Michael Marte wrote:
> (First I defined
>
> class Eager a where
> eager :: a -> a
>
> and some instances:
>
> instance Eager Int where
> eager i = i `seq` i
For Int (and other base types), forcing to WHNF is as far as you can go,
so I think you can simplify the above to
> instance Eager Int where
> eager i = i
> instance Eager a => Eager [a] where
> eager [] = []
> eager (x : xs) = eager x `seq` eager xs `seq` (x : xs)
If you want eager to preserve identity (i.e. eager x `unsafePtrEq` x),
as well as to avoid duplicating data, you might prefer:
> instance Eager a => Eager [a] where
> eager xs@[] = xs
> eager xs@(x : xs') = eager x `seq` eager xs' `seq` xs
> instance Eager a => Eager (a, a) where
> eager (x, y) = eager x `seq` eager y `seq` (x, y)
I think the following is more general:
> instance (Eager a, Eager b) => Eager (a, b) where
> eager xy@(x, y) = eager x `seq` eager y `seq` xy
Ultimately, generic Haskell may come handy.
-- Zhanyong Wan
Dept of Computer Science, Yale University
