Re: Re[Haskell-cafe] cursive to foldr

On Tue, Nov 17, 2009 at 10:01 PM, Luke Palmer wrote:

filter even [0..]    -->    [0,2,4,6,8,...] searchList even [0...]   -->   Just [0,2,4,6,8,...]

searchList gives Nothing in exactly those cases that filter gives []. They give _|_ in exactly the same situations.  searchList could well be defined as:

searchList p xs = if null ys then Nothing else Just ys    where ys = filter p xs

null is strict, so searchList is just as strict as filter.

You're right. I was thinking of traverse with an exception monad. To make up for it, I'll note that with the recently (re)proposed mfilter, you can define searchList as: searchList p = mfilter (not . null) . Just . filter p -- Dave Menendez http://www.eyrie.org/~zednenem/