Am Samstag, 15. Januar 2005 14:36 schrieben Sie:
Well, to start, the type signatures are unnecessarily restrictive. Yep, but since I always needed them for taking elements that satisfied either of two predicates from a list, that was the type that first came to mind (actually the zero'th type used ([a],[a]) ). Then, the function that also is not in the Report, but does come up quite a bit by people who get into a point-free or categorical style is the bifunctor, (***) :: (a -> b) -> (c -> d) -> (a,c) -> (b,d) f *** g = \(a,b) -> (f a,g b) this is an instance of (***) in Control.Arrow, hence the name.
That's good to know, thanks.
So, your first function is, (&) x = (x:) *** id or using another function from Control.Arrow, (&) x = first (x:)
I can say that I have wanted (***), I can't say that I've ever wanted your two functions. Also, first (x:) seems to be more self-documenting.
I haven't read Control.Arrow yet, but it seems at first glance that I should write first (x:) (xs,y) second (y:) (x,ys) instead? That looks good to me, more to type, alas, but much better to read. Many thanks, Daniel