Yes; I had a similar question, and it turns out Num is special, or rather, pattern matching on integer literals is special. See the thread http://www.nabble.com/Pattern-matching-on-numbers--td20571034.html The summary is that pattern matching on a literal integer is different than a regular pattern match; in particular:
foo 1 = print "one" foo _ = print "not one"
turns into
foo x = if x == fromInteger 1 then "one" else "not one"
whereas
bar Test = print "Test" bar _ = print "Not Test"
turns into
bar x = case x of { Test -> print "Test" ; _ -> print "Not Test" }
In the former case, the use of (y == fromInteger 1) means that "foo"
works on any argument within the class Num (which requires Eq),
whereas in the latter case, the use of the constructor Test directly
turns into a requirement for a particular type for "bar".
There's no way to get special pattern matching behavior for other
types; this overloading is specific to integer literals.
-- ryan
On Wed, Dec 3, 2008 at 3:05 PM, Anatoly Yakovenko
module Test where --why does this work: data Test = Test
class Foo t where foo :: Num v => t -> v -> IO ()
instance Foo Test where foo _ 1 = print $ "one" foo _ _ = print $ "not one"
--but this doesn't?
class Bar t where bar :: Foo v => t -> v -> IO ()
instance Bar Test where bar _ Test = print $ "test" bar _ _ = print $ "not test" _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe