Duncan Coutts wrote:
I think it's a perfectly reasonable mental model for people to believe
that:
data Char = 'a' | 'b' | 'c' | ...
data Int = ... -2 | -1 | 0 | 1 | 2 | ...
I don't see why we should remove one and not the other. Students will
ask why the can pattern match on strings, characters and booleans but
not numbers.
Numbers are special because they are overloaded. A numeric literal is an
element of many types. That clearly distinguishes them from other literals.
Perhaps primitive recursion on integers is misleading, but people will
still write
foo n | n == 0 = ...
| otherwise = ...
where they previously wrote
foo 0 = ...
foo n = ...
so what have we gained except less notational convenience?
Discourage anyone from teaching primitive recursion on integers.
Recursion on integers then has to be taught as a separate topic, giving
opportunity to point out the pitfalls. Sure, it doesn't prevent anyone
from writing anything.
Not all pattern matching on numeric literals is involved with recursion
on integers, where as virtually all n+k patterns is used for that
purpose.
I think there are very few situations where you would use k patterns
without recursion.
So there is some distinction between the two forms. n+k
patterns are a quirk of the numeric types. k patterns are regular with
other types in the language.
As I said above, they are not regular because of overloading.
It's partly the complexity of the language and partly because our latest
language spec (H98) is not the language that we all use (H98 + various
extensions). I'm sure Haskell-prime will help in this area.
I hope as well that Haskell' will be the language that most people will
use and some extensions are certainly required for practical use. I just
want to get rid of superfluous features.
Ciao,
Olaf
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