Occasionally I find it would be nice to be able to omit some or all constructor parameters from a pattern matching rule. Perhaps the compiler could put them in for you and apply them to the right hand side of the rule ? data Tree a = Leaf a| Node (Tree a) (Tree a) treeFold :: (a -> a -> a) -> Tree a -> a treeFold f = \case Leaf -> id Node -> f `on` treeFold f This could be desugared at some point to treeFold f = \case Leaf a -> id a Node a b -> (f `on` treeFold f) a b I suppose such an extension has been considered before, but I could not find such a discussion. (In http://haskell.1045720.n5.nabble.com/Partial-pattern-matching-td3087200.html the thread focused instead on case expressions over functions) Any pointers to previous discussions ? Comments ? Thank you! Patrick Prémont