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@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@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@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@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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