How do I create an IOError exception?

[Bayley, Alistair]

(I know I'm asking some noddy questions, but hey, that's what this list is
for...)

How do I create IOError exceptions? System.IO.Error has two functions that
create IOErrors: userError and mkIOError. However, both of them take a
String (I assume containing a description of the problem), and mkIOError
takes optional Handle and FilePath args.

http://www.haskell.org/ghc/docs/latest/html/base/System.IO.Error.html

What I'd like to do is stuff some more information into the exception (a bit
like Java exceptions), along the lines of:

> mkIOError :: a -> IOError

where I could choose to use it like this (ociHandleAlloc is a foreign C
function):

> handleAlloc2 handleType env ptr = do
>	rc <- ociHandleAlloc env ptr handleType 0 0
>	if rc < 0
>		then ioError (mkIOError ("Couldn't allocate handle", rc))
>		else peek ptr


When I catch the exception (currently in main) I would like to interrogate
it to get the values I stuffed in it (in this case a (String, Int) tuple),
and then do some error reporting by calling another function which decodes
the error number.

Is this the wrong way to go about error handling? Or is it relatively simple
to create your own IOErrors? Have I missed something in the docs?


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