Change the main namespace to Language.GraphQL

9 files changed, 953 insertions, 0 deletions

diff --git a/src/Language/GraphQL.hs b/src/Language/GraphQL.hs
new file mode 100644
index 0000000..8ed29cf
--- /dev/null
+++ b/src/Language/GraphQL.hs
@@ -0,0 +1,36 @@
+-- | This module provides the functions to parse and execute @GraphQL@ queries.
+module Language.GraphQL where
+
+import Control.Monad (MonadPlus)
+
+import qualified Data.Text as T
+
+import qualified Data.Aeson as Aeson
+import Text.Megaparsec ( errorBundlePretty
+                       , parse
+                       )
+
+import Language.GraphQL.Execute
+import Language.GraphQL.Parser
+import Language.GraphQL.Schema
+
+import Language.GraphQL.Error
+
+-- | Takes a 'Schema' and text representing a @GraphQL@ request document.
+--   If the text parses correctly as a @GraphQL@ query the query is
+--   executed according to the given 'Schema'.
+--
+--   Returns the response as an @Aeson.@'Aeson.Value'.
+graphql :: MonadPlus m => Schema m -> T.Text -> m Aeson.Value
+graphql = flip graphqlSubs $ const Nothing
+
+-- | Takes a 'Schema', a variable substitution function and text
+--   representing a @GraphQL@ request document.  If the text parses
+--   correctly as a @GraphQL@ query the substitution is applied to the
+--   query and the query is then executed according to the given 'Schema'.
+--
+--   Returns the response as an @Aeson.@'Aeson.Value'.
+graphqlSubs :: MonadPlus m => Schema m -> Subs -> T.Text -> m Aeson.Value
+graphqlSubs schema f =
+    either (parseError . errorBundlePretty) (execute schema f)
+    . parse document ""
diff --git a/src/Language/GraphQL/AST.hs b/src/Language/GraphQL/AST.hs
new file mode 100644
index 0000000..618bd4b
--- /dev/null
+++ b/src/Language/GraphQL/AST.hs
@@ -0,0 +1,131 @@
+-- | This module defines an abstract syntax tree for the @GraphQL@ language based on
+--   <https://facebook.github.io/graphql/ Facebook's GraphQL Specification>.
+--
+-- Target AST for Parser.
+
+module Language.GraphQL.AST where
+
+import Data.Int (Int32)
+import Data.List.NonEmpty (NonEmpty)
+import Data.Text (Text)
+
+-- * Name
+
+type Name = Text
+
+-- * Document
+
+type Document = NonEmpty Definition
+
+-- * Operations
+
+data Definition = DefinitionOperation OperationDefinition
+                | DefinitionFragment  FragmentDefinition
+                  deriving (Eq,Show)
+
+data OperationDefinition = OperationSelectionSet SelectionSet
+                         | OperationDefinition   OperationType
+                                                 (Maybe Name)
+                                                 VariableDefinitions
+                                                 Directives
+                                                 SelectionSet
+                           deriving (Eq,Show)
+
+data OperationType = Query | Mutation deriving (Eq,Show)
+
+-- * SelectionSet
+
+type SelectionSet = NonEmpty Selection
+
+type SelectionSetOpt = [Selection]
+
+data Selection = SelectionField          Field
+               | SelectionFragmentSpread FragmentSpread
+               | SelectionInlineFragment InlineFragment
+                 deriving (Eq,Show)
+
+-- * Field
+
+data Field = Field (Maybe Alias) Name Arguments Directives SelectionSetOpt
+             deriving (Eq,Show)
+
+type Alias = Name
+
+-- * Arguments
+
+type Arguments = [Argument]
+
+data Argument = Argument Name Value deriving (Eq,Show)
+
+-- * Fragments
+
+data FragmentSpread = FragmentSpread Name Directives deriving (Eq,Show)
+
+data InlineFragment = InlineFragment (Maybe TypeCondition) Directives SelectionSet
