graphql-engine/server/src-lib/Hasura/GraphQL/Schema/Remote.hs

{-# LANGUAGE ViewPatterns #-}

module Hasura.GraphQL.Schema.Remote
  ( buildRemoteParser
  , remoteField
  ) where

import           Hasura.Prelude

import qualified Data.HashMap.Strict                         as Map
import qualified Data.HashMap.Strict.InsOrd                  as OMap
import qualified Data.HashMap.Strict.InsOrd.Extended         as OMap
import qualified Data.List.NonEmpty                          as NE
import qualified Language.GraphQL.Draft.Syntax               as G

import           Data.Monoid                                 (Any (..))
import           Data.Text.Extended
import           Data.Type.Equality

import qualified Hasura.GraphQL.Parser.Internal.Parser       as P
import qualified Hasura.GraphQL.Parser.Internal.TypeChecking as P

import           Hasura.Base.Error
import           Hasura.GraphQL.Parser                       as P
import           Hasura.RQL.Types.RemoteSchema
import           Hasura.RQL.Types.SchemaCache


--------------------------------------------------------------------------------
-- Top level function

buildRemoteParser
  :: forall m n
   . (MonadSchema n m, MonadError QErr m)
  => IntrospectionResult
  -> RemoteSchemaInfo
  -> m ( [P.FieldParser n RemoteField]
       , Maybe [P.FieldParser n RemoteField]
       , Maybe [P.FieldParser n RemoteField]
       )
buildRemoteParser (IntrospectionResult sdoc queryRoot mutationRoot subscriptionRoot) info = do
  queryT <- makeParsers queryRoot
  mutationT <- makeNonQueryRootFieldParser mutationRoot $$(G.litName "Mutation")
  subscriptionT <- makeNonQueryRootFieldParser subscriptionRoot $$(G.litName "Subscription")
  return (queryT, mutationT, subscriptionT)
  where
    makeFieldParser :: G.FieldDefinition RemoteSchemaInputValueDefinition -> m (P.FieldParser n RemoteField)
    makeFieldParser fieldDef = do
      fldParser <- remoteFieldFromDefinition sdoc fieldDef
      pure $ (RemoteFieldG info) <$> fldParser
    makeParsers :: G.Name -> m [P.FieldParser n RemoteField]
    makeParsers rootName =
      case lookupType sdoc rootName of
        Just (G.TypeDefinitionObject o) ->
          traverse makeFieldParser $ G._otdFieldsDefinition o
        _ -> throw400 Unexpected $ rootName <<> " has to be an object type"

    -- | The spec says that the `schema` definition can be omitted, if the root names are the
    -- defaults (Query, Mutation and Subscription). This function is used to construct a
    -- `FieldParser` for the mutation and subscription roots.  If the user has given a custom
    -- Mutation/Subscription root name, then it will look for that and if it's not found in the
    -- schema document, then an error is thrown.  If no root name has been provided, we lookup the
    -- schema document for an object with the default name and if that's not found, we omit the said
    -- Root from the schema.
    makeNonQueryRootFieldParser :: Maybe G.Name -> G.Name -> m (Maybe [P.FieldParser n RemoteField])
    makeNonQueryRootFieldParser userProvidedRootName defaultRootName =
      case userProvidedRootName of
        Just _rootName -> traverse makeParsers userProvidedRootName
        Nothing ->
          let isDefaultRootObjectExists = isJust $ lookupObject sdoc defaultRootName
          in bool (pure Nothing) (traverse makeParsers $ Just defaultRootName) $ isDefaultRootObjectExists



--------------------------------------------------------------------------------
-- Remote schema input parsers

{- Note [Variable expansion in remote schema input parsers]

### Input parsers as lightweight type checkers

The purpose of input parsers for remote schemas is not to translate the provided input values into
an internal representation: those values will be transmitted more or less unmodified to the remote
service; their main purpose is simply to check the shape of the input against the remote schema.

Consider, for instance, the following remote schema:

    input Foo {
      bar: Int!
    }

    type Query {
      run(foo: Foo!): Int!
    }


Our parsers will need to decide which invocations of `run` are valid:

    query {
      run(null)             # invalid: foo is non-nullable
      run(foo: {baz: 0})    # invalid: Foo doesn't have a "baz" field
      run(foo: {bar: "0"})  # actually valid!
    }


That last example is surprising: why would we accept a string literal for an Int? It simply is
because we delegate the task of translating the literal into a scalar to the remote server. After
all, *we* advertise some values as Int in the schema, despite accepting string literals.


