[Haskell-cafe] MonadPlus and Alternative folds

[David Feuer]

In the case of both Maybe and [], an action that succeeds once will always
succeed. Parsers, other state transformers, and IO don't work like that,
however. I'm not sure where else these apply.
On Sep 19, 2015 1:39 PM, "Tom Ellis" <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Sat, Sep 19, 2015 at 06:13:18PM +0100, Tom Ellis wrote:
> > On Sat, Sep 19, 2015 at 01:25:00AM -0400, David Feuer wrote:
> > > It kind of seems like there should be fold-like things to match some
> and
> > > many.
> > >
> > > foldrAlt :: Alternative f => (a -> b -> b) -> b -> f a -> f b
> > > foldrAlt c n m = foldr1Alt c n m <|> pure n
> > >
> > > foldr1Alt :: Alternative f => (a -> b -> b) -> b -> f a -> f b
> > > foldr1Alt c n m = c <$> m <*> foldrAlt c n m
> > [...]
> >
> > An Alternative allows list-like behaviour when it comes to
> *construction*,
> > but offers no operation analogous to pattern-matching and therefore
> doesn't
> > allow you to look inside it or do any sort of *destruction*.
> >
> > I tried foldrAlt and it looped forever, as I expected.  I also expect the
> > others to loop forever.
>
> I now understand this can work for some Alternatives.  The operational
> reading
> is
>
> 1. Try to read an a
> 2. If it fails, return the b
> 3. If it succeeds, update the b
> 4. Go to 1
>
> However, since it doesn't work even for an Alternative as simple as Maybe
> it's worth thinking about what class of Alternatives we really want it to
> apply to.
>
> Tom
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