On Fri, Jan 16, 2004 at 11:03:21AM +0100, Tomasz Zielonka wrote:
On Fri, Jan 16, 2004 at 01:43:40AM -0800, Brandon Michael Moore wrote:
I think it would often be useful if the list functions with class constraints had versions paramaterized on a function mapping your element type into an instance of the classes, in addition to the "By" forms that take an implementation for the required class method.
Almost all of the times I use the "By" functions follow that pattern: e.g. sortFoos :: [Foo] -> [Foo] sortFoos = sortBy (\x y -> compare (f x) (f y))
No need for so many new functions. Just write a function:
composeFGxGy :: (b -> b -> c) -> (a -> b) -> a -> a -> c composeFGxGy f g x y = f (g x) (g y)
Then you can:
sortFoos = sortBy (composeFGxGy compare f)
The special case may be useful if f is expensive: -- sortImage f = sortBy (\x y -> compare (f x) (f y)) sortImage :: Ord b => (a -> b) -> [a] -> [a] sortImage f xs = map snd (sortBy cmp_fst [(f x, x) | x <- xs]) where cmp_fst (x,_) (y,_) = compare x y