Add MonadFix instance for FreshIdT

2019-03-09 16:44:40 -05:00 · 2019-03-09 16:44:40 -05:00 · a1fd246d01
parent fd63be4f96
commit a1fd246d01
1 changed files with 7 additions and 1 deletions
--- a/src/Nix/Thunk.hs
+++ b/src/Nix/Thunk.hs
@ -49,7 +49,13 @@ class Monad m => MonadFreshId i m | m -> i where
  freshId = lift freshId

 newtype FreshIdT i m a = FreshIdT { unFreshIdT :: StateT i m a }
-  deriving (Functor, Applicative, Monad, MonadTrans)
+  deriving
+    ( Functor
+    , Applicative
+    , Monad
+    , MonadTrans
+    , MonadFix
+    )

 instance (Monad m, Num i) => MonadFreshId i (FreshIdT i m) where
  freshId = FreshIdT $ get <* modify (+ 1)