[Python-Dev] type categories

Andrew Koenig ark@research.att.com
16 Aug 2002 10:18:34 -0400

>> In the ML example I showed earlier:

>> fun len([]) = 0
>> | len(h::t) = len(t) + 1

>> ordering is crucial: As long as the argument is not empty, both cases
>> match, so the language is defined to test the clauses in sequence.
>> My intuition is that people will often want to define category tests
>> to be done in a particular order.  There is no problem with such ordering
>> as long as all of the tests are specified together.

Martin> What does "not empty" mean in this context? "not []"? Does h::t match []
Martin> or does [2] match []? Why is the ordering crucial? In Haskell:

Martin> f [] = 0
Martin> f (x:xs) = 1 + f xs

Martin> is totally equivalent with:

Martin> f (x:xs) = 1 + f xs
Martin> f [] = 0

I'm sorry, you're right.  In this particular example, there is no
overlap, so order doesn't matter.  However, the general point still
stands: ML patterns are order-sensitive in cases where there is

Andrew Koenig, ark@research.att.com, http://www.research.att.com/info/ark