[Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Thu, Dec 15, 2011 at 1:40 AM, Antoine Latter <aslatter at gmail.com> wrote:
> On Thu, Dec 15, 2011 at 1:23 AM, Gregory Crosswhite
> <gcrosswhite at gmail.com> wrote:
>>
>> On Dec 15, 2011, at 3:36 PM, Antoine Latter wrote:
>>
>>> There are a lot of combinators you can build from (<|>) and empty that
>>> go terribly wrong for Maybe and List but are still quite useful.
>>>
>>
>> Yes, you *could* do that, but the whole point is that you shouldn't.
>>  Typeclasses generally come with informal laws that must be obeyed.  If your
>> instance does not obey those laws, then it should not be an instance.
>>
>

To clarify - I dropped Greg's sentence "Incidentally, exactly what use
cases do you have in mind?", which is most of what I was addressing in
my previous email.

> I said 'combinators', not 'instances'. A lot of popular parsers
> combinators can be written exclusively from (<|>) and empty, but make
> little sense for List and Maybe, and may not even function properly.
> The 'trifecta' package includes a nice reference:
>
> http://hackage.haskell.org/packages/archive/trifecta/0.49.1/doc/html/Text-Trifecta-Parser-Combinators.html
>
> See 'skipSome' through 'chainr1' - I wouldn't be surprised if most of
> these lead to the same infinite loop behavior for Maybe as the stock
> 'many' and 'some' in base.
>
> These sorts of functions are what Alternative is for.
>
> Maybe I'm missing something fundamental here.
>
> Antoine