+                      deriving (Eq,Show)
+
+data FragmentDefinition =
+  FragmentDefinition FragmentName TypeCondition Directives SelectionSet
+  deriving (Eq,Show)
+
+type FragmentName = Name
+
+type TypeCondition = Name
+
+-- Input Values
+
+data Value = ValueVariable Variable
+           | ValueInt IntValue
+           | ValueFloat FloatValue
+           | ValueString StringValue
+           | ValueBoolean BooleanValue
+           | ValueNull
+           | ValueEnum EnumValue
+           | ValueList ListValue
+           | ValueObject ObjectValue
+             deriving (Eq,Show)
+
+type IntValue = Int32
+
+-- GraphQL Float is double precison
+type FloatValue = Double
+
+type StringValue = Text
+
+type BooleanValue = Bool
+
+type EnumValue = Name
+
+type ListValue = [Value]
+
+type ObjectValue = [ObjectField]
+
+data ObjectField = ObjectField Name Value deriving (Eq,Show)
+
+-- * Variables
+
+type VariableDefinitions = [VariableDefinition]
+
+data VariableDefinition = VariableDefinition Variable Type (Maybe DefaultValue)
+                          deriving (Eq,Show)
+
+type Variable = Name
+
+type DefaultValue = Value
+
+-- * Input Types
+
+data Type = TypeNamed   Name
+          | TypeList    Type
+          | TypeNonNull NonNullType
+            deriving (Eq,Show)
+
+data NonNullType = NonNullTypeNamed Name
+                 | NonNullTypeList  Type
+                   deriving (Eq,Show)
+
+-- * Directives
+
+type Directives = [Directive]
+
+data Directive = Directive Name [Argument] deriving (Eq,Show)
diff --git a/src/Language/GraphQL/AST/Core.hs b/src/Language/GraphQL/AST/Core.hs
new file mode 100644
index 0000000..be432a8
--- /dev/null
+++ b/src/Language/GraphQL/AST/Core.hs
@@ -0,0 +1,38 @@
+-- | This is the AST meant to be executed.
+module Language.GraphQL.AST.Core where
+
+import Data.Int (Int32)
+import Data.List.NonEmpty (NonEmpty)
+import Data.String
+
+import Data.Text (Text)
+
+type Name = Text
+
+type Document = NonEmpty Operation
+
+data Operation = Query    (NonEmpty Field)
+               | Mutation (NonEmpty Field)
+                 deriving (Eq,Show)
+
+data Field = Field (Maybe Alias) Name [Argument] [Field] deriving (Eq,Show)
+
+type Alias = Name
+
+data Argument = Argument Name Value deriving (Eq,Show)
+
+data Value = ValueInt Int32
+           -- GraphQL Float is double precision
+           | ValueFloat Double
+           | ValueString Text
+           | ValueBoolean Bool
+           | ValueNull
+           | ValueEnum Name
+           | ValueList [Value]
+           | ValueObject [ObjectField]
+             deriving (Eq,Show)
+
+instance IsString Value where
+  fromString = ValueString . fromString
+
+data ObjectField = ObjectField Name Value deriving (Eq,Show)
diff --git a/src/Language/GraphQL/AST/Transform.hs b/src/Language/GraphQL/AST/Transform.hs
new file mode 100644
index 0000000..22a9c4c
--- /dev/null
+++ b/src/Language/GraphQL/AST/Transform.hs
@@ -0,0 +1,121 @@
+{-# LANGUAGE OverloadedStrings #-}
+module Language.GraphQL.AST.Transform where
+
+import Control.Applicative (empty)
+import Control.Monad ((<=<))
+import Data.Bifunctor (first)
+import Data.Either (partitionEithers)
+import Data.Foldable (fold, foldMap)
+import qualified Data.List.NonEmpty as NonEmpty
+import Data.Monoid (Alt(Alt,getAlt), (<>))
+import Data.Text (Text)
+import qualified Language.GraphQL.AST as Full
+import qualified Language.GraphQL.AST.Core as Core
+import qualified Language.GraphQL.Schema as Schema
+
+type Name = Text
+
+-- | Replaces a fragment name by a list of 'Field'. If the name doesn't match an
+--   empty list is returned.
