Fri, 7 Dec 2001 11:38:14 -0800, Mark P Jones
There's no solid technical reason for this, but Haskell doesn't allow it at the moment because there isn't an easy way to name an instance declaration.
There is another problem: even if we created a syntax to name them, if they would not be exported by default then current programs would have to be changed. An instance is usually meant to be exported, so even not considering compatibility it would be tedious to list all those instances in the export list (unless you want to export every named thing as well and omit the export list). It's easy to overlook some and the point when somebody tries to use a particular instance might be far in the future. Each instance is often a "small" part of an interface. I agree that the ability to selectively export instances would be good. Only it's hard to combine with the current policy of exporting all instances by default. Perhaps in the future it will be possible to specify the interface of a Haskell module more formally, with types of exported values for example. Then we should remember about instances and solve both problems simultaneously. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ QRCZAK