Merge pull request #233 from HuwCampbell/topic/canonical_instances

Add MonadFail instances where appropriate

Author: jacobstanley

hedgehog/hedgehog.cabal

@@ -59,6 +59,7 @@ library
     , containers                      >= 0.4        && < 0.7
     , directory                       >= 1.2        && < 1.4
     , exceptions                      >= 0.7        && < 0.11
+    , fail                            >= 4.9        && < 5
     , lifted-async                    >= 0.7        && < 0.11
     , mmorph                          >= 1.0        && < 1.2
     , monad-control                   >= 1.0        && < 1.1

hedgehog/src/Hedgehog/Internal/Gen.hs

@@ -158,6 +158,8 @@ import           Control.Monad (MonadPlus(..), filterM, replicateM, ap, join)
 import           Control.Monad.Base (MonadBase(..))
 import           Control.Monad.Catch (MonadThrow(..), MonadCatch(..))
 import           Control.Monad.Error.Class (MonadError(..))
+import           Control.Monad.Fail (MonadFail (..))
+import qualified Control.Monad.Fail as Fail
 import           Control.Monad.IO.Class (MonadIO(..))
 import           Control.Monad.Morph (MFunctor(..), MMonad(..), generalize)
 import           Control.Monad.Primitive (PrimMonad(..))
@@ -553,6 +555,13 @@ instance Monad m => Monad (GenT m) where
           runGenT size sk . k =<<
           runGenT size sm m
 
+  fail =
+    Fail.fail
+
+instance Monad m => MonadFail (GenT m) where
+  fail =
+    error
+
 instance Monad m => Alternative (GenT m) where
   empty =
     mzero

hedgehog/src/Hedgehog/Internal/Property.hs

@@ -84,6 +84,8 @@ import           Control.Monad.Base (MonadBase(..))
 import           Control.Monad.Catch (MonadThrow(..), MonadCatch(..))
 import           Control.Monad.Catch (SomeException(..), displayException)
 import           Control.Monad.Error.Class (MonadError(..))
+import           Control.Monad.Fail (MonadFail (..))
+import qualified Control.Monad.Fail as Fail
 import           Control.Monad.IO.Class (MonadIO(..))
 import           Control.Monad.Morph (MFunctor(..))
 import           Control.Monad.Primitive (PrimMonad(..))
@@ -317,6 +319,10 @@ instance Monad m => Monad (TestT m) where
       unTest m >>=
       unTest . k
 
+  fail =
+    Fail.fail
+
+instance Monad m => MonadFail (TestT m) where
   fail err =
     TestT . ExceptT . pure . Left $ Failure Nothing err Nothing