+type Fragmenter = Name -> [Core.Field]
+
+-- TODO: Replace Maybe by MonadThrow with CustomError
+document :: Schema.Subs -> Full.Document -> Maybe Core.Document
+document subs doc = operations subs fr ops
+  where
+    (fr, ops) = first foldFrags
+              . partitionEithers
+              . NonEmpty.toList
+              $ defrag subs
+            <$> doc
+
+    foldFrags :: [Fragmenter] -> Fragmenter
+    foldFrags fs n = getAlt $ foldMap (Alt . ($ n)) fs
+
+-- * Operation
+
+-- TODO: Replace Maybe by MonadThrow CustomError
+operations
+  :: Schema.Subs
+  -> Fragmenter
+  -> [Full.OperationDefinition]
+  -> Maybe Core.Document
+operations subs fr = NonEmpty.nonEmpty <=< traverse (operation subs fr)
+
+-- TODO: Replace Maybe by MonadThrow CustomError
+operation
+  :: Schema.Subs
+  -> Fragmenter
+  -> Full.OperationDefinition
+  -> Maybe Core.Operation
+operation subs fr (Full.OperationSelectionSet sels) =
+  operation subs fr $ Full.OperationDefinition Full.Query empty empty empty sels
+-- TODO: Validate Variable definitions with substituter
+operation subs fr (Full.OperationDefinition ot _n _vars _dirs sels) =
+  case ot of
+    Full.Query    -> Core.Query    <$> node
+    Full.Mutation -> Core.Mutation <$> node
+  where
+    node = traverse (hush . selection subs fr) sels
+
+selection
+  :: Schema.Subs
+  -> Fragmenter
+  -> Full.Selection
+  -> Either [Core.Field] Core.Field
+selection subs fr (Full.SelectionField fld) =
+  Right $ field subs fr fld
+selection _    fr (Full.SelectionFragmentSpread (Full.FragmentSpread n _dirs)) =
+  Left $ fr n
+selection _ _  (Full.SelectionInlineFragment _)  =
+  error "Inline fragments not supported yet"
+
+-- * Fragment replacement
+
+-- | Extract Fragments into a single Fragmenter function and a Operation
+--   Definition.
+defrag
+  :: Schema.Subs
+  -> Full.Definition
+  -> Either Fragmenter Full.OperationDefinition
+defrag _    (Full.DefinitionOperation op) =
+  Right op
+defrag subs (Full.DefinitionFragment fragDef) =
+  Left $ fragmentDefinition subs fragDef
+
+fragmentDefinition :: Schema.Subs -> Full.FragmentDefinition -> Fragmenter
+fragmentDefinition subs (Full.FragmentDefinition name _tc _dirs sels) name' =
+  -- TODO: Support fragments within fragments. Fold instead of map.
