Improve FAlternative class

1 files changed, 7 insertions, 1 deletions

diff --git a/src/Control/FAlternative.hs b/src/Control/FAlternative.hs
index 473b3df..8fd35c3 100644
--- a/src/Control/FAlternative.hs
+++ b/src/Control/FAlternative.hs
@@ -1,6 +1,8 @@
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 module Control.FAlternative where
 
 import Data.List.NonEmpty (NonEmpty(..))
@@ -24,7 +26,11 @@ class (forall fail. Applicative (f fail)) => FAlternative f where
   faempty :: f 'Fallible a
   (<|>>) :: f 'Fallible a -> f fail a -> f fail a
   noFail :: f 'Infallible a -> f fail a
-  toFallible :: KnownFallible fail => f fail a -> f 'Fallible a
+
+toFallible :: forall fail f a. (FAlternative f, KnownFallible fail) => f fail a -> f 'Fallible a
+toFallible = case knownFallible @fail of
+               SFallible -> id
+               SInfallible -> noFail
 
 faasum :: FAlternative f => [f 'Fallible a] -> f fail a -> f fail a
 faasum l p = foldr (<|>>) p l