On Tue, 2005-07-19 at 17:01 +1000, Ben Lippmeier wrote:
Hello,
I often find it useful to determine whether two objects are using the same constructor, without worrying about the constructors' arguments.
[snip]
Having some sort of generic shallowEq operator reduces the need for a host of predicates such as: (this one from Data.Maybe)
isJust x = case x of Just {} -> True _ -> False
.. which is an approach that is obviously going to be tedious when the size of the data type becomes large.
-- There is way to hack together a partial implementation of the ShallowEq class within GHC, but it leaves much to be desired:
instance Show a => ShallowEq a where (=@=) a b = (head $ words $ show a) == (head $ words $ show b)
Ouch!
Questions: 1) Does anyone know a better/existing way to implement ShallowEq that doesn't involve enumerating all the constructors in the data type?
2) If not, can anyone think of reasons why it wouldn't be a good idea for GHC to derive ShallowEq (by expanding said enumeration)?
DriFT comes to mind: http://repetae.net/john/computer/haskell/DrIFT/ it already supplies some query operators that might make shallowEq redundant. Cheers, Bernie.