IsString [Char] instance

Dan Doel dan.doel at
Mon May 18 00:08:04 UTC 2015


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

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

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
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