Dino Morelli wrote:

I was wishing I could do this:

let foo = str `or-if-empty` default

If it was a Maybe, this works with mplus:

(Just "foo") `mplus` (Just "bar") == Just "foo" Nothing `mplus` (Just "bar") == Just "bar"

But not so much for list, mplus just ain't defined that way, instead doing concatination:

"foo" `mplus` "bar" == "foobar" "" `mplus` "bar" == "bar"

The difference between Maybe-style MonadPlus and List-style MonadPlus is discussed on http://www.haskell.org/haskellwiki/MonadPlus_reform_proposal. With that proposal, you could use morelse with both Maybe and lists. Personally, I would like to call these beasts <|> (for Maybe-style mplus) and <+> (for List-style mplus). One could even have a <| b = a <|> pure b a <+ b = a <+> pure b a +> b = pure a <+> b in the style of <* and *>. Note that I assume Applicative as a superclass of Monad, while I'm talking about a better world anyway. That said, I would consider using something like the <+> sketched here for Strings as a hack, because I do not see Strings as Chars under the List functor, even if they technically may be exactly that. Since "abc" does not represent multiple results, I don't think "" should represent failure. Instead, I would wrap optional strings in Maybe, and then use fromMaybe to handle the default values: fromMaybe "bar" (Just "foo") = "foo" fromMaybe "bar" Nothing = "bar" Now, with the operators sketched above, fromMaybe = flip (<|), so one could use Just "foo" <| "bar" = "foo" Nothing <| "bar" = "bar" which seems to closely correspond to what you want. Tillmann