+  if name == name'
+  then either id pure =<< NonEmpty.toList (selection subs mempty <$> sels)
+  else empty
+
+field :: Schema.Subs -> Fragmenter -> Full.Field -> Core.Field
+field subs fr (Full.Field a n args _dirs sels) =
+    Core.Field a n (fold $ argument subs `traverse` args) (foldr go empty sels)
+  where
+    go :: Full.Selection -> [Core.Field] -> [Core.Field]
+    go (Full.SelectionFragmentSpread (Full.FragmentSpread name _dirs)) = (fr name <>)
+    go sel =  (either id pure (selection subs fr sel) <>)
+
+argument :: Schema.Subs -> Full.Argument -> Maybe Core.Argument
+argument subs (Full.Argument n v) = Core.Argument n <$> value subs v
+
+value :: Schema.Subs -> Full.Value -> Maybe Core.Value
+value subs (Full.ValueVariable n) = subs n
+value _    (Full.ValueInt      i) = pure $ Core.ValueInt i
+value _    (Full.ValueFloat    f) = pure $ Core.ValueFloat f
+value _    (Full.ValueString   x) = pure $ Core.ValueString x
+value _    (Full.ValueBoolean  b) = pure $ Core.ValueBoolean b
+value _     Full.ValueNull        = pure   Core.ValueNull
+value _    (Full.ValueEnum     e) = pure $ Core.ValueEnum e
+value subs (Full.ValueList     l) =
+  Core.ValueList   <$> traverse (value subs) l
+value subs (Full.ValueObject   o) =
+  Core.ValueObject <$> traverse (objectField subs) o
+
+objectField :: Schema.Subs -> Full.ObjectField -> Maybe Core.ObjectField
+objectField subs (Full.ObjectField n v) = Core.ObjectField n <$> value subs v
+
+hush :: Either a b -> Maybe b
+hush = either (const Nothing) Just
diff --git a/src/Language/GraphQL/Encoder.hs b/src/Language/GraphQL/Encoder.hs
new file mode 100644
index 0000000..de5e2bb
--- /dev/null
+++ b/src/Language/GraphQL/Encoder.hs
@@ -0,0 +1,177 @@
+{-# LANGUAGE OverloadedStrings #-}
+-- | This module defines a printer for the @GraphQL@ language.
+module Language.GraphQL.Encoder where
+
+import Data.Foldable (fold)
+import Data.Monoid ((<>))
+import qualified Data.List.NonEmpty as NonEmpty (toList)
+import Data.Text (Text, cons, intercalate, pack, snoc)
+import Language.GraphQL.AST
+
+-- * Document
+
+document :: Document -> Text
+document defs = (`snoc` '\n') . mconcat . NonEmpty.toList $ definition <$> defs
+
+definition :: Definition -> Text
+definition (DefinitionOperation x) = operationDefinition x
+definition (DefinitionFragment  x) = fragmentDefinition x
+
+operationDefinition :: OperationDefinition -> Text
+operationDefinition (OperationSelectionSet sels) = selectionSet sels
+operationDefinition (OperationDefinition Query    name vars dirs sels) =
+  "query " <> node (fold name) vars dirs sels
+operationDefinition (OperationDefinition Mutation name vars dirs sels) =
+  "mutation " <> node (fold name) vars dirs sels
+
+node :: Name -> VariableDefinitions -> Directives -> SelectionSet -> Text
+node name vars dirs sels =
+       name
+    <> optempty variableDefinitions vars
+    <> optempty directives dirs
+    <> selectionSet sels
+
+variableDefinitions :: [VariableDefinition] -> Text
+variableDefinitions = parensCommas variableDefinition
+
+variableDefinition :: VariableDefinition -> Text
+variableDefinition (VariableDefinition var ty dv) =
+    variable var <> ":" <> type_ ty <> maybe mempty defaultValue dv
+
+defaultValue :: DefaultValue -> Text
+defaultValue val = "=" <> value val
+
+variable :: Variable -> Text
+variable var = "$" <> var
+
+selectionSet :: SelectionSet -> Text
+selectionSet = bracesCommas selection . NonEmpty.toList
+
+selectionSetOpt :: SelectionSetOpt -> Text
+selectionSetOpt = bracesCommas selection
+
+selection :: Selection -> Text
+selection (SelectionField          x) = field x
+selection (SelectionInlineFragment x) = inlineFragment x
+selection (SelectionFragmentSpread x) = fragmentSpread x
+
+field :: Field -> Text
+field (Field alias name args dirs selso) =
+       optempty (`snoc` ':') (fold alias)
