[Haskell-cafe] Missing common function for Maybe types

[Marc Busqué]

On Tue, 31 Jul 2018, Nigel Rantor wrote:

>
> Your `foobar` function cannot work, it doesn't handle the case when the 
> URL is neither http nor https.
>
> I would use catMaybes and headMay to work this out...untested code, and 
> you'll need the `safe` library for headMay too...
>
> parse :: ByteString -> Maybe (Url Http, Option scheme)
> parse input = headMay $ catMaybes $ fmap input [parseUrlHttp, parseUrlHttps]
>
> And everything is lazy so if the input gets parsed correctly by the 
> first call you never actually perform the second one....

Thanks Nigel! The problem is that the return type could be an `Url Http`
or an `Url Https`, so that wouldn't work. But an answer before by David
Feuer worked for my case:

```
boo :: Maybe a -> Maybe b -> Maybe (Either a b)
boo ma mb = (Left <$> ma) <|> (Right <$> mb)
```

Marc Busqué
http://waiting-for-dev.github.io/about/
>
>   n
>
> On 31/07/18 08:07, Marc Busqué wrote:
>> 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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