Merge pull request #2 from erikd/master

Add module Control.Monad.Trans.Either.Exit

src/Control/Monad/Trans/Either/Exit.hs

@@ -0,0 +1,37 @@
+module Control.Monad.Trans.Either.Exit (
+    orDie
+  , orDieWithCode
+  ) where
+
+import           Control.Applicative (pure)
+import           Control.Monad ((>>=), (>>))
+
+import           Data.Either (either)
+import           Data.Function ((.))
+import           Data.Int (Int)
+import           Data.Text (Text)
+import qualified Data.Text as T
+
+import           System.Exit (ExitCode(..), exitWith)
+import           System.IO (IO, stderr, hPutStrLn)
+
+import           Control.Monad.Trans.Either
+
+
+-- | orDieWithCode with an exit code of 1 in case of an error
+--
+orDie :: (e -> Text) -> EitherT e IO a -> IO a
+orDie = orDieWithCode 1
+
+-- | An idiom for failing hard on EitherT errors.
+--
+-- *This really dies*. There is no other way to say it.
+--
+-- The reason it lives with command line parser tooling, is that it is
+-- the only valid place to actually exit like this. Be appropriately
+-- wary.
+--
+orDieWithCode :: Int -> (e -> Text) -> EitherT e IO a -> IO a
+orDieWithCode code render e =
+  runEitherT e >>=
+    either (\err -> (hPutStrLn stderr . T.unpack . render) err >> exitWith (ExitFailure code)) pure

transformers-either.cabal

@@ -1,5 +1,5 @@
 name:                  transformers-either
-version:               0.0.1
+version:               0.0.2
 license:               BSD3
 license-file:          LICENSE
 author:                Tim McGilchrist <timmcgil@gmail.com>
@@ -14,14 +14,15 @@ description:
 
             Uses a pattern synonym to maintain compatibility with the old EitherT types
             but is actually ExceptT under the covers.
-       
+
 source-repository head
   type: git
   location: https://github.com/tmcgilchrist/transformers-either.git
-                
+
 library
   build-depends:
                        base                            >= 3          && < 5
+                     , text                            == 1.2.*
                      , transformers                    >= 0.4        && < 0.6
 
   ghc-options:
@@ -32,3 +33,4 @@ library
 
   exposed-modules:
                        Control.Monad.Trans.Either
+                       Control.Monad.Trans.Either.Exit