The IO instance of MonadPlus is essentially based on being able to throw and catch errors inside IO. The definition (in Monad.Error) is: instance MonadPlus IO where mzero = ioError (userError "mzero") m `mplus` n = m `catch` \_ -> n so essentially the zero value is an error value and (m1 + m2) is the action which attempts to run m1 and then, if it fails, runs m2. HTH - Hal -- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Wed, 22 Jan 2003, Dean Herington wrote:

According to the Haddock documentation for Control.Monad at http://www.haskell.org/ghc/, `IO` is an instance of `MonadPlus`.

1. Where in documentation is this instance described? 2. Where in source code is this instance implemented? 3. Could links to the answers to 1 and 2 be added to the Haddock documentation for Control.Monad?

Obviously, these questions apply more generally than just to `MonadPlus IO`.

Dean

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