+    <> name
+    <> optempty arguments args
+    <> optempty directives dirs
+    <> optempty selectionSetOpt selso
+
+arguments :: [Argument] -> Text
+arguments = parensCommas argument
+
+argument :: Argument -> Text
+argument (Argument name v) = name <> ":" <> value v
+
+-- * Fragments
+
+fragmentSpread :: FragmentSpread -> Text
+fragmentSpread (FragmentSpread name ds) =
+    "..." <> name <> optempty directives ds
+
+inlineFragment :: InlineFragment -> Text
+inlineFragment (InlineFragment tc dirs sels) =
+    "... on " <> fold tc
+              <> directives dirs
+              <> selectionSet sels
+
+fragmentDefinition :: FragmentDefinition -> Text
+fragmentDefinition (FragmentDefinition name tc dirs sels) =
+    "fragment " <> name <> " on " <> tc
+                <> optempty directives dirs
+                <> selectionSet sels
+
+-- * Values
+
+value :: Value -> Text
+value (ValueVariable x) = variable x
+-- TODO: This will be replaced with `decimal` Builder
+value (ValueInt      x) = pack $ show x
+-- TODO: This will be replaced with `decimal` Builder
+value (ValueFloat    x) = pack $ show x
+value (ValueBoolean  x) = booleanValue x
+value ValueNull         = mempty
+value (ValueString   x) = stringValue x
+value (ValueEnum     x) = x
+value (ValueList     x) = listValue x
+value (ValueObject   x) = objectValue x
+
+booleanValue :: Bool -> Text
+booleanValue True  = "true"
+booleanValue False = "false"
+
+-- TODO: Escape characters
+stringValue :: Text -> Text
+stringValue = quotes
+
+listValue :: ListValue -> Text
+listValue = bracketsCommas value
+
+objectValue :: ObjectValue -> Text
+objectValue = bracesCommas objectField
+
+objectField :: ObjectField -> Text
+objectField (ObjectField name v) = name <> ":" <> value v
+
+-- * Directives
+
+directives :: [Directive] -> Text
+directives = spaces directive
+
+directive :: Directive -> Text
+directive (Directive name args) = "@" <> name <> optempty arguments args
+
+-- * Type Reference
+
+type_ :: Type -> Text
+type_ (TypeNamed   x) = x
+type_ (TypeList    x) = listType x
+type_ (TypeNonNull x) = nonNullType x
+
+listType :: Type -> Text
+listType x = brackets (type_ x)
+
+nonNullType :: NonNullType -> Text
+nonNullType (NonNullTypeNamed x) = x <> "!"
+nonNullType (NonNullTypeList  x) = listType x <> "!"
+
+-- * Internal
+
+spaced :: Text -> Text
+spaced = cons '\SP'
+
+between :: Char -> Char -> Text -> Text
+between open close = cons open . (`snoc` close)
+
+parens :: Text -> Text
+parens = between '(' ')'
+
+brackets :: Text -> Text
+brackets = between '[' ']'
+
+braces :: Text -> Text
+braces = between '{' '}'
+
+quotes :: Text -> Text
+quotes = between '"' '"'
+
+spaces :: (a -> Text) -> [a] -> Text
+spaces f = intercalate "\SP" . fmap f
+
+parensCommas :: (a -> Text) -> [a] -> Text
+parensCommas f = parens . intercalate "," . fmap f
+
+bracketsCommas :: (a -> Text) -> [a] -> Text
+bracketsCommas f = brackets . intercalate "," . fmap f
+
+bracesCommas :: (a -> Text) -> [a] -> Text
+bracesCommas f = braces . intercalate "," . fmap f
+
+optempty :: (Eq a, Monoid a, Monoid b) => (a -> b) -> a -> b
+optempty f xs = if xs == mempty then mempty else f xs
diff --git a/src/Language/GraphQL/Error.hs b/src/Language/GraphQL/Error.hs
new file mode 100644
index 0000000..c2338b1
--- /dev/null
+++ b/src/Language/GraphQL/Error.hs
@@ -0,0 +1,57 @@
+{-# LANGUAGE OverloadedStrings #-}
+module Language.GraphQL.Error
+  ( parseError
+  , CollectErrsT
+  , addErr
+  , addErrMsg
+  , runCollectErrs
+  , runAppendErrs
+  ) where
+
+import qualified Data.Aeson as Aeson
+import Data.Text (Text, pack)
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.State ( StateT
+                                 , modify
+                                 , runStateT
+                                 )
+
+-- | Wraps a parse error into a list of errors.
