[Haskell-cafe] Missing common function for Maybe types

[Nigel Rantor]

I hadn't noticed they were different types...I can't say I love that API 
choice.

On 31/07/18 14:58, Marc Busqué wrote:
> 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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>>
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