IsString [Char] instance

Edward Kmett ekmett at
Mon May 18 01:36:30 UTC 2015

+1 from me. Let's resolve to do something about the situation before 7.12

I'd definitely prefer some kind of smarter defaulting, because that'd also
potentially address the length "foo" overloaded string problem that got
worse with the Foldable/Traversable Proposal, but even just the

instance (a ~ Char) => IsString [a]

solution goes a long way and has the benefit that it could be implemented
today without having to figure out and test complex defaulting rules.


On Mon, May 18, 2015 at 10:08 AM, Dan Doel <dan.doel at> wrote:

> Greetings,
> Today, someone came into #haskell and asked why they couldn't type the
> equivalent of:
>     > "hi" ++ "bye"
> into GHCi with OverloadedStrings enabled. The answer is that it's
> ambiguous, because (++) only determines the strings to be [a], and not
> [Char].
> I noticed that this could actually be solved by making the instance:
>     instance (a ~ Char) => IsString [a] where ...
> Which causes [Char] to be inferred as soon as [a] is. I then searched my
> libraries mail and noticed that we'd discussed this two years ago. The
> proposal for this instance change was rejected based on
> ExtendedDefaultRules being beefed up to solve this case. But then no one
> actually implemented the better defaulting.
> So, I'm proposing that the issue be fixed for real. I'm not terribly
> concerned with how it gets fixed, but there's not a great reason for this
> to not behave better than it currently does. If someone steps up and makes
> defaulting better, than that's great. But if not, then the libraries
> committee can fix this very easily for GHC 7.12, and I think it's better to
> do so than to wait if there are no signs that the alternative is going to
> happen.
> I don't think we need to nail down which of the two solutions we're going
> to choose right now, but it'd be good to resolve that we're going to fix
> it, one way or another, by some well defined date.
> Here's a link to the previous discussion:
> Discussion period: 2 weeks
> -- Dan
> _______________________________________________
> Libraries mailing list
> Libraries at
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