+parseError :: Applicative f => String -> f Aeson.Value
+parseError s =
+  pure $ Aeson.object [("errors", Aeson.toJSON [makeErrorMsg $ pack s])]
+
+-- | A wrapper to pass error messages around.
+type CollectErrsT m = StateT [Aeson.Value] m
+
+-- | Adds an error to the list of errors.
+addErr :: Monad m => Aeson.Value -> CollectErrsT m ()
+addErr v = modify (v :)
+
+makeErrorMsg :: Text -> Aeson.Value
+makeErrorMsg s = Aeson.object [("message", Aeson.toJSON s)]
+
+-- | Convenience function for just wrapping an error message.
+addErrMsg :: Monad m => Text -> CollectErrsT m ()
+addErrMsg = addErr . makeErrorMsg
+
+-- | Appends the given list of errors to the current list of errors.
+appendErrs :: Monad m => [Aeson.Value] -> CollectErrsT m ()
+appendErrs errs = modify (errs ++)
+
+-- | Runs the given query computation, but collects the errors into an error
+--   list, which is then sent back with the data.
+runCollectErrs :: Monad m => CollectErrsT m Aeson.Value -> m Aeson.Value
+runCollectErrs res = do
+    (dat, errs) <- runStateT res []
+    if null errs
+       then return $ Aeson.object [("data", dat)]
+       else return $ Aeson.object [("data", dat), ("errors", Aeson.toJSON $ reverse errs)]
+
+-- | Runs the given computation, collecting the errors and appending them
+--   to the previous list of errors.
+runAppendErrs :: Monad m => CollectErrsT m a -> CollectErrsT m a
+runAppendErrs f = do
+    (v, errs) <- lift $ runStateT f []
+    appendErrs errs
+    return v
diff --git a/src/Language/GraphQL/Execute.hs b/src/Language/GraphQL/Execute.hs
new file mode 100644
index 0000000..eb53bba
--- /dev/null
+++ b/src/Language/GraphQL/Execute.hs
@@ -0,0 +1,38 @@
+{-# LANGUAGE OverloadedStrings #-}
+-- | This module provides the function to execute a @GraphQL@ request --
+--   according to a 'Schema'.
+module Language.GraphQL.Execute (execute) where
+
+import Control.Monad (MonadPlus(..))
+import qualified Data.List.NonEmpty as NE
+import Data.List.NonEmpty (NonEmpty((:|)))
+import qualified Data.Aeson as Aeson
+import qualified Language.GraphQL.AST as AST
+import qualified Language.GraphQL.AST.Core as AST.Core
+import qualified Language.GraphQL.AST.Transform as Transform
+import Language.GraphQL.Error
+import Language.GraphQL.Schema (Schema)
+import qualified Language.GraphQL.Schema as Schema
+
+-- | Takes a 'Schema', a variable substitution function ('Schema.Subs'), and a
+--   @GraphQL@ 'document'. The substitution is applied to the document using
+--  'rootFields', and the 'Schema''s resolvers are applied to the resulting fields.
+--
+--   Returns the result of the query against the 'Schema' wrapped in a /data/ field, or
+--   errors wrapped in an /errors/ field.
