How to catch an exception

[Bayley, Alistair]

Trying to get the hang of exceptions...

I would expect this program:


 module Main where
 import Control.Exception hiding (GHC.Prelude.catch)

 temp :: IO ()
 temp = do
   putStrLn line 1
   ioError (AssertionFailed my temp)

 handler :: Exception - IO ()
 handler e = putStrLn (exception:  ++ (show e))

 main :: IO ()
 main = catch temp handler


.. to output:
line 1
exception: AssertionFailed: my temp  (or whatever show produces for the
AssertionFailed exception)


... but all I get is:
line 1

Fail: my temp


This implies that the handler is not called. So what am I doing wrong?


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Re: How to catch an exception

[Ross Paterson]

On Mon, Jul 21, 2003 at 02:12:02PM +0100, Bayley, Alistair wrote:
  module Main where
  import Control.Exception hiding (GHC.Prelude.catch)

This hiding clause is illegal.  But anyway what you want is

 import Prelude hiding (catch)
 import Control.Exception

Prelude.catch only catches Haskell 98 exceptions; Control.Exception.catch
catches everything.

  temp :: IO ()
  temp = do
putStrLn line 1
ioError (AssertionFailed my temp)

From GHC 6.0, Exception is not the same as IOError: say throwIO instead
of ioError here.  (So GHC 6.0 flags your error as a type error.)
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