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

[John Lato]

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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