List functions - "Under" operations

[Tomasz Zielonka]

On Fri, Jan 16, 2004 at 12:01:42PM +0000, Ross Paterson wrote:
> > 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

Yes, you are right. But you are trading memory for speed, and in Haskell
more memory sometimes means less speed.

BTW. cmp_fst = composeFGxGy compare fst

Best regards,
Tom

-- 
.signature: Too many levels of symbolic links