### Inserting remote permissions presets

Where things get more complicated is with remote permissions. We allow users to specify "presets":
values that will always be provided to the remote schema, and that the user cannot customize in
their query. For instance, given the following schema with permissions:

    input Range {
      low:  Int! @preset(value: 0)
      high: Int!
    }

    type Query {
      getValues(range: Range!): [Int]
    }


a user cannot specify "low" in OUR schema, as we will insert its value when parsing the incoming
query. This is the second purpose of those input parsers: they insert remote schema presets where
required. In this case:

    # we receive
    query {
      getValues(range: {high: 42})
    }

    # we emit
    query {
      getValues(range: {low: 0, high: 42})
    }


### Variable expansion

But where this gets even more complicated is with variables. As much as possible, we simply forward
variables without interpeting them (not all JSON values are representable in GraphQL). We do so
whenever possible; for instance, using the previously established remote schema:

    # we receive
    query:
      query($h: Int!) {
        getValues(range: {high: $h})
      }
    variables:
      { "h": 42 }

    # we emit
    query:
      query($h: Int!) {
        getValues(range: {low: 0, high: $h})
      }
    variables:
      { "h": 42 }


The tricky case is when a preset field is *within a variable*. We then have no choice: we have to
expand the variable, and rewrap the value as best as we can, to minimize the amount of JSON
evaluation. For instance:

    # we receive
    query:
      query($r: Range!) {
        getValues(range: $r)
      }
    variables:
      { "r": {"high": 42} }

    # we emit
    query:
      query($hasura_json_var_1: Int!) {
        getValues(range: {low: 0, high: $hasura_json_var_1})
      }
    variables:
      { "hasura_json_var_1": 42 }


Our parsers, like all others in our model, expand the variables as they traverse the tree, and add
the preset values where required. But the downside of this is that we will create one such JSON
variable per scalar within a JSON variable!


### Short-circuiting optimization

To avoid this, we track in the parsers whether an alteration has occured: if we had to insert a
preset value. As long as we don't, we can discard the output of the parser, as it will contain the
exact same value as the input (if perhaps represented differently); by discarding the output and
just forwarding the input, we avoid expanding variables if no preset needs be inserted.
-}


-- | Helper, used to track whether an input value was altered during its parsing. At time of
-- writing, the only possible source of alteration is preset values. They might force evaluation of
-- variables, and encapsulation of sub-JSON expressions as new variables. Each parser indicates
-- whether such alteration took place within its part of the tree.
-- See Note [Variable expansion in remote schema input parsers] for more information.

newtype Altered = Altered { getAltered :: Bool }
  deriving (Show)
  deriving (Semigroup, Monoid) via Any


-- | 'inputValueDefinitionParser' accepts a 'G.InputValueDefinition' and will return an
-- 'InputFieldsParser' for it. If a non 'Input' GraphQL type is found in the 'type' of the
-- 'InputValueDefinition' then an error will be thrown.
--
-- Each parser also returns a boolean that indicates whether the parsed value was altered by
-- presets. Presets might force the evaluation of variables that would otherwise be transmitted
-- unmodified.
inputValueDefinitionParser
  :: forall n m
   . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.InputValueDefinition
  -> m (InputFieldsParser n (Maybe (Altered, G.Value RemoteSchemaVariable)))
inputValueDefinitionParser schemaDoc (G.InputValueDefinition desc name fieldType maybeDefaultVal _directives) =
  buildField fieldConstructor fieldType
  where
    doNullability
      :: forall a k . 'Input <: k
      => G.Nullability
      -> Parser k n (Maybe a)
      -> Parser k n (Maybe a)
    doNullability (G.Nullability True)  = fmap join . P.nullable
    doNullability (G.Nullability False) = id

    fieldConstructor
      :: forall k. 'Input <: k
      => Parser k n (Maybe (Altered, G.Value RemoteSchemaVariable))
      -> InputFieldsParser n (Maybe (Altered, G.Value RemoteSchemaVariable))
    fieldConstructor (shortCircuitIfUnaltered -> parser) =
      case maybeDefaultVal of
        Nothing ->
          if G.isNullable fieldType
          then join <$> fieldOptional name desc parser
          else field name desc parser
        Just defaultVal -> fieldWithDefault name desc defaultVal parser

    buildField
      :: ( forall k. 'Input <: k
           => Parser k n (Maybe (Altered, G.Value RemoteSchemaVariable))
           -> InputFieldsParser n (Maybe (Altered, G.Value RemoteSchemaVariable))
         )
      -> G.GType
      -> m (InputFieldsParser n (Maybe (Altered, G.Value RemoteSchemaVariable)))
    buildField mkInputFieldsParser = \case
     G.TypeNamed nullability typeName ->
       case lookupType schemaDoc typeName of
         Nothing -> throw400 RemoteSchemaError $ "Could not find type with name " <>> typeName
         Just typeDef ->
           case typeDef of
             G.TypeDefinitionScalar scalarTypeDefn ->
               pure $ mkInputFieldsParser $ doNullability nullability $ Just <$> remoteFieldScalarParser scalarTypeDefn
             G.TypeDefinitionEnum defn ->
               pure $ mkInputFieldsParser $ doNullability nullability $ Just <$> remoteFieldEnumParser defn
             G.TypeDefinitionObject _ ->
               throw400 RemoteSchemaError "expected input type, but got output type"
             G.TypeDefinitionInputObject defn -> do
               potentialObject <- remoteInputObjectParser schemaDoc defn
               pure $ case potentialObject of
                 Left dummyInputFieldsParser -> do
                   -- We couln't create a parser, meaning we can't create a field for this
                   -- object. Instead we must return a "pure" InputFieldsParser that always yields
                   -- the needed result without containing a field definition.
                   --
                   -- !!! WARNING #1 !!!
                   -- Since we have no input field in the schema for this field, we can't make the
                   -- distinction between it being actually present at parsing time or not. We
                   -- therefore choose to behave as if it was always present, and we always
                   -- include the preset values in the result.
                   --
                   -- !!! WARNING #2 !!!
                   -- We are re-using an 'InputFieldsParser' that was created earlier! Won't that
                   -- create new fields in the current context? No, it won't, but only because in
                   -- this case we know that it was created from the preset fields in
                   -- 'argumentsParser', and therefore contains no field definition.
                   Just <$> dummyInputFieldsParser
                 Right actualParser -> do
                   -- We're in the normal case: we do have a parser for the input object, which is
                   -- therefore valid (non-empty).
                   mkInputFieldsParser $ doNullability nullability $ Just <$> actualParser
             G.TypeDefinitionUnion     _ ->
               throw400 RemoteSchemaError "expected input type, but got output type"
             G.TypeDefinitionInterface _ ->
               throw400 RemoteSchemaError "expected input type, but got output type"
     G.TypeList nullability subType -> do
       buildField (mkInputFieldsParser . doNullability nullability . fmap (Just . fmap G.VList . aggregateListAndAlteration) . P.list) subType

