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540 lines
21 KiB
Haskell
540 lines
21 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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, mkAnyBackend
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, mapBackend
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, dispatchAnyBackend
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, dispatchAnyBackend'
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, dispatchAnyBackendArrow
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, unpackAnyBackend
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, composeAnyBackend
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, runBackend
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) where
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import Hasura.Prelude
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import Control.Arrow.Extended (ArrowChoice, arr, (|||))
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import Data.Aeson (FromJSON (..), ToJSON (..), Value (..), withObject, (.:?))
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import Data.Hashable (Hashable (hashWithSalt))
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import Data.Kind (Constraint, Type)
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import Language.Haskell.TH hiding (Type)
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import Test.QuickCheck (oneof)
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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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--------------------------------------------------------------------------------
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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 -> pure $ 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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)
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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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-- | 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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-- | 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 [| 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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-- | 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 [| (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 [| (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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(c :: BackendType -> Constraint)
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(i :: BackendType -> Type)
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(r :: Type)
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(arr :: Type -> Type -> Type)
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. (ArrowChoice arr, AllBackendsSatisfy c)
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=> (forall b. c b => arr (i b) r)
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-> arr (BackendChoice i) r
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coalesceChoice arrow =
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$(do
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-- associate the arrow to each type
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arrows <- forEachBackend $ const [| arrow |]
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-- the default case of our fold is `arr absurd` for the terminating Void
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baseCase <- [| arr absurd |]
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-- how to combine two arrows using (|||)
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let combine = \l r -> [| $(pure l) ||| $(pure r) |]
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foldrM combine baseCase arrows
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)
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-- | Dispatch variant for use with arrow syntax. The universally quantified
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-- dispatch function is an arrow instead. Since we can't express this using
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-- Template Haskell, we instead generate the arrow by combining `spreadChoice`
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-- and `coalesceChoice`.
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dispatchAnyBackendArrow'
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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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(arr :: Type -> Type -> Type)
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. (ArrowChoice arr, AllBackendsSatisfy c)
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=> (forall b. c b => arr (i b) r)
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-> arr (AnyBackend i) r
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dispatchAnyBackendArrow' arrow = spreadChoice >>> coalesceChoice @c arrow
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-- | While dispatchAnyBackendArrow' is expressed over an `AnyBackend`, in
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-- practice we need slightly more complex types. Specifically: the only call
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-- site for 'dispatchAnyBackendArrow' uses a four element tuple containing an
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-- 'AnyBackend'.
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newtype BackendArrowTuple x y z i (b :: BackendType) = BackendArrowTuple { unTuple :: (x, y, i b, z) }
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-- | Finally, we can do the dispatch on the four-elements tuple.
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-- Here's what happens, step by step:
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--
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-- ┌─────────────────────────┐
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-- │ (x, y, AnyBackend i, z) │
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-- └─┬───────────────────────┘
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-- │
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-- │ cons
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-- ▼
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-- ┌────────────────────────────────────────┐ ┌─────────────────────────────┐
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-- │ AnyBackend (BackendArrowTuple x y z i) │ ┌───► │ BackendArrowTuple x y z i b │
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-- └─┬──────────────────────────────────────┘ │ └─┬───────────────────────────┘
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-- │ │ │
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-- │ spreadChoice │ │ arr unTuple
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-- ▼ │ ▼
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-- ┌───────────────────────────────────────────┐ │ ┌────────────────┐
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-- │ BackendChoice (BackendArrowTuple x y z i) │ │ │ (x, y, i b, z) │
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-- └─┬─────────────────────────────────────────┘ │ └─┬──────────────┘
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-- │ │ │
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-- │ coalesceChoice (arr unTuple >>> arrow) ◄─────┘ │ arrow
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-- ▼ ▼
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-- ┌───┐ ┌───┐
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-- │ r │ │ r │
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-- └───┘ └───┘
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--
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dispatchAnyBackendArrow
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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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(arr :: Type -> Type -> Type)
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x y z
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. (ArrowChoice arr, AllBackendsSatisfy c)
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=> (forall b. c b => arr (x, y, i b, z) r)
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-> arr (x, y, AnyBackend i, z) r
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dispatchAnyBackendArrow arrow =
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arr cons >>> dispatchAnyBackendArrow' @c (arr unTuple >>> arrow)
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where
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cons :: (x, y, AnyBackend i, z) -> AnyBackend (BackendArrowTuple x y z i)
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cons (x, y, e, z) = mapBackend e \ib -> BackendArrowTuple (x, y, ib, z)
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--------------------------------------------------------------------------------
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-- Instances for 'AnyBackend'
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instance i `SatisfiesForAllBackends` Show => Show (AnyBackend i) where
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show e = dispatchAnyBackend' @Show e show
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instance i `SatisfiesForAllBackends` Eq => Eq (AnyBackend i) where
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e1 == e2 =
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-- generates the following case expression for all backends:
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-- (FooValue a, FooValue b) -> a == b
|
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-- ...
|
|
-- _ -> False
|
|
$(backendCase [| (e1, e2) |]
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-- the pattern for a given backend
|
|
( \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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)
|
|
-- the body for each backend
|
|
( const [| a == b |] )
|
|
-- the default case
|
|
( Just [| False |] )
|
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)
|
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|
|
instance i `SatisfiesForAllBackends` ToJSON => ToJSON (AnyBackend i) where
|
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toJSON e = dispatchAnyBackend' @ToJSON e toJSON
|
|
|
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instance i `SatisfiesForAllBackends` FromJSON => FromJSON (AnyBackend i) where
|
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parseJSON = withObject "AnyBackend" $ \o -> do
|
|
backendKind <- fromMaybe (Postgres Vanilla) <$> o .:? "kind"
|
|
-- generates the following case for all backends:
|
|
-- Foo -> FooValue <$> parseJSON (Object o)
|
|
-- Bar -> BarValue <$> parseJSON (Object o)
|
|
-- ...
|
|
$(backendCase [| backendKind |]
|
|
-- the pattern for a given backend
|
|
( \b -> do
|
|
(con:args) <- pure b
|
|
pure $ ConP con [ConP arg [] | arg <- args]
|
|
)
|
|
-- the body for each backend
|
|
( \b -> do
|
|
let valueCon = pure $ ConE $ getBackendValueName b
|
|
[| $valueCon <$> parseJSON (Object o) |]
|
|
)
|
|
-- no default case
|
|
Nothing
|
|
)
|
|
|
|
instance i `SatisfiesForAllBackends` Hashable => Hashable (AnyBackend i) where
|
|
hashWithSalt salt e = dispatchAnyBackend' @Hashable e (hashWithSalt salt)
|
|
|
|
instance i `SatisfiesForAllBackends` Arbitrary => Arbitrary (AnyBackend i) where
|
|
arbitrary = oneof
|
|
-- generates @FooValue <$> arbitrary@ for each backend
|
|
$(backendList \b -> [| $(pure $ ConE $ getBackendValueName b) <$> arbitrary |])
|