On Sun, Apr 22, 2012 at 10:37 AM, Brent Yorgey
I do not think this is a bug. Since type classes are open, GHC does not do any reasoning of the form "X is the only instance in scope, so I will pick that one". Other instances could be added at any time (perhaps in other modules). In this particular instance, GHC has no reason to choose the Text instance other than the fact that it is the only instance in scope -- that is, type inference is not enough to determine that the Text instance should be chosen.
However, I do agree that it would be nice to have a mechanism for specifying default instances for arbitrary (user-defined) type classes.
Couldn't we make a special case for IsString, like we do for Num, given it's special syntactic association with OverloadedStrings? -- Johan