-- | remoteFieldScalarParser attempts to parse a scalar value for a given remote field
--
-- We do not attempt to verify that the literal is correct! Some GraphQL implementations, including
-- ours, are a bit flexible with the intepretations of literals; for instance, there are several
-- places in our schema where we declare something to be an `Int`, but actually accept `String`
-- literals. We do however peform variable type-checking.
--
-- If we encounter a JSON value, it means that we were introspecting a query variable. To call the
-- remote schema, we need a graphql value; we therefore need to treat that JSON expression as if it
-- were a query variable of its own. To avoid ending up with one such variable per scalar in the
-- query, we also track alterations, to apply optimizations.
-- See Note [Variable expansion in remote schema input parsers] for more information.
remoteFieldScalarParser
  :: MonadParse n
  => G.ScalarTypeDefinition
  -> P.Parser 'Both n (Altered, G.Value RemoteSchemaVariable)
remoteFieldScalarParser (G.ScalarTypeDefinition description name _directives) = P.Parser
  { pType = schemaType
  , pParser = \inputValue ->
      (Altered False,) <$> case inputValue of
        JSONValue    v -> pure $ G.VVariable $ RemoteJSONValue gType v
        GraphQLValue v -> for v \var -> do
          P.typeCheck False gType var
          pure $ QueryVariable var
  }
  where
    schemaType = NonNullable $ TNamed $ mkDefinition name description TIScalar
    gType      = toGraphQLType schemaType

remoteFieldEnumParser
  :: MonadParse n
  => G.EnumTypeDefinition
  -> Parser 'Both n (Altered, G.Value RemoteSchemaVariable)
remoteFieldEnumParser (G.EnumTypeDefinition desc name _directives valueDefns) =
  let enumValDefns = valueDefns <&> \(G.EnumValueDefinition enumDesc enumName _) ->
        ( mkDefinition (G.unEnumValue enumName) enumDesc P.EnumValueInfo
        , G.VEnum enumName
        )
  in fmap (Altered False,) $ P.enum name desc $ NE.fromList enumValDefns

-- | remoteInputObjectParser returns an input parser for a given 'G.InputObjectTypeDefinition'
--
-- Now, this is tricky! We are faced with two contradicting constraints here. On one hand, the
-- GraphQL spec forbids us from creating empty input objects. This means that if all the arguments
-- have presets, we CANNOT use the parser this function creates, and the caller cannot create a
-- field for this object (and instead should use @pure@ to include the preset values in the result
-- of parsing the fields).
--
-- One way we could fix this would be to change the type of this function to return a `Maybe
-- Parser`, inspect the result of 'argumentsParser', and return @Nothing@ when we realize that there
-- aren't any actual field in it (or at least return a value that propagates the preset values). But
-- this would contradict our second constraint: this function needs to be memoized!
--
-- At time of writing, we can't memoize functions that return arbitrary functors of Parsers; so no
-- memoizing Maybe Parser or Either Presets Parser. Which means that we would need to first call
-- `argumentsParser`, then memoize the "Just" branch that builds the actual Parser. The problem is
-- that the recursive call ro remoteSchemaInputObject is within 'argumentsParser', meaning the call
-- to it MUST be in the memoized branch!
--
-- This is why, in the end, we do the following: we first test whether there is any non-preset
-- field: if yes, we memoize that branch and proceed as normal. Otherwise we can omit the
-- memoization: we know for sure that the preset fields won't generate a recursive call!
remoteInputObjectParser
  :: forall n m
  .  (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.InputObjectTypeDefinition RemoteSchemaInputValueDefinition
  -> m ( Either
           (InputFieldsParser n (Altered, G.Value RemoteSchemaVariable))
           (Parser 'Input     n (Altered, G.Value RemoteSchemaVariable))
       )
remoteInputObjectParser schemaDoc defn@(G.InputObjectTypeDefinition desc name _ valueDefns) =
  if all (isJust . _rsitdPresetArgument) valueDefns
  then
    -- All the fields are preset: we can't create a parser, that would result in an invalid type in
    -- the schema (an input object with no field). We therefore forward the InputFieldsParser
    -- unmodified. No need to memoize this branch: since all arguments are preset, 'argumentsParser'
    -- won't be recursively calling this function.
    Left . fmap (fmap G.VObject) <$> argumentsParser valueDefns schemaDoc
  else
    -- At least one field is not a preset, meaning we have the guarantee that there will be at least
    -- one field in the input object. We have to memoize this branch as we might recursively call
    -- the same parser.
    Right <$> P.memoizeOn 'remoteInputObjectParser defn do
      argsParser <- argumentsParser valueDefns schemaDoc
      pure $ fmap G.VObject <$> P.object name desc argsParser

