Hi! On Wed, Sep 29, 2004 at 09:47:14AM +0400, Serge D. Mechveliani wrote: > Simon Marlow responds on the subject of constant space `minimum': > > > > In 6.4, minimum & maximum will have specialised versions for Int & > > Integer, which will run in constant stack space. We can't do this in > > general, because minimum/maximum have the type > > > > (Ord a) => [a] -> a > > > > and we can't assume that the comparison operations for any given type > > are always strict. > > > > I meant that the implementation like > > minimum [x] = x > minimum (x:y:xs) = if x > y then minimum (y:xs) > else minimum (x:xs) > > is correct,
It seems to me that with only minimal assumptions on (>) the above is
actually equivalent to `foldl1 (\ x y -> if x > y then y else x)'.
(And preferable to it.) But does (\ x y -> if x > y then y else x)
have to be equivalent to min? What about the following?
data E a b = L a | R b deriving Eq
instance (Ord a, Ord b) => Ord (E a b) where
compare (L x) (L y) = compare x y
compare (L _) (R _) = LT
compare (R _) (L _) = GT
compare (R x) (R y) = compare x y
min (L x) (L y) = L (min x y)
min (L x) (R _) = L x
min (R _) (L x) = L x
min (R x) (R y) = R (min x y)
I think that it is completely reasonable. Does the report say
anything about this?
Greetings,
Carsten
--
Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin
http://carsten.codimi.de/
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