On Wed, Jul 07, 2010 at 08:26:50AM +0100, Stephen Tetley wrote:

Hi Michael

Untested - maybe you can use Record puns (section 7.3.15.of the GHC manual). Though they might only work if you have used field names in your data type.

{} swaps for _ _ _ _ _ _ so it would be:

...

doSomething :: Maybe Result doSomething item = case item of note@(Note {}) -> Just $ process note _ -> Nothing

Nope, this works in general, whether you've used field names or not. I use this trick all the time. Also, I see no reason to use a case; why not just doSomething item@(Note{}) = Just $ process item doSomething _ = Nothing -Brent

David Virebayre wrote:

On Wed, Jul 7, 2010 at 9:19 AM, Michael Mossey wrote:

...

doSomething :: Maybe Result doSomething item = case item of note@(Note _ _ _ _ _) -> Just $ process note _ -> Nothing

But I don't like writing "Note _ _ _ _ _"

isNote (Note _ _ _ _ _) = True isNote _ = False

doSomething :: Maybe Result doSomething item | isNote item = Just $ process note | otherwise = Nothing

But I don't like writing "Note _ _ _ _ _"

I think this is pretty good because I can use isNote with a filter in another function. Thanks, Mike