[Haskell-cafe] Still stacking monad transformers
rendel at daimi.au.dk
Mon Oct 13 16:17:22 EDT 2008
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
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
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
Just "foo" <| "bar" = "foo"
Nothing <| "bar" = "bar"
which seems to closely correspond to what you want.
More information about the Haskell-Cafe