Am Samstag 31 Oktober 2009 20:39:57 schrieb Keith Sheppard:
the compare function for lists seems to be work like (leaving off
class details):
compare [] [] = EQ compare [] _ = LT compare _ [] = GT compare (x:xt) (y:yt) = case x `compare` y of EQ -> xt `compare` yt _ -> x `compare` y
I poked around to see if I could find where this was defined in the spec or if it was an implementation specific thing (I need to be able to rely on this definition across implementations).
You can rely on lexicographic ordering for lists. Haskell Report, section 10.1 ( http://haskell.org/onlinereport/derived.html#derived- appendix ): 10.1 Derived instances of Eq and Ord The class methods automatically introduced by derived instances of Eq and Ord are (==), (/=), compare, (<), (<=), (>), (>=), max, and min. The latter seven operators are defined so as to compare their arguments lexicographically with respect to the constructor set given, with earlier constructors in the datatype declaration counting as smaller than later ones.
I found this text in the report:
-- Lists
data [a] = [] | a : [a] deriving (Eq, Ord) -- Not legal Haskell; for illustration only
Is this basically saying the same thing as my compare definition?
Yes, see above.
Thanks Keith