Re: [Haskell-cafe] Missing common function for Maybe types

alternative does not apply function to first Nothing

<|> or other functions from Alternative may not be the entire function to solve Marc's problem. It is a possibly useful function that is a bit tricky to find. ;) On 31 July 2018 at 11:26, Paul wrote:

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 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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