From 09135c581aaae471f7d964bc2a3a141bef299097 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Fri, 17 Jul 2020 07:05:03 +0200
Subject: 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.
---
 tests/Test/StarWars/Data.hs | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

(limited to 'tests/Test/StarWars/Data.hs')

diff --git a/tests/Test/StarWars/Data.hs b/tests/Test/StarWars/Data.hs
index 00a89d9..e3dd696 100644
--- a/tests/Test/StarWars/Data.hs
+++ b/tests/Test/StarWars/Data.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 module Test.StarWars.Data
     ( Character
+    , StarWarsException(..)
     , appearsIn
     , artoo
     , getDroid
@@ -16,11 +17,12 @@ module Test.StarWars.Data
     , typeName
     ) where
 
-import Data.Functor.Identity (Identity)
+import Control.Monad.Catch (Exception(..), MonadThrow(..), SomeException)
 import Control.Applicative (Alternative(..), liftA2)
-import Control.Monad.Trans.Except (throwE)
 import Data.Maybe (catMaybes)
 import Data.Text (Text)
+import Data.Typeable (cast)
+import Language.GraphQL.Error
 import Language.GraphQL.Type
 
 -- * Data
@@ -66,8 +68,20 @@ appearsIn :: Character -> [Int]
 appearsIn (Left  x) = _appearsIn . _droidChar $ x
 appearsIn (Right x) = _appearsIn . _humanChar $ x
 
-secretBackstory :: Resolve Identity
-secretBackstory = throwE "secretBackstory is secret."
+data StarWarsException = SecretBackstory | InvalidArguments
+
+instance Show StarWarsException where
+    show SecretBackstory = "secretBackstory is secret."
+    show InvalidArguments = "Invalid arguments."
+
+instance Exception StarWarsException where
+    toException = toException . ResolverException
+    fromException e = do
+        ResolverException resolverException <- fromException e
+        cast resolverException
+
+secretBackstory :: Resolve (Either SomeException)
+secretBackstory = throwM SecretBackstory
 
 typeName :: Character -> Text
 typeName = either (const "Droid") (const "Human")
-- 
cgit v1.2.3