-- | Variable expansion optimization.
-- Since each parser returns a value that indicates whether it was altered, we can detect when no
-- alteration took place, and replace the parsed and expanded value by its original.
-- See Note [Variable expansion in remote schema input parsers] for more information.
shortCircuitIfUnaltered
  :: forall k n
   . ('Input <: k, MonadParse n)
  => Parser k n (Maybe (Altered, G.Value RemoteSchemaVariable))
  -> Parser k n (Maybe (Altered, G.Value RemoteSchemaVariable))
shortCircuitIfUnaltered parser = P.Parser
  { pType   = P.pType parser
  , pParser = \value -> do
      result <- P.pParser parser value
      pure $ case result of
        -- The parser did yield a value, and it was unmodified by presets
        -- we can short-citcuit by transforming the input value, therefore
        -- "unpeeling" variables and avoiding extraneous JSON variables.
        Just (Altered False, _) -> Just $ (Altered False,) $ case castWith (P.inputParserInput @k) value of
          -- The input was a GraphQL value: just forward it.
          GraphQLValue v -> QueryVariable <$> v
          -- The input value was already a JSON value: we still have to create
          -- a new JSON variable, but it will still be more efficient than having
          -- all the leaves of said value each be their own distinct value.
          JSONValue    v -> G.VVariable $ RemoteJSONValue (toGraphQLType $ P.pType parser) v
        -- Otherwise either the parser did not yield any value, or a value
        -- that has been altered by presets and permissions; we forward it
        -- unoptimized.
        _ -> result
  }

-- | argumentsParser is used for creating an argument parser for remote fields,
-- This function is called for field arguments and input object fields. This
-- function works in the following way:
--
--   * if a field is not preset, we recursively call `inputValueDefinitionParser` on it
--   * otherwise, we use the preset
--
-- For example, consider the following input objects:
--
--   input MessageWhereInpObj {
--     id:   IntCompareObj
--     name: StringCompareObj
--   }
--
--   input IntCompareObj {
--     eq : Int @preset(value: 2)
--     gt : Int
--     lt : Int
--   }
--
-- parsing a MessageWhereInpObj will result in the following call tree:
--
--   -> argumentsParser MessageWhereInpObj
--     -> id => inputValueDefinitionParser IntCompareObj
--       -> remoteInputObjectParser IntCompareObj
--         -> argumentsParser IntCompareObj
--           -> eq => using preset, no recursion
--           -> gt => inputValueDefinitionParser Int
--             -> remoteFieldScalarParser Int
--           -> lt => inputValueDefinitionParser Int
--             -> remoteFieldScalarParser Int
--     -> name => inputValueDefinitionParser StringCompareObj
--       -> ...
--
-- Furthermore, like all other input parsers in this file, 'argumentsParser' indicates whether this
-- part of the tree was altered during parsing; if any of the fields is preset, or recursively
-- contains values that contain presets further down, then this result is labelled as altered.
argumentsParser
  :: forall n m
   . (MonadSchema n m, MonadError QErr m)
  => G.ArgumentsDefinition RemoteSchemaInputValueDefinition
  -> RemoteSchemaIntrospection
  -> m (InputFieldsParser n (Altered, HashMap G.Name (G.Value RemoteSchemaVariable)))
argumentsParser args schemaDoc = do
  -- ! DANGER !
  --
  -- This function is mutually recursive with 'inputValueDefinitionParser': if one of the non-preset
  -- arguments is an input object, then recursively we'll end up using 'argumentsParser' to parse
  -- its arguments. Note however that if all arguments have a preset value, then this function will
  -- not call 'inputValueDefinitionParser', and will simply return without any recursion.
  --
  -- This is labelled as dangerous because another function in this module,
  -- 'remoteInputObjectParser', EXPLICITLY RELIES ON THIS BEHAVIOUR. Due to limitations of the
  -- GraphQL spec and of parser memoization functions, it cannot memoize the case where all
  -- arguments are preset, and therefore relies on the assumption that 'argumentsParser' is not
  -- recursive in this edge case.
  --
  -- This assumptions is unlikely to ever be broken; but if you ever modify this function, please
  -- nonetheless make sure that it is maintained.
  argsParsers <- for args \arg -> do
    let argDef  = _rsitdDefinition arg
        argName = G._ivdName argDef
    argParser <- case _rsitdPresetArgument arg of
      Nothing     -> inputValueDefinitionParser schemaDoc argDef
      -- This is the source of all possible alterations: one of the fields is preset; everything
      -- "above" this field in the tree will be considered "altered", and the optimizations will
      -- not apply.
      Just preset -> pure $ pure $ pure (Altered True, preset)
    pure $ fmap (fmap (argName,)) <$> argParser
  pure $ sequenceA argsParsers <&> fmap Map.fromList . aggregateListAndAlteration

aggregateListAndAlteration :: [Maybe (Altered, a)] -> (Altered, [a])
aggregateListAndAlteration = first mconcat . unzip . catMaybes



