6 files changed, 162 insertions, 16 deletions
diff --git a/Data/GraphQL/AST.hs b/Data/GraphQL/AST.hs
index 2703a5b..99eaa79 100644
--- a/Data/GraphQL/AST.hs
+++ b/Data/GraphQL/AST.hs
@@ -1,3 +1,6 @@
+-- | This module defines an abstract syntax tree for the @GraphQL@ language based on
+--   <https://facebook.github.io/graphql/ Facebook's GraphQL Specification>.
+
 module Data.GraphQL.AST where
 
 import Data.Int (Int32)
@@ -39,11 +42,26 @@ data Selection = SelectionField Field
                | SelectionInlineFragment InlineFragment
                  deriving (Eq,Show)
 
+-- | A 'SelectionSet' is primarily composed of 'Field's. A 'Field' describes one
+--   discrete piece of information available to request within a 'SelectionSet'.
+--
+--   Some 'Field's describe complex data or relationships to other data. In
+--   order to further explore this data, a 'Field' may itself contain a
+--   'SelectionSet', allowing for deeply nested requests. All @GraphQL@ operations
+--   must specify their 'Selection's down to 'Field's which return scalar values to
+--   ensure an unambiguously shaped response.
+--
+--   <https://facebook.github.io/graphql/#sec-Language.Query-Document.Fields Field Specification>
 data Field = Field Alias Name [Argument] [Directive] SelectionSet
              deriving (Eq,Show)
 
 type Alias = Name
 
+-- | 'Field's are conceptually functions which return values, and occasionally accept
+--   'Argument's which alter their behavior. These 'Argument's often map directly to
+--   function arguments within a @GraphQL@ server’s implementation.
+--
+--   <https://facebook.github.io/graphql/#sec-Language.Query-Document.Arguments Argument Specification>
 data Argument = Argument Name Value deriving (Eq,Show)
 
 -- * Fragments
@@ -63,6 +81,15 @@ type TypeCondition = NamedType
 
 -- * Values
 
+-- | 'Field' and 'Directive' 'Arguments' accept input values of various literal
+--   primitives; input values can be scalars, enumeration values, lists, or input
+--   objects.
+--
+--   If not defined as constant (for example, in 'DefaultValue'), input values
+--   can be specified as a 'Variable'. List and inputs objects may also contain
+--   'Variable's (unless defined to be constant).
+--
+--   <https://facebook.github.io/graphql/#sec-Input-Values Input Value Specification>
 data Value = ValueVariable Variable
            | ValueInt Int32
            -- GraphQL Float is double precison
diff --git a/Data/GraphQL/Encoder.hs b/Data/GraphQL/Encoder.hs
index 2b7e239..86f090b 100644
--- a/Data/GraphQL/Encoder.hs
+++ b/Data/GraphQL/Encoder.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE OverloadedStrings #-}
+-- | This module defines a printer for the @GraphQL@ language.
 module Data.GraphQL.Encoder where
 
 #if !MIN_VERSION_base(4,8,0)
diff --git a/Data/GraphQL/Error.hs b/Data/GraphQL/Error.hs
new file mode 100644
index 0000000..74f08e4
--- /dev/null
+++ b/Data/GraphQL/Error.hs
@@ -0,0 +1,63 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Data.GraphQL.Error (
+  parseError,
+  CollectErrsT,
+  addErr,
+  addErrMsg,
+  runCollectErrs,
+  joinErrs,
+  errWrap
+  ) where
+
+import qualified Data.Aeson as Aeson
+import Data.Text (Text, pack)
+
+import Control.Arrow ((&&&))
+
+#if !MIN_VERSION_base(4,8,0)
+import Control.Applicative (Applicative, pure)
+import Data.Foldable (Foldable, concatMap)
+import Prelude hiding (concatMap)
+#endif
+
+-- | 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 for an 'Applicative' to pass error messages around.
+type CollectErrsT f a = f (a,[Aeson.Value])
+
+-- | Takes a (wrapped) list (foldable functor) of values and errors,
+--   joins the values into a list and concatenates the errors.
+joinErrs
+  :: (Functor m, Functor f, Foldable f)
+  => m (f (a,[Aeson.Value])) -> CollectErrsT m (f a)
+joinErrs = fmap $ fmap fst &&& concatMap snd
+
+-- | Wraps the given 'Applicative' to handle errors
+errWrap :: Functor f => f a -> f (a, [Aeson.Value])
+errWrap = fmap (flip (,) [])
+
+-- | Adds an error to the list of errors.
+addErr :: Functor f => Aeson.Value -> CollectErrsT f a -> CollectErrsT f a
+addErr v = (fmap . fmap) (v :)
+
+makeErrorMsg :: Text -> Aeson.Value
+makeErrorMsg s = Aeson.object [("message",Aeson.toJSON s)]
+
+-- | Convenience function for just wrapping an error message.
+addErrMsg :: Functor f => Text -> CollectErrsT f a -> CollectErrsT f a
+addErrMsg = addErr . makeErrorMsg
+
+-- | Runs the given query, but collects the errors into an error
+--   list which is then sent back with the data.
+runCollectErrs :: Functor f => CollectErrsT f Aeson.Value -> f Aeson.Value
+runCollectErrs = fmap finalD
+  where
+    finalD (dat,errs) =
+        Aeson.object
+      $ if null errs
+           then [("data",dat)]
+           else [("data",dat),("errors",Aeson.toJSON $ reverse errs)]
diff --git a/Data/GraphQL/Execute.hs b/Data/GraphQL/Execute.hs
index 47d1d03..86887ff 100644
--- a/Data/GraphQL/Execute.hs
+++ b/Data/GraphQL/Execute.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE CPP #-}
+-- | This module provides the function to execute a @GraphQL@ request --
+--   according to a 'Schema'.
 module Data.GraphQL.Execute (execute) where
 
