Proposal: Add IsString instance for (Maybe a) to base

[Carter Schonwald]

ok, framed, that way, the proposal seems much more reasonable.

When framed wrt Maybe, I immediately think "nullable values", though now
that I think about it more, any use of IsString there would always be  f
a =  Just a, so even then, its philosophically kosher.

I withdraw my -1, at least for the applicative version


On Thu, Jul 11, 2013 at 8:19 PM, John Lato <jwlato at gmail.com> wrote:

> To play devil's advocate, why?
>
> What does limiting IsString in this fashion gain anyone?  It doesn't
> complicate type inference any more than it already is.  For any
> applicative, there's the trivial instance
>
> instance (IsString a, Applicative t) => IsString (t a) where
>     fromString = pure . fromString
>
> which is conceptually a very simple step and is always total.
>
> Of course some functors admit other instances.  I think some parser
> libraries already provide IsString instances as a nice syntax on matching
> string literals.  A rule like this would invalidate those instances for no
> particularly good reason I can see.
>
>
> On Fri, Jul 12, 2013 at 4:22 AM, Sjoerd Visscher <sjoerd at w3future.com>wrote:
>
>> Could this be formalised by saying that there should be a function
>> toString such that
>>
>>   fromString . toString = id
>>
>> There's no such toString for "Maybe a" with fromString = Just.
>>
>> Sjoerd
>>
>> On Jul 10, 2013, at 4:02 PM, David Luposchainsky <
>> dluposchainsky at googlemail.com> wrote:
>>
>> > On 2013-07-10 15:55, Joachim Breitner wrote:
>> >> Hi,
>> >>
>> >> Am Mittwoch, den 10.07.2013, 09:21 +1000 schrieb Ivan Lazar Miljenovic:
>> >>> -0.5 from me; I think it's too easy to forget that it's a Maybe value
>> >>> there when refactoring, etc.
>> >>
>> >> same here; even with OverloadedStrings Haskell should feel as strongly
>> >> typed as it is.
>> >
>> > Agreed. Overloaded strings should be reserved for things that *are*
>> > strings in a certain way. A "Maybe String" is not a special way of
>> > storing a string (like for example a "Vector Char"), it's a String with
>> > an additional value. Providing a Maybe instance goes too far in the
>> > direction of having a general "'return' with strings" function, so I'm
>> > -1 on this.
>> >
>> > David
>> >
>> >
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