--------------------------------------------------------------------------------
-- Remote schema output parsers

-- | 'remoteSchemaObject' returns a output parser for a given 'ObjectTypeDefinition'.
remoteSchemaObject
  :: forall n m
  . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.ObjectTypeDefinition RemoteSchemaInputValueDefinition
  -> m (Parser 'Output n [G.Field G.NoFragments RemoteSchemaVariable])
remoteSchemaObject schemaDoc defn@(G.ObjectTypeDefinition description name interfaces _directives subFields) =
  P.memoizeOn 'remoteSchemaObject defn do
  subFieldParsers <- traverse (remoteFieldFromDefinition schemaDoc) subFields
  interfaceDefs <- traverse getInterface interfaces
  implements <- traverse (remoteSchemaInterface schemaDoc) interfaceDefs
  -- TODO: also check sub-interfaces, when these are supported in a future graphql spec
  traverse_ validateImplementsFields interfaceDefs
  pure $ P.selectionSetObject name description subFieldParsers implements <&>
    toList . OMap.mapWithKey (\alias -> \case
        P.SelectField fld  -> fld
        P.SelectTypename _ ->
          G.Field (Just alias) $$(G.litName "__typename") mempty mempty mempty)
  where
    getInterface :: G.Name -> m (G.InterfaceTypeDefinition [G.Name] RemoteSchemaInputValueDefinition)
    getInterface interfaceName =
      onNothing (lookupInterface schemaDoc interfaceName) $
        throw400 RemoteSchemaError $ "Could not find interface " <> squote interfaceName
        <> " implemented by Object type " <> squote name
    validateImplementsFields :: G.InterfaceTypeDefinition [G.Name] RemoteSchemaInputValueDefinition -> m ()
    validateImplementsFields interface =
      traverse_ (validateImplementsField (G._itdName interface)) (G._itdFieldsDefinition interface)
    validateImplementsField :: G.Name -> G.FieldDefinition RemoteSchemaInputValueDefinition -> m ()
    validateImplementsField interfaceName interfaceField =
      case lookup (G._fldName interfaceField) (zip (fmap G._fldName subFields) subFields) of
        Nothing -> throw400 RemoteSchemaError $
          "Interface field " <> squote interfaceName <> "." <> dquote (G._fldName interfaceField)
          <> " expected, but " <> squote name <> " does not provide it"
        Just f -> do
          unless (validateSubType (G._fldType f) (G._fldType interfaceField)) $
            throw400 RemoteSchemaError $
            "The type of Object field " <> squote name <> "." <> dquote (G._fldName f)
            <> " (" <> G.showGT (G._fldType f)
            <> ") is not the same type/sub type of Interface field "
            <> squote interfaceName <> "." <> dquote (G._fldName interfaceField)
            <> " (" <> G.showGT (G._fldType interfaceField) <> ")"
          traverse_
            (validateArgument
               (map _rsitdDefinition (G._fldArgumentsDefinition f)) . _rsitdDefinition)
            (G._fldArgumentsDefinition interfaceField)
          traverse_
            (validateNoExtraNonNull
               (map _rsitdDefinition (G._fldArgumentsDefinition interfaceField)) . _rsitdDefinition)
            (G._fldArgumentsDefinition f)
            where
              validateArgument :: [G.InputValueDefinition] -> G.InputValueDefinition -> m ()
              validateArgument objectFieldArgs ifaceArgument =
                case lookup (G._ivdName ifaceArgument) (zip (fmap G._ivdName objectFieldArgs) objectFieldArgs) of
                  Nothing ->
                    throw400 RemoteSchemaError $
                      "Interface field argument " <> squote interfaceName <> "." <> dquote (G._fldName interfaceField)
                      <> "(" <> dquote (G._ivdName ifaceArgument) <> ":) required, but Object field " <> squote name <> "." <> dquote (G._fldName f)
                      <> " does not provide it"
                  Just a ->
                    unless (G._ivdType a == G._ivdType ifaceArgument) $
                      throw400 RemoteSchemaError $
                      "Interface field argument " <> squote interfaceName <> "." <> dquote (G._fldName interfaceField)
                      <> "(" <> dquote (G._ivdName ifaceArgument) <> ":) expects type "
                      <> G.showGT (G._ivdType ifaceArgument)
                      <> ", but " <> squote name <> "." <> dquote (G._fldName f) <> "("
                      <> dquote (G._ivdName ifaceArgument) <> ":) has type "
                      <> G.showGT (G._ivdType a)
              validateNoExtraNonNull :: [G.InputValueDefinition] -> G.InputValueDefinition -> m ()
              validateNoExtraNonNull ifaceArguments objectFieldArg =
                case lookup (G._ivdName objectFieldArg) (zip (fmap G._ivdName ifaceArguments) ifaceArguments) of
                  Just _ -> pure ()
                  Nothing ->
                    unless (G.isNullable (G._ivdType objectFieldArg)) $
                    throw400 RemoteSchemaError $
                    "Object field argument " <>  squote name <> "." <> dquote (G._fldName f) <> "("
                      <> dquote (G._ivdName objectFieldArg) <> ":) is of required type "
                      <> G.showGT (G._ivdType objectFieldArg) <> ", but is not provided by Interface field "
                      <> squote interfaceName <> "." <> dquote (G._fldName interfaceField)
    validateSubType :: G.GType -> G.GType -> Bool
    -- TODO this ignores nullability which is probably wrong, even though the GraphQL spec is ambiguous
    validateSubType (G.TypeList _ x) (G.TypeList _ y) = validateSubType x y
    -- It is OK to "upgrade" the strictness
    validateSubType (G.TypeNamed (G.Nullability False) x) (G.TypeNamed (G.Nullability True) y) =
      validateSubType (G.TypeNamed (G.Nullability True) x) (G.TypeNamed (G.Nullability True) y)
    validateSubType (G.TypeNamed nx x) (G.TypeNamed ny y) =
      case (lookupType schemaDoc x , lookupType schemaDoc y) of
        (Just x' , Just y') -> nx == ny && validateSubTypeDefinition x' y'
        _                   -> False
    validateSubType _ _ = False
    validateSubTypeDefinition x' y' | x' == y' = True
    validateSubTypeDefinition (G.TypeDefinitionObject otd) (G.TypeDefinitionInterface itd)
      = G._otdName otd `elem` G._itdPossibleTypes itd
    validateSubTypeDefinition (G.TypeDefinitionObject _otd) (G.TypeDefinitionUnion _utd)
      = True -- TODO write appropriate check (may require saving 'possibleTypes' in Syntax.hs)
    validateSubTypeDefinition _ _ = False

{- Note [Querying remote schema interfaces]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When querying Remote schema interfaces, we need to re-construct
the incoming query to be compliant with the upstream remote.
We need to do this because the `SelectionSet`(s) that are
inputted to this function have the fragments (if any) flattened.
(Check `flattenSelectionSet` in 'Hasura.GraphQL.Parser.Collect' module)
The `constructInterfaceSelectionSet` function makes a valid interface query by:
1. Getting the common interface fields in all the selection sets
2. Remove the common fields obtained in #1 from the selection sets
3. Construct a selection field for every common interface field
4. Construct inline fragments for non-common interface fields
   using the result of #2 for every object
5. Construct the final selection set by combining #3 and #4

Example: Suppose an interface 'Character' is defined in the upstream
and two objects 'Human' and 'Droid' implement the 'Character' Interface.

Suppose, a field 'hero' returns 'Character'.

{
   hero {
     id
     name
     ... on Droid {
       primaryFunction
     }
     ... on Human {
       homePlanet
     }
   }
}

When we parse the selection set of the `hero` field, we parse the selection set
twice: once for the `Droid` object type, which would be passed a selection set
containing the field(s) defined in the `Droid` object type and similarly once
for the 'Human' object type. The result of the interface selection set parsing
would then be the results of the parsing of the object types when passed their
corresponding flattened selection sets and the results of the parsing of the
interface fields.

After we parse the above GraphQL query, we get a selection set containing
the interface fields and the selection sets of the objects that were queried
in the GraphQL query. Since, we have the selection sets of the objects that
were being queried, we can convert them into inline fragments resembling
the original query and then query the remote schema with the newly
constructed query.
-}

-- | 'remoteSchemaInterface' returns a output parser for a given 'InterfaceTypeDefinition'.
--   Also check Note [Querying remote schema interfaces]
remoteSchemaInterface
  :: forall n m
  . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.InterfaceTypeDefinition [G.Name] RemoteSchemaInputValueDefinition
  -> m (Parser 'Output n (G.SelectionSet G.NoFragments RemoteSchemaVariable))
remoteSchemaInterface schemaDoc defn@(G.InterfaceTypeDefinition description name _directives fields possibleTypes) =
  P.memoizeOn 'remoteSchemaObject defn do
  subFieldParsers <- traverse (remoteFieldFromDefinition schemaDoc) fields
  objs <- traverse (getObjectParser schemaDoc getObject) possibleTypes
  -- In the Draft GraphQL spec (> June 2018), interfaces can themselves
  -- implement superinterfaces.  In the future, we may need to support this
  -- here.
  when (null subFieldParsers) $
    throw400 RemoteSchemaError $ "List of fields cannot be empty for interface " <> squote name
  -- TODO: another way to obtain 'possibleTypes' is to lookup all the object
  -- types in the schema document that claim to implement this interface.  We
  -- should have a check that expresses that that collection of objects is equal
  -- to 'possibleTypes'.
  pure $ P.selectionSetInterface name description subFieldParsers objs <&> constructInterfaceSelectionSet
  where
    getObject :: G.Name -> m (G.ObjectTypeDefinition RemoteSchemaInputValueDefinition)
    getObject objectName =
      onNothing (lookupObject schemaDoc objectName) $
        case lookupInterface schemaDoc objectName of
          Nothing -> throw400 RemoteSchemaError $ "Could not find type " <> squote objectName
            <> ", which is defined as a member type of Interface " <> squote name
          Just _  -> throw400 RemoteSchemaError $ "Interface type " <> squote name <>
            " can only include object types. It cannot include " <> squote objectName

    -- 'constructInterfaceQuery' constructs a remote interface query.
    constructInterfaceSelectionSet
      :: [(G.Name, [G.Field G.NoFragments RemoteSchemaVariable])]
      -> G.SelectionSet G.NoFragments RemoteSchemaVariable
    constructInterfaceSelectionSet objNameAndFields =
      let -- common interface fields that exist in every
          -- selection set provided
          -- #1 of Note [Querying remote schema Interfaces]
          commonInterfaceFields =
            OMap.elems $
            OMap.mapMaybe (allTheSame . toList) $
            OMap.groupListWith G._fName $
            concatMap (filter ((`elem` interfaceFieldNames) . G._fName) . snd) $
            objNameAndFields

          interfaceFieldNames = map G._fldName fields

          allTheSame (x:xs) | all (== x) xs = Just x
          allTheSame _ = Nothing

          -- #2 of Note [Querying remote schema interface fields]
          nonCommonInterfaceFields =
            catMaybes $ flip map objNameAndFields $ \(objName, objFields) ->
              let nonCommonFields = filter (not . flip elem commonInterfaceFields) objFields
              in mkObjInlineFragment (objName, map G.SelectionField nonCommonFields)

          -- helper function for #4 of Note [Querying remote schema interface fields]
          mkObjInlineFragment (_, []) = Nothing
          mkObjInlineFragment (objName, selSet) =
            Just $ G.SelectionInlineFragment $
              G.InlineFragment (Just objName) mempty selSet

      -- #5 of Note [Querying remote schema interface fields]
      in fmap G.SelectionField commonInterfaceFields <> nonCommonInterfaceFields

-- | 'remoteSchemaUnion' returns a output parser for a given 'UnionTypeDefinition'.
remoteSchemaUnion
  :: forall n m
  . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.UnionTypeDefinition
  -> m (Parser 'Output n (G.SelectionSet G.NoFragments RemoteSchemaVariable))
remoteSchemaUnion schemaDoc defn@(G.UnionTypeDefinition description name _directives objectNames) =
  P.memoizeOn 'remoteSchemaObject defn do
  objs <- traverse (getObjectParser schemaDoc getObject) objectNames
  when (null objs) $
    throw400 RemoteSchemaError $ "List of member types cannot be empty for union type " <> squote name
  pure $ P.selectionSetUnion name description objs <&>
    (\objNameAndFields ->
       catMaybes $ objNameAndFields <&> \(objName, fields) ->
        case fields of
          -- The return value obtained from the parsing of a union selection set
          -- specifies, for each object in the union type, a fragment-free
          -- selection set for that object type. In particular, if, for a given
          -- object type, the selection set passed to the union type did not
          -- specify any fields for that object type (i.e. if no inline fragment
          -- applied to that object), the selection set resulting from the parsing
          -- through that object type would be empty, i.e. []. We exclude such
          -- object types from the reconstructed selection set for the union
          -- type, as selection sets cannot be empty.
          [] -> Nothing
          _  ->
            Just (G.SelectionInlineFragment
                    $ G.InlineFragment (Just objName) mempty $ fmap G.SelectionField fields))
  where
    getObject :: G.Name -> m (G.ObjectTypeDefinition RemoteSchemaInputValueDefinition)
    getObject objectName =
      onNothing (lookupObject schemaDoc objectName) $
        case lookupInterface schemaDoc objectName of
          Nothing -> throw400 RemoteSchemaError $ "Could not find type " <> squote objectName
            <> ", which is defined as a member type of Union " <> squote name
          Just _  -> throw400 RemoteSchemaError $ "Union type " <> squote name <>
            " can only include object types. It cannot include " <> squote objectName

remoteFieldFromDefinition
  :: forall n m
  . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.FieldDefinition RemoteSchemaInputValueDefinition
  -> m (FieldParser n (G.Field G.NoFragments RemoteSchemaVariable))
remoteFieldFromDefinition schemaDoc (G.FieldDefinition description name argsDefinition gType _) =
  let
    addNullableList :: FieldParser n (G.Field G.NoFragments RemoteSchemaVariable) -> FieldParser n (G.Field G.NoFragments RemoteSchemaVariable)
    addNullableList (P.FieldParser (Definition name' un desc (FieldInfo args typ)) parser)
      = P.FieldParser (Definition name' un desc (FieldInfo args (Nullable (TList typ)))) parser

    addNonNullableList :: FieldParser n (G.Field G.NoFragments RemoteSchemaVariable) -> FieldParser n (G.Field G.NoFragments RemoteSchemaVariable)
    addNonNullableList (P.FieldParser (Definition name' un desc (FieldInfo args typ)) parser)
      = P.FieldParser (Definition name' un desc (FieldInfo args (NonNullable (TList typ)))) parser