 #if !MIN_VERSION_base(4,8,0)
@@ -13,16 +15,32 @@ import Data.GraphQL.AST
 import Data.GraphQL.Schema (Schema(..))
 import qualified Data.GraphQL.Schema as Schema
 
-execute
-  :: Alternative f
+import Data.GraphQL.Error
+
+-- | 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 :: Alternative f
   => Schema.Schema f -> Schema.Subs -> Document -> f Aeson.Value
-execute (Schema resolvs) subs = Schema.resolvers resolvs . rootFields subs
+execute (Schema resolvs) subs doc = runCollectErrs res
+  where res = Schema.resolvers resolvs $ rootFields subs doc
 
+-- | Takes a variable substitution function and a @GraphQL@ document.
+--   If the document contains one query (and no other definitions)
+--   it applies the substitution to the query's set of selections
+--   and then returns their fields.
 rootFields :: Schema.Subs -> Document -> [Field]
 rootFields subs (Document [DefinitionOperation (Query (Node _varDefs _ _ sels))]) =
     Schema.fields $ substitute subs <$> sels
 rootFields _ _ = []
 
+-- | Takes a variable substitution function and a selection. If the
+--   selection is a field it applies the substitution to the field's
+--   arguments using 'subsArg', and recursively applies the substitution to
+--   the arguments of fields nested in the primary field.
 substitute :: Schema.Subs -> Selection -> Selection
 substitute subs (SelectionField (Field alias name args directives sels)) =
     SelectionField $ Field
@@ -35,6 +53,9 @@ substitute subs (SelectionField (Field alias name args directives sels)) =
 substitute _ sel = sel
 
 -- TODO: Support different value types
+-- | Takes a variable substitution function and an argument. If the
+--   argument's value is a variable the substitution is applied to the
+--   variable's name.
 subsArg :: Schema.Subs -> Argument -> Maybe Argument
 subsArg subs (Argument n (ValueVariable (Variable v))) =
     Argument n . ValueString <$> subs v
diff --git a/Data/GraphQL/Parser.hs b/Data/GraphQL/Parser.hs
index 6658af0..227c9a8 100644
--- a/Data/GraphQL/Parser.hs
+++ b/Data/GraphQL/Parser.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE OverloadedStrings #-}
+-- | This module defines a parser for @GraphQL@ request documents.
 module Data.GraphQL.Parser where
 
 import Prelude hiding (takeWhile)
diff --git a/Data/GraphQL/Schema.hs b/Data/GraphQL/Schema.hs
index 1c45af2..7966392 100644
--- a/Data/GraphQL/Schema.hs
+++ b/Data/GraphQL/Schema.hs
@@ -1,5 +1,8 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE LambdaCase #-}
+-- | This module provides a representation of a @GraphQL@ Schema in addition to
+--   functions for defining and manipulating Schemas.
 module Data.GraphQL.Schema
   ( Schema(..)
   , Resolver
@@ -21,14 +24,16 @@ module Data.GraphQL.Schema
   ) where
 
 #if !MIN_VERSION_base(4,8,0)
-import Control.Applicative (pure, (<|>))
+import Control.Applicative (pure)
+import Control.Arrow (first)
 import Data.Foldable (foldMap)
 import Data.Traversable (traverse)
 import Data.Monoid (Monoid(mempty,mappend))
 #else
+import Data.Bifunctor (first)
 import Data.Monoid (Alt(Alt,getAlt))
 #endif
-import Control.Applicative (Alternative, empty)
+import Control.Applicative (Alternative((<|>), empty))
 import Data.Maybe (catMaybes)
 import Data.Foldable (fold)
 
@@ -36,77 +41,105 @@ 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 (null)
+import qualified Data.Text as T (null, unwords)
 
 import Data.GraphQL.AST
+import Data.GraphQL.Error
 
+-- | A GraphQL schema.
+--   @f@ is usually expected to be an instance of 'Alternative'.
 data Schema f = Schema [Resolver f]
 
