[Python-Dev] type categories

[Martin Sjögren]

fre 2002-08-16 klockan 15.37 skrev Andrew Koenig:
> David> Oh, easily solved: "in the face of ambiguity, refuse the
> David> temptation to guess".  There should be a best match rule, and
> David> if there are two best matches, it's an error.
> 
> 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.

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

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

is totally equivalent with:

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

Of course, if the different patterns overlap, THEN ordering is crucial,
I just find it odd that [] and h::t would overlap...


Martin

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