feat: Initial Implementation.

.gitignore

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+/.stack-work

LICENSE

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+Copyright 2021 Owens Murray, LLC.
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# exceptiont
+
+This package provides a monad transformer that implements `MonadError`
+using IO exceptions. In particular, this is useful when you want your
+stack to be an instance of both `MonadError` and `MonadUnliftIO`, which
+you can't do with `ExceptT`.

Setup.hs

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+import Distribution.Simple
+main = defaultMain

exceptiont.cabal

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+-- Initial exceptiont.cabal generated by cabal init.  For further
+-- documentation, see http://haskell.org/cabal/users-guide/
+
+name:                exceptiont
+version:             0.1.0.0
+synopsis:            A monad transformer providing `MonadError` using the IO exception mechanism.
+description:         A monad transformer providing `MonadError` using the IO exception mechanism.
+homepage:            https://github.com/owensmurray/exceptiont
+license:             MIT
+license-file:        LICENSE
+author:              Rick Owens
+maintainer:          [email protected]
+copyright:           2021 Owens Murray, LLC.
+-- category:
+build-type:          Simple
+extra-source-files:  README.md
+cabal-version:       >=1.10
+
+library
+  exposed-modules:
+    Control.Monad.ExceptionT
+  -- other-modules:
+  -- other-extensions:
+  build-depends:
+    base         >= 4.13     && < 4.14,
+    mtl          >= 2.2.2    && < 2.3,
+    transformers >= 0.5.6.2  && < 0.6,
+    unliftio     >= 0.2.13.1 && < 0.3
+  hs-source-dirs:      src
+  default-language:    Haskell2010
+

src/Control/Monad/ExceptionT.hs

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+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE StandaloneDeriving #-}
+
+module Control.Monad.ExceptionT (
+  ExceptionT,
+  runExceptionT,
+) where
+
+
+import Control.Monad.Except (MonadError(catchError, throwError))
+import Control.Monad.IO.Class (MonadIO)
+import Control.Monad.Reader (MonadReader)
+import Control.Monad.Trans.Class (MonadTrans, lift)
+import Data.Coerce (Coercible)
+import Data.Kind (Constraint)
+import UnliftIO (UnliftIO(UnliftIO), Exception, MonadUnliftIO, catch,
+  throwIO, try)
+
+
+{- |
+  A monad transformer whose `MonadError` isntances is implemented using
+  raw Haskell exceptions. This is particularly useful if you want
+  your transformer stack to be an instance of both `MonadError` and
+  `MonadUnliftIO`, which you can't do with `ExceptT`.
+-}
+newtype ExceptionT e m a = ExceptionT (m a)
+  deriving newtype
+    ( Applicative
+    , Functor
+    , Monad
+    , MonadIO
+    , MonadReader r
+    )
+type Repr m = forall a b. (Coercible a b) => Coercible (m a) (m b) :: Constraint
+deriving newtype instance (MonadIO m, MonadUnliftIO m, Repr m) => MonadUnliftIO (ExceptionT e m)
+instance MonadTrans (ExceptionT e) where
+  lift = ExceptionT
+instance (MonadIO m, MonadUnliftIO m, Exception e, Repr m) => MonadError e (ExceptionT e m) where
+  throwError = throwIO
+  catchError = catch
+
+
+runExceptionT
+  :: ( MonadUnliftIO m
+     , Exception e
+     )
+  => ExceptionT e m a
+  -> m (Either e a)
+runExceptionT (ExceptionT action) = try action
+
+

stack.yaml

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+flags: {}
+packages:
+- .
+extra-deps: []
+resolver: lts-16.26