[Haskell-cafe] Missing common function for Maybe types

[David Feuer]

I think you're probably after something like

boo :: Maybe a -> Maybe b -> Maybe (Either a b)
boo ma mb = (Left <$> ma) <|> (Right <$> mb)

On Tue, Jul 31, 2018, 3:47 AM Marc Busqué <marc at lamarciana.com> wrote:

> Apologies, I clearly asked the question in a rushed way...
>
> I'm working with [req](http://hackage.haskell.org/package/req) package.
> I need to parse a url from a string, but I don't know whether its schema
> will be `http` or `https`. However, `req` just provides me with two
> functions:
>
> ```
> parseUrlHttp :: ByteString -> Maybe (Url Http, Option scheme)
> parseUrlHttps :: ByteString -> Maybe (Url Https, Option scheme)
> ```
>
> As I don't know the schema beforehand, I have to apply one of the two
> functions, do a case analysis on its result and, depending on the
> result, call the second one or return the first result.
>
> What I think is a common case here (perhaps I'm wrong) is the need to
> choose between two or more `Maybe` values where at most one of them will
> be a `Just`. `maybe` does not do that. `catMaybes` could be used for
> that when all the `Maybe` have the same inner type, but it is not the
> exact abstraction and it would need more code (just like simply doing a
> case analysis).
>
> Thanks and, again, sorry for the hurried question before (I don't like
> when I see it from others :()
>
> Marc Busqué
> http://waiting-for-dev.github.io/about/
>
> On Tue, 31 Jul 2018, Paul wrote:
>
> >
> > Something like "maybe" function?
> >
> >
> > 31.07.2018 10:07, Marc Busqué wrotes:
> >       Hi!
> >
> >       I have two functions:
> >
> >       ```
> >       foo :: a -> Maybe b
> >       bar :: a -> Maybe c
> >       ```
> >
> >       From which I want to build a higher order function:
> >
> >       ```
> >       foobar :: a -> (a -> Maybe b) -> (a -> Maybe c) -> Either b c
> >       ```
> >
> >       The implementation I need is:
> >
> >       ```
> >       foobar x f g =
> >         case (f x) of
> >           Nothing -> g x
> >           Just y  -> Just y
> >       ```
> >
> >       I'm a bit surprised that looking at hoogle I don't find a built-in
> >       solution for this quite common need for `Maybe` types (or perhaps
> for
> >       any monad).
> >
> >       Am I looking in the wrong way? Does it exist a similar abstraction
> but
> >       with a different shape?
> >
> >       Thanks in advance,
> >
> >       Marc Busqué
> >       http://waiting-for-dev.github.io/about/
> >
> >
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