On Mon, May 23, 2011 at 6:21 PM, Richard O'Keefe <ok@cs.otago.ac.nz> wrote:

On 24/05/2011, at 5:49 AM, Alexander Solla wrote:
> There's a library function for it, but also:
>
> > filter ((/=) Nothing)

The problem with that in general is that it only
applies to [Maybe t] if Eq t, but you don't
actually _need_ t to support equality.
filter isJust
will do the job, where isJust is in Data.Maybe.

Indeed, isJust will do the job. Sometimes it is acceptable to just use the tools you know. For example, filtering a list of (Maybe Int)s. This is a balance that is tough to get right.

Personally, I find non-functional values without Eq instances to be degenerate. So I really do not mind superfluous Eq constraints. I would not hesitate to use filter ((/=) Nothing) in a function whose type has no free type variables. It's just a bit of plumbing inside of a more complex function.

But the point of avoiding unnecessary constraints is a good one, especially for constraints that constrain more strongly, e.g., Ord.