[Haskell-cafe] lexicographic order
simeon.mattes at gmail.com
Sun Mar 30 16:45:54 EDT 2008
I would like to ask something that I found in the ebook "a Gentle
Introduction to Haskell".
data Tree a = Leaf a | Branch (Tree a) (Tree a)
instance (Ord a) => Ord (Tree a) where
(Leaf _) <= (Branch _) = True
(Leaf x) <= (Leaf y) = x <= y
(Branch _) <= (Leaf _) = False
(Branch l r) <= (Branch l' r') = l == l' && r <= r' || l <= l'
The latter specifies a lexicographic order: Constructors are ordered by the
order of their appearance the data declaration, and the arguments of a
constructor are compared from left to right.
Although I have tried to make sense what lexicographic order means I haven't
figured out. Maybe an example with a simple application of this would be
helpful. To be honest I can't understand what the symbol <= really means.
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