, simply merging changes from `main` will result in merge conflicts. To avoid this, update your branch using the following instructions. Replace `<format-commit>` by the hash of *this* commit. $ git checkout my-feature-branch $ git merge <format-commit>^ # and resolve conflicts normally $ make format $ git commit -a -m "reformat with ormolu" $ git merge -s ours post-ormolu https://github.com/hasura/graphql-engine-mono/pull/2404 GitOrigin-RevId: 75049f5c12f430c615eafb4c6b8e83e371e01c8e
601 lines
24 KiB
Haskell
601 lines
24 KiB
Haskell
{-# LANGUAGE Arrows #-}
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{-# LANGUAGE UndecidableInstances #-}
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module Hasura.SQL.AnyBackend
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( AnyBackend,
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liftTag,
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mkAnyBackend,
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mapBackend,
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traverseBackend,
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dispatchAnyBackend,
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dispatchAnyBackend',
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dispatchAnyBackendArrow,
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dispatchAnyBackendWithTwoConstraints,
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unpackAnyBackend,
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composeAnyBackend,
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runBackend,
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parseAnyBackendFromJSON,
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debugAnyBackendToJSON,
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)
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where
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import Control.Arrow.Extended (ArrowChoice, arr, (|||))
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import Data.Aeson
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import Data.Aeson.Types (Parser)
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import Data.Kind (Constraint, Type)
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import Hasura.Incremental (Cacheable)
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import Hasura.Prelude
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import Hasura.SQL.Backend
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import Hasura.SQL.TH
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import Hasura.SQL.Tag
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import Language.Haskell.TH hiding (Type)
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--------------------------------------------------------------------------------
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-- Types and constraints
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-- | This type is essentially an unlabeled box for types indexed by BackendType.
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-- Given some type defined as 'data T (b :: BackendType) = ...', we can define
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-- 'AnyBackend T' without mentioning any 'BackendType'.
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--
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-- This is useful for having generic containers of potentially different types
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-- of T. For instance, @SourceCache@ is defined as a
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-- @HashMap SourceName (AnyBackend SourceInfo)@.
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--
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-- This type is generated with Template Haskell to have one constructor per
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-- backend. This declaration generates the following type:
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--
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-- data AnyBackend (i :: BackendType -> Type)
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-- = PostgresValue (i 'Postgres)
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-- | MSSQLValue (i 'MSSQL)
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-- | ...
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$( do
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-- the kind of the type variable, expressed with a quote
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varKind <- [t|BackendType -> Type|]
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-- how to build a basic type: no UNPACK, no strict!, just a name
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let normalType = (Bang NoSourceUnpackedness NoSourceStrictness,)
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-- the name of the type variable
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let typeVarName = mkName "i"
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backendData
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-- the name of the type
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(mkName "AnyBackend")
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-- the type variable
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[KindedTV typeVarName varKind]
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-- the constructor for each backend
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( \b ->
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pure $
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NormalC
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-- the name of the constructor: `FooValue`
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(getBackendValueName b)
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-- one argument: `i 'Foo`
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-- (we Apply a type Variable to a Promoted name)
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[normalType $ AppT (VarT typeVarName) (getBackendTypeValue b)]
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)
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-- classes in the deriving clause
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[''Generic]
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)
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-- | Generates a constraint for all backends.
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-- This Template Haskell expression generates the following constraint type:
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--
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-- type AllBackendsSatisfy (c :: BackendType -> Constraint) =
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-- ( c 'Postgres
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-- , c 'MSSQL
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-- , ...
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-- )
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--
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-- That is, given a class C, this creates the constraint that dictates that all
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-- backend must satisfy C.
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type AllBackendsSatisfy (c :: BackendType -> Constraint) =
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$( do
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-- the constraint for each backend: `c 'Foo`
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-- (we Apply a type Variable to a Promoted name)
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constraints <- forEachBackend \b ->
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pure $ AppT (VarT $ mkName "c") (getBackendTypeValue b)
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-- transforms a list of constraints into a tuple of constraints
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-- by folding the "type application" constructor:
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--
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-- > apply (,,) [c 'Foo, c 'Bar, c 'Baz]
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-- > apply (c 'Foo,,) [c 'Bar, c 'Baz]
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-- > apply (c 'Foo, c 'Bar,) [c 'Baz]
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-- > apply (c 'Foo, c 'Bar, c 'Baz) []
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-- = (c 'Foo, c 'Bar, c 'Baz)
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let tupleConstructor = TupleT $ length constraints
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pure $ foldl AppT tupleConstructor constraints
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)
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-- | Generates a constraint for a generic type over all backends.
