On Tue, 2008-08-05 at 20:28 +0100, Andrew Coppin wrote:
Suppose we have the following:
data Foo x list_foo :: [x] -> Foo x foo_list :: Foo x -> [x]
What would be the best way to write a Show instance?
The thing that I came up with is
instance (Show x) => Show (Foo x) where show foo = "list_foo " ++ show (foo_list foo)
But apparently you're supposed to use the strange "showsPrec" method that I don't understand. So can somebody show me a correct instance definition for this type please?
instance Show x => Show (Foo x) where showsPrec n foo = ("list_foo "++) . shows (foo_list foo) You use the n parameter if you've got an infix operator in your syntax and you want to put parentheses around an instance of it if n has the wrong value (either too high or too low, I can never remember). jcc