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

John Lato jwlato at
Tue Jul 16 02:44:50 CEST 2013

On Tue, Jul 16, 2013 at 1:53 AM, Roman Cheplyaka <roma at> wrote:

> * MightyByte <mightybyte at> [2013-07-15 08:26:13-0400]
> > I'll just throw out a data point related to this discussion.  In one of
> my
> > previous commercial Haskell projects we ended up writing a similar Num
> > instance for Maybe a.  I think I asked about it in #haskell back then and
> > got pretty vocal distaste about the idea from several people who spoke
> up.
> >  "Maybe is not a number!"  But we did end up using the instance in our
> > project because it greatly simplified a section of our code.  I'm
> certainly
> > not going to recommend that we also add the Num instance for Maybe.  But
> I
> > think it's a useful data point that this happened in real-world
> production
> > code at a company where writing Haskell code was my full-time job.
> >
> > As for my personal opinion on this proposal, I'm on the fence.  I can see
> > how it would be nice to have this kind of convenience, but the arguments
> on
> > the opposing side also resonate with me as well.
> I don't see any contradiction here.
> It's perfectly fine if you decided that you need that instance and
> introduced it in your code. It would be very strange if someone said you
> shouldn't do that.
> I also don't mind package authors (ab)using IsString to make their
> libraries more convenient to use. If someone doesn't like that library,
> they doesn't have to use it.
> What I am against is putting that instance into base, thus making it
> impossible to opt-out.

It's opt-in, not opt-out.  You opt-in by turning on OverloadedStrings.  If
you don't want your strings to be other than Strings, don't enable the

I get the sense that a lot of people feel OverloadedStrings really means
ProperStringLiterals, and want to use the extension as a proxy for a
language with String type that isn't [Char].  I don't think it's correct to
co-opt a generalizing extension to try to cover up a prior issue with the

We probably should add a ProperStringLiterals extension, actually.
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