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-- This Template Haskell expression generates the following constraint type:
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--
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-- type SatisfiesForAllBackends
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-- (i :: BackendType -> Type)
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-- (c :: Type -> Constraint)
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-- = ( c (i 'Postgres)
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-- , c (i 'MSSQL)
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-- , ...
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-- )
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--
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-- That is, given a type I and a class C, this creates the constraint that
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-- dictates that for all backends b, @I b@ must satisfy C.
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type SatisfiesForAllBackends
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(i :: BackendType -> Type)
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(c :: Type -> Constraint) =
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$( do
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-- the constraint for each backend: `c (i 'Foo)`
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constraints <- forEachBackend \b ->
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pure $ AppT (VarT $ mkName "c") $ AppT (VarT $ mkName "i") (getBackendTypeValue b)
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-- transforms a list of constraints into a tuple of constraints
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-- by folding the type application constructor
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-- by folding the "type application" constructor:
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--
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-- > apply (,,) [c (i 'Foo), c (i 'Bar), c (i 'Baz)]
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-- > apply (c (i 'Foo),,) [c (i 'Bar), c (i 'Baz)]
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-- > apply (c (i 'Foo), c (i 'Bar),) [c (i 'Baz)]
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-- > apply (c (i 'Foo), c (i 'Bar), c (i 'Baz)) []
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-- = (c (i 'Foo), c (i 'Bar), c (i 'Baz))
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let tupleConstructor = TupleT $ length constraints
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pure $ foldl AppT tupleConstructor constraints
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)
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--------------------------------------------------------------------------------
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-- Functions on AnyBackend
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-- | How to obtain a tag from a runtime value. This function is generated with
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-- Template Haskell for each 'Backend'. The case switch looks like this:
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--
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-- Postgres -> PostgresValue PostgresTag
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-- MSSQL -> MSSQLValue MSSQLTag
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-- ...
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liftTag :: BackendType -> AnyBackend BackendTag
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liftTag t =
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$( backendCase
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-- the expression on which we do the case switch
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[|t|]
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-- the pattern for a given backend: the backend type itself
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(\(con :| args) -> pure $ ConP con [ConP a [] | a <- args])
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-- the body for a given backend: creating and wrapping the tag
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(\b -> [|$(pure $ ConE $ getBackendValueName b) $(pure $ ConE $ getBackendTagName b)|])
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-- no default case: every constructor should be handled
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Nothing
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)
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-- | Transforms an `AnyBackend i` into an `AnyBackend j`.
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mapBackend ::
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forall
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(i :: BackendType -> Type)
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(j :: BackendType -> Type).
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AnyBackend i ->
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(forall b. i b -> j b) ->
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AnyBackend j
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mapBackend e f =
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-- generates a case switch that, for each constructor, applies the provided function
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-- case e of
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-- FooValue x -> FooValue (f x)
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-- BarValue x -> BarValue (f x)
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$( do
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-- we create a case match for each backend
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matches <- forEachBackend \b -> do
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-- the name of the constructor
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let consName = getBackendValueName b
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-- the patterrn we match: `FooValue x`
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let matchPattern = ConP consName [VarP $ mkName "x"]
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-- the body of the match: `FooValue (f x)`
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matchBody <- [|$(pure $ ConE consName) (f x)|]
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pure $ Match matchPattern (NormalB matchBody) []
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-- the expression on which we do the case
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caseExpr <- [|e|]
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-- return the the expression of the case switch
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pure $ CaseE caseExpr matches
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)
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-- | Traverse an `AnyBackend i` into an `f (AnyBackend j)`.
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traverseBackend ::
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forall
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(c :: BackendType -> Constraint)
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(i :: BackendType -> Type)
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(j :: BackendType -> Type)
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f.