+execute
+  :: (MonadPlus m)
+  => Schema m -> Schema.Subs -> AST.Document -> m Aeson.Value
+execute schema subs doc = do
+    coreDocument <- maybe mzero pure (Transform.document subs doc)
+    document schema coreDocument
+
+document :: MonadPlus m => Schema m -> AST.Core.Document -> m Aeson.Value
+document schema (op :| []) = operation schema op
+document _ _ = error "Multiple operations not supported yet"
+
+operation :: MonadPlus m => Schema m -> AST.Core.Operation -> m Aeson.Value
+operation schema (AST.Core.Query flds)
+    = runCollectErrs (Schema.resolve (NE.toList schema) (NE.toList flds))
+operation schema (AST.Core.Mutation flds)
+    = runCollectErrs (Schema.resolve (NE.toList schema) (NE.toList flds))
diff --git a/src/Language/GraphQL/Parser.hs b/src/Language/GraphQL/Parser.hs
new file mode 100644
index 0000000..215da73
--- /dev/null
+++ b/src/Language/GraphQL/Parser.hs
@@ -0,0 +1,183 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Language.GraphQL.Parser where
+
+import Control.Applicative ( Alternative(..)
+                           , optional
+                           )
+import Data.List.NonEmpty (NonEmpty(..))
+import Language.GraphQL.AST
+import Language.GraphQL.Lexer
+import Text.Megaparsec ( lookAhead
+                       , option
+                       , try
+                       , (<?>)
+                       )
+
+document :: Parser Document
+document = spaceConsumer >> lexeme (manyNE definition)
+
+definition :: Parser Definition
+definition = DefinitionOperation <$> operationDefinition
+         <|> DefinitionFragment  <$> fragmentDefinition
+         <?> "definition error!"
+
+operationDefinition :: Parser OperationDefinition
+operationDefinition = OperationSelectionSet <$> selectionSet
+                  <|> OperationDefinition   <$> operationType
+                                            <*> optional name
+                                            <*> opt variableDefinitions
+                                            <*> opt directives
+                                            <*> selectionSet
+                  <?> "operationDefinition error"
+
+operationType :: Parser OperationType
+operationType = Query <$ symbol "query"
+    <|> Mutation <$ symbol "mutation"
+    <?> "operationType error"
+
+-- * SelectionSet
+
+selectionSet :: Parser SelectionSet
+selectionSet = braces $ manyNE selection
+
+selectionSetOpt :: Parser SelectionSetOpt
+selectionSetOpt = braces $ some selection
+
+selection :: Parser Selection
+selection = SelectionField          <$> field
+        <|> try (SelectionFragmentSpread <$> fragmentSpread)
+        <|> SelectionInlineFragment <$> inlineFragment
+        <?> "selection error!"
+
+-- * Field
+
+field :: Parser Field
+field = Field <$> optional alias
+              <*> name
+              <*> opt arguments
+              <*> opt directives
+              <*> opt selectionSetOpt
+
+alias :: Parser Alias
+alias = try $ name <* colon
+
+-- * Arguments
+
+arguments :: Parser Arguments
+arguments = parens $ some argument
+
+argument :: Parser Argument
+argument = Argument <$> name <* colon <*> value
+
+-- * Fragments
+
+fragmentSpread :: Parser FragmentSpread
+fragmentSpread = FragmentSpread <$  spread
+                                <*> fragmentName
+                                <*> opt directives
+
+inlineFragment :: Parser InlineFragment
+inlineFragment = InlineFragment <$  spread
+                                <*> optional typeCondition
+                                <*> opt directives
+                                <*> selectionSet
+
+fragmentDefinition :: Parser FragmentDefinition
+fragmentDefinition = FragmentDefinition
+                 <$  symbol "fragment"
+                 <*> name
+                 <*> typeCondition
+                 <*> opt directives
+                 <*> selectionSet
+
+fragmentName :: Parser FragmentName
+fragmentName = but (symbol "on") *> name
+
+typeCondition :: Parser TypeCondition
+typeCondition = symbol "on" *> name
+
+-- * Input Values
+
+value :: Parser Value
+value = ValueVariable <$> variable
+    <|> ValueFloat    <$> try float
+    <|> ValueInt      <$> integer
+    <|> ValueBoolean  <$> booleanValue
+    <|> ValueNull     <$  symbol "null"
+    <|> ValueString   <$> string
+    <|> ValueString   <$> blockString
+    <|> ValueEnum     <$> try enumValue
+    <|> ValueList     <$> listValue
+    <|> ValueObject   <$> objectValue
+    <?> "value error!"
