Rough implementation of `execute`

The first end-to-end test taken from `graphql-js` passes but this still
needs to be extended to support more general cases.

- `Data.GraphQL.Schema` has been heavily modified to support the
  execution model. More drastic changes are expected in this module.
- When defining a `Schema` ordinary functions taking fields as input are
  being used instead of maps. This makes the implementation of `execute`
  easier, and, arguably, makes `Schema` definitions more *Haskellish*.
- Drop explicit `unordered-containers` dependency. `Aeson.Value`s and
  field functions should be good enough for now.

1 files changed, 19 insertions, 3 deletions

diff --git a/Data/GraphQL/Execute.hs b/Data/GraphQL/Execute.hs
index 06dc9a6..40eb122 100644
--- a/Data/GraphQL/Execute.hs
+++ b/Data/GraphQL/Execute.hs
@@ -1,16 +1,32 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE OverloadedLists #-}
 module Data.GraphQL.Execute where
 
 #if !MIN_VERSION_base(4,8,0)
 import Control.Applicative (Applicative)
 #endif
 
-import qualified Data.Aeson as Aeson (Value)
+import qualified Data.Aeson as Aeson
 
 import Data.GraphQL.AST
 import Data.GraphQL.Schema
 
 type Response = Aeson.Value
 
-execute :: Applicative f => Schema f -> Document -> f Response
-execute _schema _doc = undefined
+execute :: Schema -> Document -> Maybe Response
+execute (Schema resolv0) doc = go resolv0 =<< root doc
+  where
+
+    root :: Document -> Maybe Selection
+    root (Document [DefinitionOperation (Query (Node _ _ _ [sel]))]) = Just sel
+    root _ = error "root: Not implemented yet"
+
+    go :: Resolver -> Selection -> Maybe Response
+    go resolv (SelectionField (Field _ n _ _ sfs)) =
+        case resolv (InputField n) of
+            (OutputScalar s) -> if null sfs
+                                   then Just $ Aeson.Object [(n, Aeson.toJSON s)]
+                                   else Nothing
+            (OutputResolver resolv') -> (\r-> Aeson.Object [(n, r)]) <$> go resolv' (head sfs)
+            _ -> error "go case resolv: Not implemented yet"
+    go _ _ = error "go: Not implemented yet"