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(AllBackendsSatisfy c, Applicative f) =>
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AnyBackend i ->
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(forall b. c b => i b -> f (j b)) ->
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f (AnyBackend j)
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traverseBackend e f =
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-- generates a case switch that, for each constructor, applies the provided function
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-- case e of
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-- FooValue x -> FooValue <$> f x
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-- BarValue x -> BarValue <$> f x
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$( do
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-- we create a case match for each backend
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matches <- forEachBackend \b -> do
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-- the name of the constructor
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let consName = getBackendValueName b
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-- the patterrn we match: `FooValue x`
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let matchPattern = ConP consName [VarP $ mkName "x"]
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-- the body of the match: `FooValue <$> f x`
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matchBody <- [|$(pure $ ConE consName) <$> f x|]
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pure $ Match matchPattern (NormalB matchBody) []
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-- the expression on which we do the case
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caseExpr <- [|e|]
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-- return the the expression of the case switch
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pure $ CaseE caseExpr matches
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)
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-- | Creates a new @AnyBackend i@ for a given backend @b@ by wrapping the given @i b@.
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mkAnyBackend ::
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forall
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(b :: BackendType)
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(i :: BackendType -> Type).
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HasTag b =>
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i b ->
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AnyBackend i
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mkAnyBackend =
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-- generates a case switch that associates a tag constructor to a value constructor
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-- case backendTag @b of
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-- FooTag -> FooValue
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-- BarTag -> BarValue
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$( backendCase
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[|backendTag @b|]
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-- the pattern for a backend
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(\b -> pure $ ConP (getBackendTagName b) [])
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-- the body for a backend
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(pure . ConE . getBackendValueName)
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-- no default case
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Nothing
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)
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-- | Dispatch a function to the value inside the @AnyBackend@, that does not
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-- require bringing into scope a new class constraint.
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runBackend ::
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forall
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(i :: BackendType -> Type)
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(r :: Type).
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AnyBackend i ->
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(forall (b :: BackendType). i b -> r) ->
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r
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runBackend b f = $(mkDispatch 'f 'b)
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-- | Dispatch an existential using an universally quantified function while
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-- also resolving a different constraint.
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-- Use this to dispatch Backend* instances.
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-- This is essentially a wrapper around 'runAnyBackend f . repackAnyBackend @c'.
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dispatchAnyBackend ::
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forall
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(c :: BackendType -> Constraint)
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(i :: BackendType -> Type)
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(r :: Type).
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AllBackendsSatisfy c =>
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AnyBackend i ->
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(forall (b :: BackendType). c b => i b -> r) ->
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r
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dispatchAnyBackend e f = $(mkDispatch 'f 'e)
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dispatchAnyBackendWithTwoConstraints ::
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forall
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(c1 :: BackendType -> Constraint)
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(c2 :: BackendType -> Constraint)
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(i :: BackendType -> Type)
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(r :: Type).
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AllBackendsSatisfy c1 =>
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AllBackendsSatisfy c2 =>
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AnyBackend i ->
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(forall (b :: BackendType). c1 b => c2 b => i b -> r) ->
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r
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dispatchAnyBackendWithTwoConstraints e f = $(mkDispatch 'f 'e)
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-- | Unlike 'dispatchAnyBackend', the expected constraint has a different kind.
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-- Use for classes like 'Show', 'ToJSON', etc.
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dispatchAnyBackend' ::
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forall
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(c :: Type -> Constraint)
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(i :: BackendType -> Type)
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(r :: Type).
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i `SatisfiesForAllBackends` c =>
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AnyBackend i ->
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(forall (b :: BackendType). c (i b) => i b -> r) ->
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r
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dispatchAnyBackend' e f = $(mkDispatch 'f 'e)
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-- | Sometimes we need to run operations on two backends of the same type.
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-- If the backends don't contain the same type, the given 'r' value is returned.
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-- Otherwise, the function is called with the two wrapped values.
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composeAnyBackend ::
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forall
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(c :: BackendType -> Constraint)
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(i :: BackendType -> Type)
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(r :: Type).
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AllBackendsSatisfy c =>
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(forall (b :: BackendType). c b => i b -> i b -> r) ->
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AnyBackend i ->
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AnyBackend i ->
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r ->
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r
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composeAnyBackend f e1 e2 owise =
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-- generates the following case expression for all backends:
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-- (FooValue a, FooValue b) -> f a b
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-- (BarValue a, BarValue b) -> f a b
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-- ...
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-- _ -> owise
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$( backendCase
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[|(e1, e2)|]
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-- the pattern for a given backend: `(FooValue a, FooValue b)`
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( \b -> do
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let valueCon n = pure $ ConP (getBackendValueName b) [VarP $ mkName n]
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[p|($(valueCon "a"), $(valueCon "b"))|]
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)
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-- the body for each backend: `f a b`
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(const [|f a b|])
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-- the default case
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(Just [|owise|])
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)
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-- | Try to unpack the type of an existential.
