On Sat, Nov 21, 2009 at 9:33 PM, Philip Scott
For the other newbies, 'second' takes a function and a tuple, it applies the function to the second thing in your tuple and returns a tuple with the first value unchanged, and the result of applying 'f' to the second:
That's what it does on a specific arrow, though generally that's the idea.
What I am struggling to understand is what on earth the type signature means:
:t second second :: (Arrow a) => a b c -> a (d, b) (d, c)
How can (\x -> "fish") be an 'a b c' when it really looks like this:
:t (\x->"fish") (\x->"fish") :: t -> [Char]
Right, but you must understand that (->) is a type constructor, just like Maybe or Either or your Ts. It takes two types parameter and return a function type. So "a -> b" is the infix syntax, but you could write that "(->) a b" just like you can write "3 + 5" as "(+) 3 5". Once you've done that on your function you get "(->) t Char" which looks a bit more like "a b c"... The final piece is that (->) is an Arrow, the most basic one but still an Arrow, so if you replace a by (->) in the type of second, you get : second :: (->) b c -> (->) (d, b) (d, c) which is just second :: (b -> c) -> (d, b) -> (d, c) which corresponds exactly to the action of second you described (that's the only function that could have this type, except bottom of course). -- Jedaï