On 27 Jan 2009, at 18:42, Tom Poliquin wrote:
I was reading "Arrows and Computation"
http://www.soi.city.ac.uk/~ross/papers/fop.ps.gz
(trying to lose my 'beginner' status) when I saw (on page one)
add :: (b -> Int) -> (b -> Int) -> (b -> Int) add f g b = f b + g b
It seemed like the type definition was wrong (short at least).
I tried it anyway ..
module Main where add :: (b -> Int) -> (b -> Int) -> (b -> Int) add f g b = f b + g b main = do x <- return $ add (+2) (+3) 7 print x
The program compiles and runs and produces '19' !
For fun I loaded into ghci and got what I believe is the proper type ..
*Main> :t add add :: (b -> Int) -> (b -> Int) -> b -> Int
When I try the same thing with something simpler (leaving a bit off the type definition) I get the expected error (by me) ..
module Main where dog :: Int -> Int dog a b = a + b
main = do x <- return $ dog 2 3 print x
Main.hs:5:0: The equation(s) for `dog' have two arguments, but its type `Int -> Int' has only one
What am I missing? .. Apparently something fundamental about type definitions ..
What you're observing is "currying" at work When in Haskell we write the type a -> b, we mean "a function, which accepts items of type 'a', and returns items of type 'b'". Importantly, the -> type operator is right associative. Now, when we say a -> b -> c, right associativity of -> means that this *really* is a -> (b -> c), so, our function takes a single argument, of type a, and produces a new function, which accepts a single argument of type b, and produces something of type c. We can see this at work, lets define this function: f = (+ 1) plus gets applied to a single argument, and returns a new function. We can investigate the type of the new function 'f', and discover that it's a => a -> a – it takes a single numeric argument, and returns something of the same type. Now lets go back and look at your examples: You expected the type of add to be (b -> Int) -> (b -> Int) -> b -> Int, but instead saw the type signature (b -> Int) -> (b -> Int) -> (b -> Int). *but*, thanks to right associativity, those two types are equal! The only reason it's written with parentheses is that the author is trying to draw your eye to the fact that it's a "function transformer" – it takes two functions, and produces a new function based on them. Your dog function gives you an error, because indeed, it takes one argument (a), and returns a new function, which accepts another argument (b), and returns a + b. Thus, it's type signature is Int -> (Int -> Int), which can be written without parentheses as Int -> Int -
Int.
Hope that helps Bob