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-- Returns @Just x@ upon a succesful match, @Nothing@ otherwise.
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unpackAnyBackend ::
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forall
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(b :: BackendType)
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(i :: BackendType -> Type).
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HasTag b =>
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AnyBackend i ->
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Maybe (i b)
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unpackAnyBackend exists =
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-- generates the following case expression for all backends:
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-- (FooTag, FooValue a) -> Just a
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-- ...
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-- _ -> Nothing
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$( backendCase
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[|(backendTag @b, exists)|]
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-- the pattern for a given backend
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( \b -> do
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let tagConstructor = pure $ ConP (getBackendTagName b) []
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valConstructor = pure $ ConP (getBackendValueName b) [VarP $ mkName "a"]
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[p|($tagConstructor, $valConstructor)|]
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)
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-- the body for each backend
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(const [|Just a|])
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-- the default case
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(Just [|Nothing|])
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)
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--------------------------------------------------------------------------------
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-- Special case for arrows
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-- Sadly, we CAN'T mix template haskell and arrow syntax... Meaning we can't
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-- generate a `backendCase` within proc syntax. What we have to do instead is to
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-- MANUALLY DESUGAR the arrow code, to manually construct the following
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-- pipeline.
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--
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-- ┌────────────┐ ┌────────────────────┐ ┌───┐
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-- │ AnyBackend ├─┬──────►│ Left PostgresValue ├───────────────►│ f ├────────┐
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-- └────────────┘ │ └────────────────────┘ └───┘ │
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-- │ │
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-- │ ┌─────────────────────────┐ ┌───┐ │
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-- └─┬────►│ Right (Left MSSQLValue) ├──────────►│ f ├─────┐ │
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-- │ └─────────────────────────┘ └───┘ │ │
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-- │ │ │
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-- │ ┌─────────────────────────────────┐ ┌───┐ │ │
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-- └─┬──►│ Right (Right (Left MongoValue)) ├───┤ f ├──┐ │ │
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-- │ └─────────────────────────────────┘ └───┘ │ │ │
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-- │ │ │ │
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-- │ ┌───────────────────────────┐ ┌───┐ │ │ │ ┌───┐
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-- └──►│ Right (Right (Right ...)) ├─────────┤ f ├──┴──┴──┴─►│ r │
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-- └───────────────────────────┘ └───┘ └───┘
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--
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-- This is what, internally, GHC would translate an arrow case-switch into: the
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-- only tool it has is:
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-- (|||) :: a b d -> a c d -> a (Either b c) d
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--
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-- It must therefore encode the case switch as an arrow from the original value
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-- to this tree of Either, and then coalesce them using (|||). This is what we
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-- do here.
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-- | First, we create a type to represent our complicated Either type. We use
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-- `Void` as a terminating case for our recursion. This declaration creates the
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-- following type:
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--
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-- type BackendChoice (i :: BackendType -> Type)
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-- = Either (i 'Postgres)
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-- ( Either (i 'MSSQL)
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-- ( Either ...
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-- Void
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type BackendChoice (i :: BackendType -> Type) =
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$( do
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-- creates the type (i b) for each backend b
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types <- forEachBackend \b ->
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pure $ AppT (VarT $ mkName "i") (getBackendTypeValue b)
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-- generate the either type by folding over that list
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let appEither l r = [t|Either $(pure l) $(pure r)|]
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foldrM appEither (ConT ''Void) types
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)
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-- | Spread a 'AnyBackend' into a 'BackendChoice'.
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--
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-- Given backends Foo, Bar, Baz, the type of `BackendChoice c` will be:
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-- ( Either (c 'Foo)
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-- ( Either (c 'Bar)
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-- ( Either (c 'Baz)
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-- Void )))
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--
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-- Accordingly, the following Template Haskell splice generates the following code:
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--
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-- case e of
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-- FooValue x -> Left x
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-- BarValue x -> Right (Left x)
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-- BazValue x -> Right (Right (Left x))
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spreadChoice ::
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forall
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(i :: BackendType -> Type)
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(arr :: Type -> Type -> Type).
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(ArrowChoice arr) =>
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arr (AnyBackend i) (BackendChoice i)
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spreadChoice = arr $ \e ->
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$( do
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-- to each backend we match a 'BackendChoice' constructor
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-- in order: Left, Right . Left, Right . Right . Left...
