Perhaps this thought exercise will make things clear:
class Show a => Foo a where toFoo :: String -> a
foos :: (Foo a) => [(String, a)] foos = map (\f -> (show f, f)) [toFoo "a", toFoo "b", toFoo "c"]
data Foo1 = Foo1 instance Show Foo1 where show _ = "1" instance Foo Foo1 where toFoo _ = Foo1 data Foo2 = Foo2 instance Show Foo2 where show _ = "2" instance Foo Foo2 where toFoo _ = Foo2
exercise :: [String] exercise = map fst foos
Exercise for the reader: what should the contents of "exercise" be?
Keep in mind that your question is exactly the same as this one, from
the compiler's point of view.
-- ryan
On Wed, Mar 3, 2010 at 10:48 PM, Marcus Uneson
Thanks. I realize there are many ways to make it compile. However, I am trying to understand the mechanism behind -- why does the first example compile and what constraints does enumerateMethodNames add on a (which it does not inspect)?