Yet Another Haskell Tutorial/Type basics/Solutions

Simple Types String or [Char] type error: lists are homogenous Num{a} => (a, Char) Int type error: cannot add values of different types Polymorphic Types The types: (a,b) -> b [a] -> a [a] -> Bool [a] -> a [[a]] -> a Type Classes Motivation Equality Testing The Num Class The Show Class Function Types Lambda Calculus Higher-Order Types That Pesky IO Type Explicit Type Declarations Functional Arguments The types: a -> [a]. This function takes an element and returns the list containing only that element. a -> b -> b -> (a,[b]). The second and third argument must be of the same type, since they go into the same list. The first element can be of any type. (Num a) => a -> a. Since we apply (+) to a, it must be an instance of Num. a -> String, or a -> [Char]. This ignores the first argument, so it can be any type. (Char -> a) -> a. In this expression, x must be a function which takes a Char as an argument. We don't know anything about what it produces, though, so we call it a. Type error. Here, we assume x has type a. But x is applied to itself, so it must have type b -> c. But then it must have type (b -> c) -> c, but then it must have type ((b -> c) -> c) -> c and so on, leading to an infinite type. (Num a) => a -> a. Again, since we apply (+), this must be an instance of Num. Data Types Pairs The definitions will be something like: data Triple a b c = Triple a b c tripleFst (Triple x y z) = x tripleSnd (Triple x y z) = y tripleThr (Triple x y z) = zThe code, with type signatures, is: data Quadruple a b = Quadruple a a b b firstTwo :: Quadruple a b -> [a] firstTwo (Quadruple x y z t) = [x,y] lastTwo :: Quadruple a b -> [b] lastTwo (Quadruple x y z t) = [z,t] We note here that there are only two type variables, a and b associated with Quadruple. Multiple Constructors The code: data Tuple a b c d = One a | Two a b | Three a b c | Four a b c d tuple1 (One a ) = Just a tuple1 (Two a b ) = Just a tuple1 (Three a b c ) = Just a tuple1 (Four a b c d) = Just a tuple2 (One a ) = Nothing tuple2 (Two a b ) = Just b tuple2 (Three a b c ) = Just b tuple2 (Four a b c d) = Just b tuple3 (One a ) = Nothing tuple3 (Two a b ) = Nothing tuple3 (Three a b c ) = Just c tuple3 (Four a b c d) = Just c tuple4 (One a ) = Nothing tuple4 (Two a b ) = Nothing tuple4 (Three a b c ) = Nothing tuple4 (Four a b c d) = Just dThe code: fromTuple :: Tuple a b c d -> Either (Either a (a,b)) (Either (a,b,c) (a,b,c,d)) fromTuple (One a ) = Left (Left a ) fromTuple (Two a b ) = Left (Right (a,b) ) fromTuple (Three a b c ) = Right (Left (a,b,c) ) fromTuple (Four a b c d) = Right (Right (a,b,c,d)) Here, we use embedded Eithers to represent the fact that there are four (instead of two) options. Recursive Datatypes The code: listHead (Cons x xs) = x listTail (Cons x xs) = xs listFoldl f y Nil = y listFoldl f y (Cons x xs) = listFoldl f (f y x) xs listFoldr f y Nil = y listFoldr f y (Cons x xs) = f x (listFoldr f y xs) Binary Trees The code: elements (Leaf x) = [x] elements (Branch lhs x rhs) = elements lhs ++ [x] ++ elements rhs The code: treeFoldr :: (a -> b -> b) -> b -> BinaryTree a -> b treeFoldr f i (Leaf x) = f x i treeFoldr f i (Branch left x right) = treeFoldr f (f x (treeFoldr f i right)) left elements2 = treeFoldr (:) [] or: elements2 tree = treeFoldr (\a b -> a:b) [] tree The first elements2 is simply a more compact version of the second. The code: treeFoldl :: (a -> b -> a) -> a -> BinaryTree b -> a treeFoldl f i (Leaf x) = f i x treeFoldl f i (Branch left x right) = treeFoldl f (f (treeFoldl f i left) x) right elements3 t = treeFoldl (\i a -> i ++ [a]) [] t Enumerated Sets The Unit type Continuation Passing Style It mimicks neither exactly. Its behavior most closely resembles foldr, but differs slightly in its treatment of the initial value. We can observe the difference in an interpreter: CPS> foldr (-) 0 [1,2,3] 2 CPS> foldl (-) 0 [1,2,3] -6 CPS> fold (-) 0 [1,2,3] -2 Clearly it behaves differently. By writing down the derivations of fold and foldr we can see exactly where they diverge: foldr (-) 0 [1,2,3] ==> 1 - foldr (-) 0 [2,3] ==> ... ==> 1 - (2 - (3 - foldr (-) 0 [])) ==> 1 - (2 - (3 - 0)) ==> 2 fold (-) 0 [1,2,3] ==> fold' (-) (\y -> 0 - y) [1,2,3] ==> 0 - fold' (-) (\y -> 1 - y) [2,3] ==> 0 - (1 - fold' (-) (\y -> 2 - y) [3]) ==> 0 - (1 - (2 - 3)) ==> -2 Essentially, the primary difference is that in the foldr case, the "initial value" is used at the end (replacing []), whereas in the CPS case, the initial value is used at the beginning. The function map in CPS: map' :: (a -> [b] -> [b]) -> [a] -> [b] map' f [] = [] map' f (x:xs) = f x (map' f xs) map2 :: (a -> b) -> [a] -> [b] map2 f l = map' (\x y -> f x : y) l Point elimination: map2 f = map' (\x y -> (:) (f x) y) map2 f = map' (\x -> (:) (f x)) map2 f = map' (\x -> ((:) . f) x) map2 f = map' ((:) . f) The function filter in CPS: (needs to be double checked) filter' :: (a -> [b] -> [b]) -> [a] -> [b] filter' f [] = [] filter' f (x:xs) = f x (filter' f xs) filter2 :: (a -> Bool) -> [a] -> [a] filter2 f l = filter' (\x y -> if (f x) then x:y else y) l