    -- TODO add directives, deprecation
    convertType :: G.GType -> m (FieldParser n (G.Field G.NoFragments RemoteSchemaVariable))
    convertType gType' = do
        case gType' of
          G.TypeNamed (G.Nullability True) fieldTypeName ->
            P.nullableField <$> remoteFieldFromName schemaDoc name description fieldTypeName argsDefinition
          G.TypeList (G.Nullability True) gType'' ->
            addNullableList <$> convertType gType''
          G.TypeNamed (G.Nullability False) fieldTypeName -> do
            P.nonNullableField <$> remoteFieldFromName schemaDoc name description fieldTypeName argsDefinition
          G.TypeList (G.Nullability False) gType'' ->
            addNonNullableList <$> convertType gType''
  in convertType gType

-- | 'remoteFieldFromName' accepts a GraphQL name and searches for its definition
--   in the 'RemoteSchemaIntrospection'.
remoteFieldFromName
  :: forall n m
   . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.Name
  -> Maybe G.Description
  -> G.Name
  -> G.ArgumentsDefinition RemoteSchemaInputValueDefinition
  -> m (FieldParser n (G.Field G.NoFragments RemoteSchemaVariable))
remoteFieldFromName sdoc fieldName description fieldTypeName argsDefns =
  case lookupType sdoc fieldTypeName of
    Nothing      -> throw400 RemoteSchemaError $ "Could not find type with name " <>> fieldTypeName
    Just typeDef -> remoteField sdoc fieldName description argsDefns typeDef

-- | 'remoteField' accepts a 'G.TypeDefinition' and will returns a 'FieldParser' for it.
--   Note that the 'G.TypeDefinition' should be of the GraphQL 'Output' kind, when an
--   GraphQL 'Input' kind is provided, then error will be thrown.
remoteField
  :: forall n m
   . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> G.Name
  -> Maybe G.Description
  -> G.ArgumentsDefinition RemoteSchemaInputValueDefinition
  -> G.TypeDefinition [G.Name] RemoteSchemaInputValueDefinition
  -> m (FieldParser n (G.Field G.NoFragments RemoteSchemaVariable))
remoteField sdoc fieldName description argsDefn typeDefn = do
  -- TODO add directives
  argsParser <- argumentsParser argsDefn sdoc
  case typeDefn of
    G.TypeDefinitionObject objTypeDefn -> do
      remoteSchemaObjFields <- remoteSchemaObject sdoc objTypeDefn
      -- converting [Field NoFragments Name] to (SelectionSet NoFragments G.Name)
      let remoteSchemaObjSelSet = fmap G.SelectionField <$> remoteSchemaObjFields
      pure remoteSchemaObjSelSet <&> mkFieldParserWithSelectionSet argsParser
    G.TypeDefinitionScalar scalarTypeDefn ->
      pure $ mkFieldParserWithoutSelectionSet argsParser $ void $ remoteFieldScalarParser scalarTypeDefn
    G.TypeDefinitionEnum enumTypeDefn ->
      pure $ mkFieldParserWithoutSelectionSet argsParser $ void $ remoteFieldEnumParser enumTypeDefn
    G.TypeDefinitionInterface ifaceTypeDefn ->
      remoteSchemaInterface sdoc ifaceTypeDefn <&> mkFieldParserWithSelectionSet argsParser
    G.TypeDefinitionUnion unionTypeDefn ->
      remoteSchemaUnion     sdoc unionTypeDefn <&> mkFieldParserWithSelectionSet argsParser
    _ -> throw400 RemoteSchemaError "expected output type, but got input type"
  where
    mkFieldParserWithoutSelectionSet
      :: InputFieldsParser n (Altered, HashMap G.Name (G.Value RemoteSchemaVariable))
      -> Parser 'Both n ()
      -> FieldParser n (G.Field G.NoFragments RemoteSchemaVariable)
    mkFieldParserWithoutSelectionSet argsParser outputParser =
      P.rawSelection fieldName description argsParser outputParser
        <&> \(alias, _, (_, args)) -> G.Field alias fieldName args mempty []

    mkFieldParserWithSelectionSet
      :: InputFieldsParser n (Altered, HashMap G.Name (G.Value RemoteSchemaVariable))
      -> Parser 'Output n (G.SelectionSet G.NoFragments RemoteSchemaVariable)
      -> FieldParser n (G.Field G.NoFragments RemoteSchemaVariable)
    mkFieldParserWithSelectionSet argsParser outputParser =
      P.rawSubselection fieldName description argsParser outputParser
        <&> \(alias, _, (_, args), selSet) -> G.Field alias fieldName args mempty selSet

-- | helper function to get a parser of an object with it's name
--   This function is called from 'remoteSchemaInterface' and
--   'remoteSchemaObject' functions. Both of these have a slightly
--   different implementation of 'getObject', which is the
--   reason 'getObject' is an argument to this function
getObjectParser
  :: forall n m
  . (MonadSchema n m, MonadError QErr m)
  => RemoteSchemaIntrospection
  -> (G.Name -> m (G.ObjectTypeDefinition RemoteSchemaInputValueDefinition))
  -> G.Name
  -> m (Parser 'Output n (G.Name, [G.Field G.NoFragments RemoteSchemaVariable]))
getObjectParser schemaDoc getObject objName = do
  obj <- remoteSchemaObject schemaDoc =<< getObject objName
  return $ (objName,) <$> obj