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let choiceCons = iterate (UInfixE (ConE 'Right) (VarE '(.))) (ConE 'Left)
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backendCons <- backendConstructors
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-- we then construct the case match for each of them
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matches <- for (zip backendCons choiceCons) \(b, c) -> do
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-- name of the constructor: FooValue
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let consName = getBackendValueName b
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-- pattern of the match: `FooValue x`
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let matchPattern = ConP consName [VarP $ mkName "x"]
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-- expression of the match: applying the 'BackendChoice' constructor to x
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matchBody <- [|$(pure c) x|]
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pure $ Match matchPattern (NormalB matchBody) []
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-- the expression on which we do the case
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caseExpr <- [|e|]
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-- we return the case expression
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pure $ CaseE caseExpr matches
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)
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-- | Coalesce a 'BackendChoice' into a result, given an arrow from each
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-- possibilty to a common result.
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--
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-- Given backends Foo, Bar, Baz, the type of `BackendChoice c` will be:
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-- ( Either (c 'Foo)
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-- ( Either (c 'Bar)
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-- ( Either (c 'Baz)
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-- Void )))
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--
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-- Accordingly, the following Template Haskell splice generates the following code:
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--
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-- ( arrow |||
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-- ( arrow |||
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-- ( arrow |||
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-- absurd )))
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coalesceChoice ::
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forall
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(c1 :: BackendType -> Constraint)
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(c2 :: BackendType -> Constraint)
|
|
(i :: BackendType -> Type)
|
|
(r :: Type)
|
|
(arr :: Type -> Type -> Type).
|
|
(ArrowChoice arr, AllBackendsSatisfy c1, AllBackendsSatisfy c2) =>
|
|
(forall b. c1 b => c2 b => arr (i b) r) ->
|
|
arr (BackendChoice i) r
|
|
coalesceChoice arrow =
|
|
$( do
|
|
-- associate the arrow to each type
|
|
arrows <- forEachBackend $ const [|arrow|]
|
|
-- the default case of our fold is `arr absurd` for the terminating Void
|
|
baseCase <- [|arr absurd|]
|
|
-- how to combine two arrows using (|||)
|
|
let combine = \l r -> [|$(pure l) ||| $(pure r)|]
|
|
foldrM combine baseCase arrows
|
|
)
|
|
|
|
-- | Dispatch variant for use with arrow syntax. The universally quantified
|
|
-- dispatch function is an arrow instead. Since we can't express this using
|
|
-- Template Haskell, we instead generate the arrow by combining `spreadChoice`
|
|
-- and `coalesceChoice`.
|
|
dispatchAnyBackendArrow' ::
|
|
forall
|
|
(c1 :: BackendType -> Constraint)
|
|
(c2 :: BackendType -> Constraint)
|
|
(i :: BackendType -> Type)
|
|
(r :: Type)
|
|
(arr :: Type -> Type -> Type).
|
|
(ArrowChoice arr, AllBackendsSatisfy c1, AllBackendsSatisfy c2) =>
|
|
(forall b. c1 b => c2 b => arr (i b) r) ->
|
|
arr (AnyBackend i) r
|
|
dispatchAnyBackendArrow' arrow = spreadChoice >>> coalesceChoice @c1 @c2 arrow
|
|
|
|
-- | While dispatchAnyBackendArrow' is expressed over an `AnyBackend`, in
|
|
-- practice we need slightly more complex types. Specifically: the only call
|
|
-- site for 'dispatchAnyBackendArrow' uses a four element tuple containing an
|
|
-- 'AnyBackend'.
|
|
newtype BackendArrowTuple x i (b :: BackendType) = BackendArrowTuple {unTuple :: (i b, x)}
|
|
|
|
-- | Finally, we can do the dispatch on the four-elements tuple.
