Thanks for your help.
On Wed, Dec 3, 2008 at 3:47 PM, Ryan Ingram <[EMAIL PROTECTED]> wrote: > 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 <[EMAIL PROTECTED]> wrote: >> 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 >> [email protected] >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
