[Haskell-cafe] Missing common function for Maybe types

[Imants Cekusins]

> 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 <aquagnu at gmail.com> 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 <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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>>>
>>>
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>
>
>
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