On Fri, Jul 25, 2008 at 4:45 PM, Anatoly Vorobey
1. I initially tried putStrLn("Hello, " ++ show $ sumIt args), but that didn't compile. Why not?
You see, Prelude> :i (++) (++) :: [a] -> [a] -> [a] -- Defined in GHC.Base infixr 5 ++ Prelude> :i ($) ($) :: (a -> b) -> a -> b -- Defined in GHC.Base infixr 0 $ so "Hello, " ++ show $ sumIt args is parsed as ("Hello, " ++ show) $ sumIt args which is an obvious error.
2. I initially tried
where sumIt = sum $ map read
(the "point-free" style, I believe it's called?) but that didn't compile. Why not? A friend suggested
where sumIt = sum . map read
and that does work; I guess my real problem, then, is that I don't really understand the difference between the two and why the former doesn't work.
It helps to see the types and the definitions ($) :: (a -> b) -> a -> b f $ x = f x (.) :: (b -> c) -> (a -> b) -> (a -> c) f . g = \x -> f (g x) so ($) applies an argument, while (.) composes functions. For example, sumIt x = sum (map read x) so sumIt x = sum $ map read x by ($)'s definition. On the other hand sum $ map read is sum (map read) [by ($)'s definition again] which is, again, a mistake. However, we may write sumIt as sumIt = \x -> sum (map read x) so it becames clear that sumIt = sum . map read Did I help at all? =) -- Felipe.