From 97bb518220d8ede752d59207e6cbd9715334788d Mon Sep 17 00:00:00 2001
From: Michael Snoyman <michael@snoyman.com>
Date: Sat, 5 Dec 2009 23:05:32 +0200
Subject: [PATCH] Imported from c-m-f, new Failure based class

---
 Control/Failure.hs | 62 ++++++++++++++++++++++++++++++++++++++++++++++
 failure.cabal      | 24 ++++++++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 Control/Failure.hs
 create mode 100644 failure.cabal

diff --git a/Control/Failure.hs b/Control/Failure.hs
new file mode 100644
index 0000000..d01e9e3
--- /dev/null
+++ b/Control/Failure.hs
@@ -0,0 +1,62 @@
+{-# LANGUAGE Rank2Types #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE FlexibleContexts #-}
+-- | Type classes for returning failures.
+module Control.Failure
+    ( -- * Type classes
+      Failure (..)
+    , FunctorFailure
+    , ApplicativeFailure
+    , MonadFailure
+      -- * Wrapping failures
+    , WrapFailure (..)
+      -- * Convenience 'String' failure
+    , StringException (..)
+    , failureString
+    ) where
+
+import Control.Exception (throw, Exception)
+import Data.Typeable (Typeable)
+import Control.Applicative (Applicative)
+
+class Failure e f where
+    failure :: e -> f v
+class (Functor f, Failure e f) => FunctorFailure e f
+class (Applicative f, Failure e f) => ApplicativeFailure e f
+class (Monad f, Applicative f, Failure e f) => MonadFailure e f
+
+-- These introduce the need to use undecidables and overlapping.
+-- However, since they are merely type synonyms, this is acceptable.
+instance (Functor f, Failure e f) => FunctorFailure e f
+instance (Applicative f, Failure e f) => ApplicativeFailure e f
+instance (Monad f, Applicative f, Failure e f) => MonadFailure e f
+
+class Failure e f => WrapFailure e f where
+    -- | Wrap the failure value, if any, with the given function. This is
+    -- useful in particular when you want all the exceptions returned from a
+    -- certain library to be of a certain type, even if they were generated by
+    -- a different library.
+    wrapFailure :: (forall eIn. Exception eIn => eIn -> e) -> f a -> f a
+
+-- | Call 'failure' with a 'String'.
+failureString :: MonadFailure StringException m => String -> m a
+failureString = failure . StringException
+
+newtype StringException = StringException String
+    deriving Typeable
+instance Show StringException where
+    show (StringException s) = "StringException: " ++ s
+instance Exception StringException
+
+-- --------------
+-- base instances
+-- --------------
+
+instance Failure e Maybe where failure _ = Nothing
+instance Failure e []    where failure _ = []
+
+instance Exception e => Failure e IO where
+  failure = Control.Exception.throw
diff --git a/failure.cabal b/failure.cabal
new file mode 100644
index 0000000..21dd395
--- /dev/null
+++ b/failure.cabal
@@ -0,0 +1,24 @@
+name: failure
+version: 0.0.0
+Cabal-Version:  >= 1.6
+build-type: Simple
+license: PublicDomain
+author: Pepe Iborra, Michael Snoyman, Nicolas Pouillard
+maintainer: pepeiborra@gmail.com
+homepage: http://github.com/snoyberg/failure
+description: Typeclass for representing failures
+synopsis: Typeclass for representing failures
+category: Control, Monads, Failure
+stability: stable
+
+Library
+  buildable: True
+  build-depends: base >= 4 && < 5
+  ghc-options: -Wall
+
+  exposed-modules:
+     Control.Failure
+
+source-repository head
+  type:     git
+  location: git://github.com/snoyberg/failure.git