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