-type Resolver  f = Field -> f Aeson.Object
+-- | Resolves a 'Field' into an @Aeson.@'Aeson.Object' with error information
+--   (or 'empty'). @f@ is usually expected to be an instance of 'Alternative'.
+type Resolver f = Field -> CollectErrsT f Aeson.Object
 
+-- | Variable substitution function.
 type Subs = Text -> Maybe Text
 
+-- | Create a named 'Resolver' from a list of 'Resolver's.
 object :: Alternative f => Text -> [Resolver f] -> Resolver f
 object name resolvs = objectA name $ \case
      [] -> resolvs
      _  -> empty
 
+-- | Like 'object' but also taking 'Argument's.
 objectA
   :: Alternative f
   => Text -> ([Argument] -> [Resolver f]) -> Resolver f
 objectA name f fld@(Field _ _ args _ sels) =
     withField name (resolvers (f args) $ fields sels) fld
 
+-- | A scalar represents a primitive value, like a string or an integer.
 scalar :: (Alternative f, Aeson.ToJSON a) => Text -> a -> Resolver f
 scalar name s = scalarA name $ \case
     [] -> pure s
     _  -> empty
 
+-- | Like 'scalar' but also taking 'Argument's.
 scalarA
   :: (Alternative f, Aeson.ToJSON a)
   => Text -> ([Argument] -> f a) -> Resolver f
-scalarA name f fld@(Field _ _ args _ []) = withField name (f args) fld
+scalarA name f fld@(Field _ _ args _ []) = withField name (errWrap $ f args) fld
 scalarA _ _ _ = empty
 
+-- | Like 'object' but taking lists of 'Resolver's instead of a single list.
 array :: Alternative f => Text -> [[Resolver f]] -> Resolver f
 array name resolvs = arrayA name $ \case
     [] -> resolvs
     _  -> empty
 
+-- | Like 'array' but also taking 'Argument's.
 arrayA
   :: Alternative f
   => Text -> ([Argument] -> [[Resolver f]]) -> Resolver f
 arrayA name f fld@(Field _ _ args _ sels) =
-     withField name (traverse (flip resolvers $ fields sels) $ f args) fld
+     withField name (joinErrs $ traverse (flip resolvers $ fields sels) $ f args) fld
 
+-- | Represents one of a finite set of possible values.
+--   Used in place of a 'scalar' when the possible responses are easily enumerable.
 enum :: Alternative f => Text -> f [Text] -> Resolver f
 enum name enums = enumA name $ \case
      [] -> enums
      _  -> empty
 
+-- | Like 'enum' but also taking 'Argument's.
 enumA :: Alternative f => Text -> ([Argument] -> f [Text]) -> Resolver f
-enumA name f fld@(Field _ _ args _ []) = withField name (f args) fld
+enumA name f fld@(Field _ _ args _ []) = withField name (errWrap $ f args) fld
 enumA _ _ _ = empty
 
+-- | Helper function to facilitate 'Argument' handling.
 withField
   :: (Alternative f, Aeson.ToJSON a)
-  => Text -> f a -> Field -> f (HashMap Text Aeson.Value)
+  => Text -> CollectErrsT f a -> Field -> CollectErrsT f (HashMap Text Aeson.Value)
 withField name f (Field alias name' _ _ _) =
      if name == name'
-        then fmap (HashMap.singleton aliasOrName . Aeson.toJSON) f
+        then fmap (first $ HashMap.singleton aliasOrName . Aeson.toJSON) f
         else empty
      where
        aliasOrName = if T.null alias then name' else alias
 
-resolvers :: Alternative f => [Resolver f] -> [Field] -> f Aeson.Value
+-- | 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.
+resolvers :: Alternative f => [Resolver f] -> [Field] -> CollectErrsT f Aeson.Value
 resolvers resolvs =
-    fmap (Aeson.toJSON . fold)
-  . traverse (\fld -> getAlt $ foldMap (Alt . ($ fld)) resolvs)
-
+    fmap (first Aeson.toJSON . fold)
+  . traverse (\fld -> getAlt (foldMap (Alt . ($ fld)) resolvs) <|> errmsg fld)
+  where
+    errmsg (Field alias name _ _ _) = addErrMsg msg $ (errWrap . pure) val
+       where
+         val = HashMap.singleton aliasOrName Aeson.Null
+         msg = T.unwords ["field", name, "not resolved."]
+         aliasOrName = if T.null alias then name else alias
+
+-- | Checks whether the given 'Selection' contains a 'Field' and
+--   returns the 'Field' if so, else returns 'Nothing'.
 field :: Selection -> Maybe Field
 field (SelectionField x) = Just x
 field _ = Nothing
 
+-- | Returns a list of the 'Field's contained in the given 'SelectionSet'.
 fields :: SelectionSet -> [Field]
 fields = catMaybes . fmap field