[Haskell-cafe] Splitting off many/some from Alternative

[Antoine Latter]

On Wed, Dec 14, 2011 at 8:03 PM, Ross Paterson <ross at soi.city.ac.uk> wrote:
> The current definition says that some and many should be the least
> solutions of the equations
>
>    some v = (:) <$> v <*> many v
>    many v = some v <|> pure []
>
> We could relax that to just requiring that they satisfy these equations
> (which I think is what John wants).  In that case there would be another
> possible definition for Maybe:
>
>    some Nothing = Nothing
>    some (Just x) = Just (repeat x)
>
>    many Nothing = Just []
>    many (Just x) = Just (repeat x)
>

This seems to generalize to list:

    some [] = []
    some xs = [cycle xs]

    many [] = [[]]
    many xs = [cycle xs]

Although I'm still not sure why I would be using these operations in
maybe or list.

Antoine

> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe