Initial support for variable substitution

The correspondent end-to-end test has been ported. The variable
definition still needs to be checked.

Data/GraphQL/Schema.hs

@@ -4,10 +4,11 @@ module Data.GraphQL.Schema where
 #if !MIN_VERSION_base(4,8,0)
 import Control.Applicative ((<$>))
 #endif
+import Data.String (IsString(fromString))
 
 import Data.Aeson (ToJSON(toJSON))
 import Data.HashMap.Strict (HashMap)
-import Data.Text (Text)
+import Data.Text (Text, pack)
 
 data Schema f = Schema (QueryRoot f) -- (Maybe  MutationRoot)
 
@@ -25,6 +26,8 @@ data Output = OutputObject (HashMap Text Output)
 
 type Argument = (Text, Scalar)
 
+type Subs = Text -> Maybe Scalar
+
 data Input = InputField Text [Argument] [Input]
              deriving (Show)
 
@@ -36,6 +39,9 @@ data Scalar = ScalarInt     Int
             | ScalarID      Text
               deriving (Show)
 
+instance IsString Scalar where
+  fromString = ScalarString . pack
+
 instance ToJSON Scalar where
     toJSON (ScalarInt     x) = toJSON x
     toJSON (ScalarFloat   x) = toJSON x