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|
{-# LANGUAGE OverloadedStrings #-}
-- | This module provides a representation of a @GraphQL@ Schema in addition to
-- functions for defining and manipulating schemas.
module Language.GraphQL.Schema
( Resolver
, Subs
, object
, scalar
, resolve
, wrappedObject
, wrappedScalar
-- * AST Reexports
, Field
, Value(..)
) where
import Control.Monad.IO.Class (MonadIO(..))
import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Except (runExceptT)
import Control.Monad.Trans.Reader (runReaderT)
import Data.Foldable (find, fold)
import Data.Maybe (fromMaybe)
import qualified Data.Aeson as Aeson
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Sequence (Seq)
import Data.Text (Text)
import qualified Data.Text as T
import Language.GraphQL.AST.Core
import Language.GraphQL.Error
import Language.GraphQL.Trans
import qualified Language.GraphQL.Type as Type
-- | Resolves a 'Field' into an @Aeson.@'Data.Aeson.Types.Object' with error
-- information (if an error has occurred). @m@ is usually expected to be an
-- instance of 'MonadIO'.
data Resolver m = Resolver
Text -- ^ Name
(Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
-- | Contains variables for the query. The key of the map is a variable name,
-- and the value is the variable value.
type Subs = HashMap Name Value
-- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
object :: MonadIO m => Name -> ActionT m [Resolver m] -> Resolver m
object name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ flds) resolver = withField (resolve resolver flds) fld
-- | Like 'object' but can be null or a list of objects.
wrappedObject ::
MonadIO m =>
Name ->
ActionT m (Type.Wrapping [Resolver m]) ->
Resolver m
wrappedObject name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ sels) resolver
= withField (traverse (`resolve` sels) resolver) fld
-- | A scalar represents a primitive value, like a string or an integer.
scalar :: (MonadIO m, Aeson.ToJSON a) => Name -> ActionT m a -> Resolver m
scalar name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld result = withField (return result) fld
-- | Like 'scalar' but can be null or a list of scalars.
wrappedScalar ::
(MonadIO m, Aeson.ToJSON a) =>
Name ->
ActionT m (Type.Wrapping a) ->
Resolver m
wrappedScalar name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld (Type.Named result) = withField (return result) fld
resolveRight fld Type.Null
= return $ HashMap.singleton (aliasOrName fld) Aeson.Null
resolveRight fld (Type.List result) = withField (return result) fld
resolveFieldValue ::
MonadIO m =>
ActionT m a ->
(Field -> a -> CollectErrsT m (HashMap Text Aeson.Value)) ->
Field ->
CollectErrsT m (HashMap Text Aeson.Value)
resolveFieldValue f resolveRight fld@(Field _ _ args _) = do
result <- lift $ reader . runExceptT . runActionT $ f
either resolveLeft (resolveRight fld) result
where
reader = flip runReaderT $ Context {arguments=args}
resolveLeft err = do
_ <- addErrMsg err
return $ HashMap.singleton (aliasOrName fld) Aeson.Null
-- | Helper function to facilitate error handling and result emitting.
withField :: (MonadIO m, Aeson.ToJSON a)
=> CollectErrsT m a -> Field -> CollectErrsT m (HashMap Text Aeson.Value)
withField v fld
= HashMap.singleton (aliasOrName fld) . Aeson.toJSON <$> runAppendErrs v
-- | Takes a list of 'Resolver's and a list of 'Field's and applies each
-- 'Resolver' to each 'Field'. Resolves into a value containing the
-- resolved 'Field', or a null value and error information.
resolve :: MonadIO m
=> [Resolver m] -> Seq Selection -> CollectErrsT m Aeson.Value
resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
where
resolveTypeName (Resolver "__typename" f) = do
value <- f $ Field Nothing "__typename" mempty mempty
return $ HashMap.lookupDefault "" "__typename" value
resolveTypeName _ = return ""
tryResolvers (SelectionField fld@(Field _ name _ _))
= maybe (errmsg fld) (tryResolver fld) $ find (compareResolvers name) resolvers
tryResolvers (SelectionFragment (Fragment typeCondition selections')) = do
that <- traverse resolveTypeName (find (compareResolvers "__typename") resolvers)
if maybe True (Aeson.String typeCondition ==) that
then fmap fold . traverse tryResolvers $ selections'
else return mempty
compareResolvers name (Resolver name' _) = name == name'
tryResolver fld (Resolver _ resolver) = resolver fld
errmsg fld@(Field _ name _ _) = do
addErrMsg $ T.unwords ["field", name, "not resolved."]
return $ HashMap.singleton (aliasOrName fld) Aeson.Null
aliasOrName :: Field -> Text
aliasOrName (Field alias name _ _) = fromMaybe name alias
|
