[Haskell-cafe] ErrorT and catchError question

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