[Haskell-cafe] ErrorT and catchError question

[Adam Smyczek]

Hi,

I'm trying to use ErrorT transformer based
on Don Stewart's Shell example:
http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10#programmable-semicolons
It's probably a trivial question, but I cannot figure out
how to implement the catchError function in:

instance MonadError String Shell where
	throwError      = error . ("Shell failed: "++)
	catchError l h = ???

Thanks for help,
Adam