[Haskell-cafe] Missing common function for Maybe types

[Marc Busqué]

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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