"Elvis" operator (?:) as a reverse infix form of "fromMaybe"

[Nikita Volkov]

Dan Burton, `maybeA <|> maybeB |> c` has type `Maybe a`, while `maybeA ?:
maybeB ?: c` has type `a`. The sole purpose of `?:` is in extracting a
value from `Maybe`, not lifting to it.


2013/10/12 Dan Burton <danburton.email at gmail.com>

> It's not a completely different thing, it's the very same thing, but more
> general.
>
> -- Dan Burton
>
>
> On Sat, Oct 12, 2013 at 12:41 PM, Nikita Volkov <nikita.y.volkov at gmail.com
> > wrote:
>
>> John Lato, you're offering a completely different thing and I would
>> totally vote against it. Please don't flood.
>>
>>
>> 2013/10/12 John Lato <jwlato at gmail.com>
>>
>>> I'd prefer it if we had an operator in Control.Applicative such as
>>>
>>>   |> :: Alternative f => f a -> a -> f a
>>>   l |> r = l <|> pure r
>>>
>>> if it doesn't already exist, this would generalize fromMaybe, and
>>> complicated cases could be written as
>>>
>>>   maybeA <|> maybeB |> c
>>>
>>> but even this could be easily written using <|> and pure, so I'm not
>>> sure it pulls its weight.
>>>
>>> John L.
>>>
>>>
>>> On Sat, Oct 12, 2013 at 7:40 AM, Nikita Volkov <
>>> nikita.y.volkov at gmail.com> wrote:
>>>
>>>> I suggest to include this operator in "Data.Maybe" and "Prelude".
>>>>
>>>> *Implementation:*
>>>>
>>>>     (?:) :: Maybe a -> a -> a
>>>>     maybeA ?: b = fromMaybe b maybeA
>>>>
>>>> *Use cases:*
>>>>
>>>>     1.
>>>>         maybeValue ?: error "Value is unexpectedly empty. This is a
>>>> bug."
>>>>
>>>>     instead of
>>>>
>>>>         fromMaybe (error "Value is unexpectedly empty. This is a bug.")
>>>> maybeValue
>>>>
>>>>     2.
>>>>         maybeA ?: maybeB ?: c
>>>>
>>>>     instead of
>>>>
>>>>         fromMaybe (fromMaybe c maybeB) maybeA
>>>>
>>>> *Name collisions:*
>>>>
>>>>     Hayoo search gives only 6 collisions with 5 hardly fundamental
>>>> libraries.
>>>>
>>>>
>>>>
>>>> * The nickname and the operator itself are inspired by the ones from
>>>> Groovy language.
>>>>
>>>> ** The symbols of the operator are a reminder of plain old ternary
>>>> construct.
>>>>
>>>> *** To understand the nickname look at the operator as on emoticon.
>>>>
>>>>
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>>>>
>>>
>>
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