+  where
+    booleanValue :: Parser Bool
+    booleanValue = True  <$ symbol "true"
+               <|> False <$ symbol "false"
+
+    enumValue :: Parser Name
+    enumValue = but (symbol "true") *> but (symbol "false") *> but (symbol "null") *> name
+
+    listValue :: Parser [Value]
+    listValue = brackets $ some value
+
+    objectValue :: Parser [ObjectField]
+    objectValue = braces $ some objectField
+
+objectField :: Parser ObjectField
+objectField = ObjectField <$> name <* symbol ":" <*> value
+
+-- * Variables
+
+variableDefinitions :: Parser VariableDefinitions
+variableDefinitions = parens $ some variableDefinition
+
+variableDefinition :: Parser VariableDefinition
+variableDefinition = VariableDefinition <$> variable
+                                        <*  colon
+                                        <*> type_
+                                        <*> optional defaultValue
+variable :: Parser Variable
+variable = dollar *> name
+
+defaultValue :: Parser DefaultValue
+defaultValue = equals *> value
+
+-- * Input Types
+
+type_ :: Parser Type
+type_ = try (TypeNamed <$> name <* but "!")
+    <|> TypeList       <$> brackets type_
+    <|> TypeNonNull    <$> nonNullType
+    <?> "type_ error!"
+
+nonNullType :: Parser NonNullType
+nonNullType = NonNullTypeNamed <$> name <* bang
+          <|> NonNullTypeList  <$> brackets type_  <* bang
+          <?> "nonNullType error!"
+
+-- * Directives
+
+directives :: Parser Directives
+directives = some directive
+
+directive :: Parser Directive
+directive = Directive
+        <$  at
+        <*> name
+        <*> opt arguments
+
+-- * Internal
+
+opt :: Monoid a => Parser a -> Parser a
+opt = option mempty
+
+-- Hack to reverse parser success
+but :: Parser a -> Parser ()
+but pn = False <$ lookAhead pn <|> pure True >>= \case
+    False -> empty
+    True  -> pure ()
+
+manyNE :: Alternative f => f a -> f (NonEmpty a)
+manyNE p = (:|) <$> p <*> many p
diff --git a/src/Language/GraphQL/Schema.hs b/src/Language/GraphQL/Schema.hs
new file mode 100644
index 0000000..5569155
--- /dev/null
+++ b/src/Language/GraphQL/Schema.hs
@@ -0,0 +1,172 @@
+{-# 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
+  , 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(..))
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.Except (runExceptT)
+import Data.Foldable ( find
+                     , fold
+                     )
+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)
+import qualified Data.Text as T
+import Language.GraphQL.Error
+import Language.GraphQL.Trans
+import Language.GraphQL.Type
+import Language.GraphQL.AST.Core
+
+-- | A GraphQL schema.
+--   @f@ is usually expected to be an instance of 'Alternative'.
+type Schema m = NonEmpty (Resolver m)
+
+-- | 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
+    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
+  where
+    resolveRight fld (Named result) = withField (return result) fld
+    resolveRight fld Null
+        = return $ HashMap.singleton (aliasOrName fld) Aeson.Null
+    resolveRight fld (List result) = withField (return result) fld
+
+-- | 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
+    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
+  where
+    resolveRight fld (Named resolver) = withField (return resolver) fld
+    resolveRight fld Null
+        = 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)
+resolveFieldValue f resolveRight fld@(Field _ _ args _) = do
+    result <- lift $ runExceptT . runActionT $ f args
+    either resolveLeft (resolveRight fld) result
+      where
+        resolveLeft err = do
+            _ <- addErrMsg err
+            return $ HashMap.singleton (aliasOrName fld) Aeson.Null
+
+-- | Helper function to facilitate 'Argument' handling.
+withField :: (MonadPlus 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 :: MonadPlus m
+    => [Resolver m] -> Fields -> CollectErrsT m Aeson.Value
+resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
+  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
+    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