[Haskell-cafe] Missing common function for Maybe types

[Paul]

But alternative does not apply function to first Nothing. `maybe` can 
exec function on Nothing or apply another one on Just.

maybe (g x) id (f x) ?


31.07.2018 11:10, Imants Cekusins wrotes:
> Salut Marc,
>
> Are you looking for Alternative by any chance:
> http://hackage.haskell.org/package/base-4.11.1.0/docs/Control-Applicative.html#t:Alternative
>
>
> Prelude> :m Control.Applicative
> Prelude Control.Applicative> Just 1 <|> Just 2::Maybe Int
> Just 1
> Prelude Control.Applicative> Nothing <|> Just 2::Maybe Int
> Just 2
>
>
>
>
> On 31 July 2018 at 10:47, Marc Busqué <marc at lamarciana.com 
> <mailto: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
>     <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/
>     <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/
>         <http://waiting-for-dev.github.io/about/>
>
>
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