I know I'm going to kick myself when the answer is revealed by one of you kind folks, but after staring at this a while I just can't figure out what's going on. The compiler (ghci) is trying so hard to tell me what's wrong, but I am failing to understand. I thought for the most part one could take a function definition like f x = (blah blah blah) x and turn it into points-free style by f = (blah blah blah) But from the dialog below, my assumption is incorrect. Help? I. J. Kennedy
sortBy (compare `on` fst) [(2,3),(0,1),(1,5)] -- test a little sort expression [(0,1),(1,5),(2,3)] let sortpairs xss = sortBy (compare `on` fst) xss -- make it into a function :t sortpairs sortpairs :: (Ord a) => [(a, b)] -> [(a, b)] sortpairs [(2,3),(0,1),(1,5)] [(0,1),(1,5),(2,3)] let sortpairs = sortBy (compare `on` fst) -- points-free style sortpairs [(2,3),(0,1),(1,5)] <interactive>:1:24: No instance for (Num ()) arising from the literal `1' at <interactive>:1:24 Possible fix: add an instance declaration for (Num ()) In the expression: 1 In the expression: (1, 5) In the first argument of `sortpairs', namely `[(2, 3), (0, 1), (1, 5)]' :t sortpairs sortpairs :: [((), b)] -> [((), b)] -- what?!