graphql/src/Data/GraphQL/Schema.hs

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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 Data.GraphQL.Schema
( Resolver
, Schema
, Subs
, object
, objectA
, scalar
, scalarA
, enum
, enumA
, resolve
, wrappedEnum
, wrappedEnumA
, wrappedObject
, wrappedObjectA
, wrappedScalar
, wrappedScalarA
-- * AST Reexports
, Field
, Argument(..)
, Value(..)
) where
import Control.Monad (MonadPlus(..))
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import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Except (runExceptT)
import Data.Foldable ( find
, fold
)
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import Data.GraphQL.Error
import Data.List.NonEmpty (NonEmpty)
import Data.Maybe (fromMaybe)
import qualified Data.Aeson as Aeson
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Text (Text)
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import qualified Data.Text as T
import Language.GraphQL.Trans
import Language.GraphQL.Type
import Data.GraphQL.AST.Core
-- | A GraphQL schema.
-- @f@ is usually expected to be an instance of 'Alternative'.
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type Schema m = NonEmpty (Resolver m)
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-- | Resolves a 'Field' into an @Aeson.@'Aeson.Object' with error information
-- (or 'empty'). @m@ is usually expected to be an instance of 'MonadPlus'.
data Resolver m = Resolver
Text -- ^ Name
(Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
type Fields = [Field]
type Arguments = [Argument]
-- | Variable substitution function.
type Subs = Name -> Maybe Value
-- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
object :: MonadPlus m => Name -> ActionT m [Resolver m] -> Resolver m
object name = objectA name . const
-- | Like 'object' but also taking 'Argument's.
objectA :: MonadPlus m
=> Name -> (Arguments -> ActionT m [Resolver m]) -> Resolver m
objectA name f = Resolver name $ resolveFieldValue f resolveRight
where
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resolveRight fld@(Field _ _ _ flds) resolver = withField (resolve resolver flds) fld
-- | Like 'object' but also taking 'Argument's and can be null or a list of objects.
wrappedObjectA :: MonadPlus m
=> Name -> (Arguments -> ActionT m (Wrapping [Resolver m])) -> Resolver m
wrappedObjectA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld@(Field _ _ _ sels) resolver
= withField (traverse (`resolve` sels) resolver) fld
-- | Like 'object' but can be null or a list of objects.
wrappedObject :: MonadPlus m
=> Name -> ActionT m (Wrapping [Resolver m]) -> Resolver m
wrappedObject name = wrappedObjectA name . const
-- | A scalar represents a primitive value, like a string or an integer.
scalar :: (MonadPlus m, Aeson.ToJSON a) => Name -> ActionT m a -> Resolver m
scalar name = scalarA name . const
-- | Like 'scalar' but also taking 'Argument's.
scalarA :: (MonadPlus m, Aeson.ToJSON a)
=> Name -> (Arguments -> ActionT m a) -> Resolver m
scalarA name f = Resolver name $ resolveFieldValue f resolveRight
where
resolveRight fld result = withField (return result) fld
-- | Lika 'scalar' but also taking 'Argument's and can be null or a list of scalars.
wrappedScalarA :: (MonadPlus m, Aeson.ToJSON a)
=> Name -> (Arguments -> ActionT m (Wrapping a)) -> Resolver m
wrappedScalarA name f = Resolver name $ resolveFieldValue f resolveRight
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where
resolveRight fld (Named result) = withField (return result) fld
resolveRight fld Null
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= return $ HashMap.singleton (aliasOrName fld) Aeson.Null
resolveRight fld (List result) = withField (return result) fld
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-- | Like 'scalar' but can be null or a list of scalars.
wrappedScalar :: (MonadPlus m, Aeson.ToJSON a)
=> Name -> ActionT m (Wrapping a) -> Resolver m
wrappedScalar name = wrappedScalarA name . const
-- | Represents one of a finite set of possible values.
-- Used in place of a 'scalar' when the possible responses are easily enumerable.
enum :: MonadPlus m => Name -> ActionT m [Text] -> Resolver m
enum name = enumA name . const
-- | Like 'enum' but also taking 'Argument's.
enumA :: MonadPlus m => Name -> (Arguments -> ActionT m [Text]) -> Resolver m
enumA name f = Resolver name $ resolveFieldValue f resolveRight
where
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resolveRight fld resolver = withField (return resolver) fld
-- | Like 'enum' but also taking 'Argument's and can be null or a list of enums.
wrappedEnumA :: MonadPlus m
=> Name -> (Arguments -> ActionT m (Wrapping [Text])) -> Resolver m
wrappedEnumA name f = Resolver name $ resolveFieldValue f resolveRight
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where
resolveRight fld (Named resolver) = withField (return resolver) fld
resolveRight fld Null
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= return $ HashMap.singleton (aliasOrName fld) Aeson.Null
resolveRight fld (List resolver) = withField (return resolver) fld
-- | Like 'enum' but can be null or a list of enums.
wrappedEnum :: MonadPlus m => Name -> ActionT m (Wrapping [Text]) -> Resolver m
wrappedEnum name = wrappedEnumA name . const
resolveFieldValue :: MonadPlus m
=> ([Argument] -> ActionT m a)
-> (Field -> a -> CollectErrsT m (HashMap Text Aeson.Value))
-> Field
-> CollectErrsT m (HashMap Text Aeson.Value)
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resolveFieldValue f resolveRight fld@(Field _ _ args _) = do
result <- lift $ runExceptT . runActionT $ f args
either resolveLeft (resolveRight fld) result
where
resolveLeft err = do
_ <- addErrMsg err
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return $ HashMap.singleton (aliasOrName fld) Aeson.Null
-- | Helper function to facilitate 'Argument' handling.
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withField :: (MonadPlus m, Aeson.ToJSON a)
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=> 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 :: MonadPlus m
=> [Resolver m] -> Fields -> CollectErrsT m Aeson.Value
resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
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where
tryResolvers fld = mplus (maybe mzero (tryResolver fld) $ find (compareResolvers fld) resolvers) $ errmsg fld
compareResolvers (Field _ name _ _) (Resolver name' _) = name == name'
tryResolver fld (Resolver _ resolver) = resolver fld
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errmsg fld@(Field _ name _ _) = do
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addErrMsg $ T.unwords ["field", name, "not resolved."]
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return $ HashMap.singleton (aliasOrName fld) Aeson.Null
aliasOrName :: Field -> Text
aliasOrName (Field alias name _ _) = fromMaybe name alias