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

[Edward A Kmett]

Even if we wanted to we cannot use that as an actual instance.

It conflicts with the most important IsString instance of all: String!

Sent from my iPhone

On Jul 11, 2013, at 9:20 PM, Carter Schonwald <carter.schonwald at gmail.com> wrote:

> 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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>> 
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