Constrain base monad to MonadCatch

Let's try MonadThrow/MonadCatch. It looks nice at a first glance. The
monad transformer stack contains only the ReaderT, less lifts are
required. Exception subtyping is easier, the user can (and should)
define custom error types and throw them. And it is still possible to
use pure error handling, if someone doesn't like runtime exceptions or
need to run a query in a pure environment.

Fixes #42.

src/Language/GraphQL.hs

@@ -7,6 +7,7 @@ module Language.GraphQL
     , graphqlSubs
     ) where
 
+import Control.Monad.Catch (MonadCatch)
 import qualified Data.Aeson as Aeson
 import qualified Data.Aeson.Types as Aeson
 import Data.Either (fromRight)
@@ -20,7 +21,7 @@ import Text.Megaparsec (parse)
 
 -- | If the text parses correctly as a @GraphQL@ query the query is
 -- executed using the given 'Schema'.
-graphql :: Monad m
+graphql :: MonadCatch m
     => Schema m -- ^ Resolvers.
     -> Text -- ^ Text representing a @GraphQL@ request document.
     -> m Aeson.Value -- ^ Response.
@@ -29,7 +30,7 @@ graphql schema = graphqlSubs schema mempty mempty
 -- | If the text parses correctly as a @GraphQL@ query the substitution is
 -- applied to the query and the query is then executed using to the given
 -- 'Schema'.
-graphqlSubs :: Monad m
+graphqlSubs :: MonadCatch m
     => Schema m -- ^ Resolvers.
     -> Maybe Text -- ^ Operation name.
     -> Aeson.Object -- ^ Variable substitution function.