|
|
-- Here's what happens, step by step:
|
|
--
|
|
-- ┌─────────────────────────┐
|
|
-- │ (x, y, AnyBackend i, z) │
|
|
-- └─┬───────────────────────┘
|
|
-- │
|
|
-- │ cons
|
|
-- ▼
|
|
-- ┌────────────────────────────────────────┐ ┌─────────────────────────────┐
|
|
-- │ AnyBackend (BackendArrowTuple x y z i) │ ┌───► │ BackendArrowTuple x y z i b │
|
|
-- └─┬──────────────────────────────────────┘ │ └─┬───────────────────────────┘
|
|
-- │ │ │
|
|
-- │ spreadChoice │ │ arr unTuple
|
|
-- ▼ │ ▼
|
|
-- ┌───────────────────────────────────────────┐ │ ┌────────────────┐
|
|
-- │ BackendChoice (BackendArrowTuple x y z i) │ │ │ (x, y, i b, z) │
|
|
-- └─┬─────────────────────────────────────────┘ │ └─┬──────────────┘
|
|
-- │ │ │
|
|
-- │ coalesceChoice (arr unTuple >>> arrow) ◄─────┘ │ arrow
|
|
-- ▼ ▼
|
|
-- ┌───┐ ┌───┐
|
|
-- │ r │ │ r │
|
|
-- └───┘ └───┘
|
|
--
|
|
-- NOTE: The below function accepts two constraints, if the arrow
|
|
-- you want to dispatch only has one constraint then repeat the constraint twice.
|
|
-- For example:
|
|
-- ```AB.dispatchAnyBackendArrow @BackendMetadata @BackendMetadata (proc (sourceMetadata, invalidationKeys)```
|
|
dispatchAnyBackendArrow ::
|
|
forall
|
|
(c1 :: BackendType -> Constraint)
|
|
(c2 :: BackendType -> Constraint)
|
|
(i :: BackendType -> Type)
|
|
(r :: Type)
|
|
(arr :: Type -> Type -> Type)
|
|
x.
|
|
(ArrowChoice arr, AllBackendsSatisfy c1, AllBackendsSatisfy c2) =>
|
|
(forall b. c1 b => c2 b => arr (i b, x) r) ->
|
|
arr (AnyBackend i, x) r
|
|
dispatchAnyBackendArrow arrow =
|
|
arr cons >>> dispatchAnyBackendArrow' @c1 @c2 (arr unTuple >>> arrow)
|
|
where
|
|
cons :: (AnyBackend i, x) -> AnyBackend (BackendArrowTuple x i)
|
|
cons (e, x) = mapBackend e \ib -> BackendArrowTuple (ib, x)
|
|
|
|
--------------------------------------------------------------------------------
|
|
-- JSON functions
|
|
|
|
-- | Attempts to parse an 'AnyBackend' from a JSON value, using the provided
|
|
-- backend information.
|
|
parseAnyBackendFromJSON ::
|
|
i `SatisfiesForAllBackends` FromJSON =>
|
|
BackendType ->
|
|
Value ->
|
|
Parser (AnyBackend i)
|
|
parseAnyBackendFromJSON backendKind value = do
|
|
-- generates the following case for all backends:
|
|
-- Foo -> FooValue <$> parseJSON value
|
|
-- Bar -> BarValue <$> parseJSON value
|
|
-- ...
|
|
$( backendCase
|
|
[|backendKind|]
|
|
-- the pattern for a given backend
|
|
(\(con :| args) -> pure $ ConP con [ConP arg [] | arg <- args])
|
|
-- the body for each backend
|
|
( \b -> do
|
|
let valueCon = pure $ ConE $ getBackendValueName b
|
|
[|$valueCon <$> parseJSON value|]
|
|
)
|
|
-- no default case
|
|
Nothing
|
|
)
|
|
|
|
-- | Outputs a debug JSON value from an 'AnyBackend'. This function must only be
|
|
-- used for debug purposes, as it has no way of inserting the backend kind in
|
|
-- the output, since there's no guarantee that the output will be an object.
|
|
debugAnyBackendToJSON ::
|
|
i `SatisfiesForAllBackends` ToJSON =>
|
|
AnyBackend i ->
|
|
Value
|
|
debugAnyBackendToJSON e = dispatchAnyBackend' @ToJSON e toJSON
|
|
|
|
--------------------------------------------------------------------------------
|
|
-- Instances for 'AnyBackend'
|
|
|
|
deriving instance i `SatisfiesForAllBackends` Show => Show (AnyBackend i)
|
|
|
|
deriving instance i `SatisfiesForAllBackends` Eq => Eq (AnyBackend i)
|
|
|
|
instance i `SatisfiesForAllBackends` Hashable => Hashable (AnyBackend i)
|
|
|
|
instance i `SatisfiesForAllBackends` Cacheable => Cacheable (AnyBackend i)
|