selective 0.6 → 0.6.1

raw patch · 3 files changed

+43/−4 lines, 3 filesdep ~basedep ~transformers

Dependency ranges changed: base, transformers

Files

• CHANGES.md +5/−0
• selective.cabal +1/−1
• src/Control/Selective/Trans/Except.hs +37/−3

@@ -1,5 +1,10 @@ # Change log +## 0.6.1++* Drop `MonadTrans (ExceptT e)` instance to allow `transformers-0.6.1`.+  See #70.+ ## 0.6  * Start supporting GHC 9.4. See #66.

@@ -1,5 +1,5 @@ name:          selective-version:       0.6+version:       0.6.1 synopsis:      Selective applicative functors license:       MIT license-file:  LICENSE

@@ -38,7 +38,6 @@ #endif  import qualified Control.Monad.Trans.Except as T-import Control.Monad.Trans.Class  import Control.Selective import Control.Monad.Signatures@@ -48,9 +47,44 @@ newtype ExceptT e f a = ExceptT { unwrap :: T.ExceptT e f a }   deriving     ( Functor, Foldable, Traversable, Monad, Contravariant, Eq, Ord, Read, Show-    , MonadTrans, MonadFix, MonadFail, MonadZip, MonadIO, MonadPlus, Eq1, Ord1-    , Read1, Show1 )+    , MonadFix, MonadFail, MonadZip, MonadIO, MonadPlus, Eq1, Ord1, Read1, Show1 )   deriving (Applicative, Selective, Alternative) via (ComposeEither f e)++{- Why don't we provide a `MonadTrans (ExceptT e)` instance?++   Recall the definition of the MonadTrans type class:++     class (forall m. Monad m => Monad (t m)) => MonadTrans t where+         lift :: Monad m => m a -> t m a++   If we instantiate `t` to `ExceptT e` in the constraint, we get++     forall m. Monad m => Monad (ExceptT e m)++   but the `Applicative (ExceptT e m)` instance comes with the `Selective m`+   constraint, and since Selective is not a superclass of Monad, we're stuck.+   In other words, `ExceptT` is really not a universal monad transformer: it+   works only for monads `m` that also happen to have a `Selective m` instance.++   I can see three possible solutions but none of them has a chance of working+   in practice:++     * Change the constraint in the definition of MonadTrans to++         forall m. (Selective m, Monad m) => Monad (ExceptT e m)++     * Make Selective a superclass of Monad+     * Revert the "Applicative is a superclass of Monad" proposal (lol!)++   And so we just don't provide `MonadTrans (ExceptT e)` instance.++   We could provide a SelectiveTrans instance instead, where++     class (forall f. Selective f => Selective (t f)) => SelectiveTrans t where+         lift :: Selective f => f a -> t f a++   Sounds fun!+-}  -- | Inject an 'T.ExceptT' value into the newtype wrapper. wrap :: T.ExceptT e m a -> ExceptT e m a