Add a MonadFail instance for InputT. (#117)

There are also a couple of other monads that could plausibly have this
instance, but since none of them seem to be exported I left them alone.

Author: quasicomputational

Changelog

@@ -1,3 +1,6 @@
+Next
+   * Added a `MonadFail` instance for `InputT`.
+
 Changed in version 0.7.5.0:
    * Add the new function `fallbackCompletion` to combine
      multiple `CompletionFunc`s

System/Console/Haskeline/InputT.hs

@@ -13,6 +13,7 @@ import System.Console.Haskeline.Term
 
 import Control.Exception (IOException)
 import Control.Monad.Catch
+import Control.Monad.Fail as Fail
 import Control.Monad.Fix
 import Data.IORef
 import System.Directory(getHomeDirectory)
@@ -58,6 +59,9 @@ newtype InputT m a = InputT {unInputT ::
 instance MonadTrans InputT where
     lift = InputT . lift . lift . lift . lift . lift
 
+instance ( Fail.MonadFail m ) => Fail.MonadFail (InputT m) where
+    fail = lift . Fail.fail
+
 instance ( MonadFix m ) => MonadFix (InputT m) where
     mfix f = InputT (mfix (unInputT . f))