begin Mike Dillon quotation:
begin Vo Minh Thu quotation:
I guess it is short because you make use of second... so you can define second' for your B data type, or make B an instance of Arrow.
I don't think that's the case. The code for "f" is making use of the Arrow instance for (->):
second :: Arrow a => a b c -> a (d, b) (d, c) (str ++) :: [Char] -> [Char] second (str ++) :: (d, [Char]) -> (d, [Char])
All the caller can control here is what sort of "d" is passed through unchanged, not the fact that the resulting function expects a pair and returns a pair.
BTW, I was only addressing whether making B an instance of Arrow would help somehow. Creating a second' function could indeed help, provided it had the right signature: second' :: (String -> String) -> B -> B second' f (B i s) = B i (f s) Then you can indeed write "g" like so: g :: String -> B -> B g = second' . (++) Also, I'm pretty sure it isn't possible to write an instance of Arrow for B in principle because the type constructor has the kind "*" and the Arrow class has functions whose types include "Arrow a => a b c", implying that instances of arrow have kind "* -> * -> *". In fact, GHC will tell you as much even without trying to give a body to "instance Arrow B": Kind mis-match Expected kind `* -> * -> *', but `B' has kind `*' In the instance declaration for `Arrow B' -md