1 files changed, 46 insertions, 13 deletions

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