19 Nov
2008
19 Nov
'08
midnight
On Tue, Nov 18, 2008 at 6:56 PM, Henning Thielemann
On Tue, 18 Nov 2008, Ryan Ingram wrote:
How does this work?
fac n = case n of 0 -> 1 _ -> n * fac (n-1)
ghci> :t fac fac :: (Num t) => t -> t
The first line of "fac" pattern matches on 0. So how does this work over any value of the Num typeclass? I know that the "1" on the rhs of fac are replaced with (fromInteger 1), but what about numeric literals in patterns? Does it turn into a call to (==)?
As far as I know, yes. It is even possible to trap into an error on pattern matching this way if fromInteger generates an 'undefined'.
As I understand it, the use of (==) in numeric pattern matching is why
Num requires Eq.
--
Dave Menendez