How to catch an exception

Bayley, Alistair
Mon, 21 Jul 2003 14:12:02 +0100

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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