Alternatively, since you can't control the export of instance declarations, is there a way to declare a deriving separate from the definition of a type? For example, Network.URI should not have an Eq instance because some applications may legitimately want to define URI equivalence differently (e.g. case sensitivity). OTOH, there are lots of times when the default behavior is just fine and having to define readsPrec/showsPrec manually is just a PITA. If there isn't already a way to do this how about this strawman syntax: instance Eq URI default And (going slightly OT) as long we are are simplifying instance declarations, how about making it easier to specialize them? For example in my DBMS code, I had stuff like this: instance Property BasicProp String where emptyProp x = bpEmpty deleteRecordId = bpDeleteRecordId insertRecordId = bpInsertRecordId instance Property BasicProp Integer where emptyProp x = bpEmpty deleteRecordId = bpDeleteRecordId insertRecordId = bpInsertRecordId instance Property BasicProp Double where emptyProp x = bpEmpty deleteRecordId = bpDeleteRecordId insertRecordId = bpInsertRecordId The actually list of functions is actually much longer. For some types, I may want to specialize some of these functions, but for most of them, the default behavior is good enough. I'd like to declare default implementations e.g. instance Property BasicProp a where And then only if I have a more interesting type do: instance Property BasicProp CoolType where And only define the functions that are different from the default behavior for (Propry BasicProp a) -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com On Fri, 30 Apr 2004, Simon Marlow wrote:
On 27 April 2004 20:58, Sven Panne wrote:
Playing around with System.Posix.Terminal, I realized that there are no Read, Show, Eq, Enum, ... instances for the data types defined there (TerminalState, TerminalMode, ...). This is not a very difficult problem to solve in one's own program, but it's not very handy, either.
By all means go ahead and add these instances.
This raises a general question: What instances should be defined in the standard libraries for types like this? A general policy would be a good thing. IMHO, at least Read, Show, Typeable and Eq could be expected, having Ord, Enum and Bounded, too, would be nice.
Good point. I've been meaning to create a separate document so we have somewhere to collect general library policies like this. It's on my whiteboard...
Cheers, Simon _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries