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### Re^11: 99 Problems in Perl6

by ambrus (Abbot)
 on Dec 17, 2006 at 10:23 UTC ( #590292=note: print w/ replies, xml ) Need Help??

in reply to Re^10: 99 Problems in Perl6
in thread 99 Problems in Perl6

I agree with you here that ordered matching of sigs is convenient, even though it isn't as clear.

Standard ML guarantees ordered matching in functions (and also in lambdas and case expressions), so you can say this:

```fun fact 0 = 1 |
fact n = n * fact(n - 1);
(this is not the best way to define factorial though).

Prolog also matches the heads of predicate definitions in order.

Mathematica is a bit different. It matches argument lists to more specific patterns first, and when it is not clear which pattern is more specific, it matches them in order.

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Re^12: 99 Problems in Perl6
by gaal (Parson) on Dec 17, 2006 at 18:04 UTC
Similar syntax exists in Haskell and is called a guard.

```-- Here the |s should be read as one vertical line, like a mathematica
+l function def.
fact n      | n == 0    = 1
| otherwise = n * fact (n - 1)

-- Here's the same thing in different style, the same one as I'd used
+earlier and
-- called "declarative", sometimes called direct pattern matching, bec
+ause each
-- variant gets to bind whatever it managed to match.
fact 0 = 1
fact n = n * fact (n - 1)

There are plenty of non-syntactic variations on this theme. The scanl one is my favorite.

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