[Haskell-cafe] Missing common function for Maybe types

[Marc Busqué]

On Tue, 31 Jul 2018, David Feuer wrote:

> I think you're probably after something like
> 
> boo :: Maybe a -> Maybe b -> Maybe (Either a b)
> boo ma mb = (Left <$> ma) <|> (Right <$> mb)

Thanks! That seems the right function for my case :) I see we need a
little bit of handcrafting and nothing exists out of the box in base
Haskell.

Marc Busqué
http://waiting-for-dev.github.io/about/
> 
> 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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> 
>