Re: [Haskell-cafe] Inverting a Monad

6 Feb 2008

      On Feb 6, 2008 12:51 PM, Bas van Dijk  wrote:
...
I will try out requiring 'm' to have a 'MonadError' constraint and see how far I come
with that.
I'm now trying to define 'inv' using 'catchError` but I can't get it to work.

The following obviously doesn't work:

import Control.Monad.Error

inv :: MonadError e m => m a -> m ()
inv m = (m >> fail "") `catchError` \_ -> (return ())

Bas