Add convertMay, and some git/cabal additions

.gitignore

@@ -0,0 +1,3 @@
+.cabal-sandbox/
+cabal.sandbox.config
+dist/

Data/Convertible/Utils.hs

@@ -17,11 +17,13 @@ For license and copyright information, see the file LICENSE
    Portability: portable
 
 -}
-
 module Data.Convertible.Utils(boundedConversion,
-                             convertVia
+                             convertVia,
+                             convertMay
                              )
 where
+import Control.Monad.Error (throwError)
+import Control.Monad ((<=<))
 import Data.Convertible.Base
 import Data.Typeable
 
@@ -32,7 +34,7 @@ the source via 'safeConvert', comparing to the source value.  Results in an erro
 if the conversion is out of bounds. -}
 boundedConversion :: (Ord a, Bounded b, Show a, Show b, Convertible a Integer,
                       Convertible b Integer,
-                      Typeable a, Typeable b) => 
+                      Typeable a, Typeable b) =>
                      (a -> ConvertResult b) -- ^ Function to do the conversion
                   -> a                      -- ^ Input data
                   -> ConvertResult b        -- ^ Result
@@ -70,3 +72,23 @@ convertVia :: (Convertible a b, Convertible b c) =>
 convertVia dummy inp =
     do r1 <- safeConvert inp
        safeConvert (asTypeOf r1 dummy)
+
+{- | Make an intermediary conversion based on a function (a -> Maybe b). For instance:
+
+>data Foo
+>instance Show Foo where ...
+>
+>data Bar
+instance Read Bar where ...
+>
+>instance Convertible Foo [Char] where ...
+>
+>instance Convertible Foo Bar where
+>    safeConvert = convertMay (strMsg "Invalid Bar") readMay
+ -}
+convertMay :: Convertible a b =>
+              ConvertError    -- ^ Error if the conversion fails
+           -> (b -> Maybe c)  -- ^ Conversion from intermediate type to end result
+           -> a               -- ^ Input value
+           -> ConvertResult c -- ^ Result
+convertMay e f = maybe (throwError e) return . f <=< safeConvert

convertible.cabal

@@ -12,6 +12,7 @@ extra-source-files: LICENSE, utils/genCinstances.hs,
                     testsrc/TestNum.hs,
                     testsrc/TestTime.hs
 homepage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/convertible
+bug-reports: https://github.com/hdbc/convertible/issues
 Category: Data
 synopsis: Typeclasses and instances for converting between types
 Description: convertible provides a typeclass with a single function
@@ -36,8 +37,10 @@ Description: convertible provides a typeclass with a single function
  on GHC and Hugs.
 Stability: Stable
 Build-Type: Simple
-
 Cabal-Version: >=1.2
+source-repository head
+  type: git
+  location: https://github.com/hdbc/convertible.git
 
 flag splitBase
   description: Choose the new smaller, split-up base package.
@@ -75,7 +78,7 @@ library
   Extensions: ExistentialQuantification, MultiParamTypeClasses,
               UndecidableInstances, FlexibleInstances,
               FlexibleContexts, TypeSynonymInstances, CPP
-  
+
 Executable runtests
    if flag(buildtests)
       Buildable: True