Merge pull request #4 from ffaf1/magic-number

Improve “magic number” error

Author: portnov

lib/Data/Gettext/GmoFile.hs

@@ -10,9 +10,11 @@ module Data.Gettext.GmoFile
 import Control.Monad
 import Data.Binary
 import Data.Binary.Get
-import qualified Data.ByteString.Lazy as L
 import Text.Printf
 
+import qualified Data.ByteString.Lazy as L
+import qualified Numeric as N
+
 -- | This structure describes the binary structure of Gettext @.mo/.gmo@ file.
 data GmoFile = GmoFile {
     fMagic :: Word32                       -- ^ Magic number (must be @0x950412de@ or @0xde120495@)
@@ -38,8 +40,10 @@ parseGmo = do
   getWord32 <- case magic of
                  0x950412de -> return getWord32le
                  0xde120495 -> return getWord32be
-                 _ -> fail "Invalid magic number"
-  
+                 _ -> fail $ "parseGmo: Invalid magic number " <>
+                             "0x" <> N.showHex magic "" <>
+                             " (not an .mo file?)"
+
   let getPair :: Get (Word32, Word32)
       getPair = do
         x <- getWord32