On May 4, 2011, at 3:26 PM, David McBride wrote:
I still have a question about this. If you look at the Text.Regex.Posix library you can do this:
"asdf" =~ "asdf" :: String "asdf"
"asdf" =~ "asdf" :: Int 1 :t "asdf" =~ "asdf" "asdf" =~ "asdf" :: (RegexContext Regex [Char] target) => target
So, there is something similar to what the op was asking for. Why is this possible, but his is not?
Because unlike the Foldable class, the RegexContext class does provide functions that return values of its type parameters. RegexContext is defined as follows:
class RegexLike regex source => RegexContext regex source target where match :: regex -> source -> target matchM :: Monad m => regex -> source -> m target
The 'match' function returns a value of type 'target'. For Foldable, though:
class Foldable t where fold :: Monoid m => t m -> m foldMap :: Monoid m => (a -> m) -> t a -> m foldr :: (a -> b -> b) -> b -> t a -> b foldl :: (a -> b -> a) -> a -> t b -> a foldr1 :: (a -> a -> a) -> t a -> a foldl1 :: (a -> a -> a) -> t a -> a
None of those functions return anything that mentions "t", so the Foldable class is not able to construct values of an unknown Foldable type. For a simpler example, the same ideas applies to more-familiar typeclasses defined in the Prelude. For example, you can have expressions of type "Num a => a", because the Num class defines a function "fromInteger :: Integer -> a" - so if you have an Integer, you can make a value of any type that is an instance of Num. But you cannot have a useful expression of type "Show a => a", because Show doesn't define any functions that produce values. -- James