Cherry-pick 76734205 into master

tests/raytrace/Eval.hs

@@ -33,7 +33,6 @@ instance Applicative Pure where
 instance Monad Pure where
     Pure x >>= k = k x
     return       = pure
-    fail s       = error s
 
 instance MonadEval Pure where
   doOp   = doPureOp
@@ -306,7 +305,9 @@ instance Monad Abs where
 			           AbsState r s' -> runAbs (k r) s'
                                    AbsFail m     -> AbsFail m)
     return       = pure
-    fail s       = Abs (\ n -> AbsFail s)
+
+instance MonadFail Abs where
+    fail s = Abs (\ n -> AbsFail s)
 
 instance MonadEval Abs where
   doOp = doAbsOp