Modules in Haskell

1. Modules in Haskell Using modules to structure a large program has a number of advantages: • Parts of the system can be built separately from each other. • Parts of the system can be compiled separately. • Libraries of components can be reused, by importing the appropriate module containing them. module Ant where type Ants = … antEater x = … The convention for file names is that a module Ant resides in the Haskell file Ant.hs or Ant.lhs.

2. Importing a module module Bee where import Ant beeKeeper = … Importing the module Ant means that the visible definitions from the module can be used in Bee. By default the visible definitions in a module are those which appear in the module itself. module Cow where import Bee The definitions of Ant are not visible in Cow.

3. Export control We can control what is exported by following the name of the module with a list of what is to be exported. • module Bee (beeKeeper, Ants, antEater) where … • module Bee (beeKeeper, module Ant) where … • module Fish where type Fish = (String,Size) • module Fish (Fish(..),…) where newtype Fish = F (String,Size) • module Fish (Fish,…) where newtype Fish = F (String,Size)

4. Import control Examples: • import Ant (Ants) • import Ant hiding (antEater) • module Bear where import qualified Ant antEater x = … Ant.antEater x … • import Insect as Ant • import Prelude hiding (words) • import qualified Prelude

5. Overloading and type classes A polymorphic function such as length has a single definition which works over all its types. length :: [a] -> Int length = foldl' (\n _ -> n + 1) 0 An overloaded function like equality (==), + and show can be used at a variety of types, but with different definitions being used at different types.

6. Why overloading? elemBool :: Bool -> [Bool] -> Bool elemBool x [] = False elemBool x (y:ys) = (x ==Bool y) || elemBool x ys elemInt :: Int -> [Int] -> Bool elemInt x [] = False elemInt x (y:ys) = (x ==Int y) || elemInt x ys Generalization may lead to the definition: elemGen :: (a -> a -> Bool) -> a -> [a] -> Bool elemGen p x [] = False elemGen p x (y:ys) = p x y || elemGen x ys but this is too general in a sense, because it can be used with any parameter of type a -> a -> Bool rather than just an equality check.

7. Why overloading? (cont’d) Generalization in the following way will not work. The definition elem :: a -> [a] -> Bool elem x [] = False elem x (y:ys) = (x == y) || elem x ys will cause an error No instance for (Eq a) arising from a use of `==' In the first argument of `(||)', namely `x == y' In the expression: x == y || elem x ys In an equation for `elem`: elem x (y : ys) = x == y || elem x ys because this definition requires that (==) :: a -> a -> Bool is already defined.

8. Classes A type class or simply a class defines a collection of types over which specific functions are defined. class Eq a where (==) :: a -> a -> Bool Members of a class are called instances. Built-in instances of Eq include the base types Int, Float, Bool, Char, tuples and lists built from types which are themselves instances of Eq, e.g., (Int,Bool) and [[Char]]. elem :: Eq a => a -> [a] -> Bool elem x [] = False elem x (y:ys) = (x == y) || elem x ys

9. Classes (cont’d) (+1) :: Int -> Int elem (+1) [] causes an error No instance for (Eq (a0 -> a0)) arising from a use of `elem' Possible fix: add an instance declaration for (Eq (a0 -> a0)) In the expression: elem (+ 1) [] In an equation for `it': it = elem (+ 1) [] which conveys the fact that Int -> Int is not an instance of the Eq class.

10. Instances of classes Examples: instance Eq Bool where True == True = True False == False = True _ == _ = False instance Eq a => Eq [a] where [] == [] = True (x:xs) == (y:ys) = x==y && xs==ys _ == _ = False instance (Eq a, Eq b) => Eq (a,b) where (x,y) == (z,w) = x==z && y==w

11. Default definitions The Haskell Eq class is in fact defined by class Eq a where (==), (/=) :: a -> a -> Bool x /= y = not (x==y) x == y = not (x/=y) Both functions have default definitions in terms of the other function. At any instance a definition of at least one of == and /= needs to be provided.

12. Derived classes To be ordered, a type must carry operations >, >= and so on, as well as the equality operations. class (Eq a) => Ord a where compare :: a -> a -> Ordering (<), (<=), (>=), (>) :: a -> a -> Bool max, min :: a -> a -> a

13. Example class Visible a where toString :: a -> String size :: a -> Int instance Visible Bool where toString True = ”True” toString False = ”False” size _ = 1 instance Visible a => Visible [a] where toString = concat . map toString size = foldr (+) 1 . map size

14. Example (cont’d) sort :: Ord a => [a] -> [a] vSort :: (Ord a, Visible a) => [a] -> String vSort = toString . sort class (Ord a, Visible a) => OrdVis a vSort' :: OrdVis a => [a] -> String vSort' = toString . sort

15. A tour of the built-in Haskell classes Many of the Haskell built-in classes are numeric, and are built to deal with overloading of the numerical operations. We will not study those classes. Class Eq: class Eq a where (==), (/=) :: a -> a -> Bool x /= y = not (x==y) x == y = not (x/=y) Instances: All except of IO, ->

16. Class Ord class (Eq a) => Ord a where compare :: a -> a -> Ordering (<), (<=), (>=), (>) :: a -> a -> Bool max, min :: a -> a -> a -- Minimal complete definition: (<=) or compare -- using compare can be more efficient for complex types compare x y | x==y = EQ | x<=y = LT | otherwise = GT x <= y = compare x y /= GT x < y = compare x y == LT x >= y = compare x y /= LT x > y = compare x y == GT max x y | x <= y = y | otherwise = x min x y | x <= y = x | otherwise = y Instances: All except IO, IOError, ->

17. Class Enum class Enum a where succ, pred :: a -> a toEnum :: Int -> a fromEnum :: a -> Int enumFrom :: a -> [a] -- [n..] enumFromThen :: a -> a -> [a] -- [n,m..] enumFromTo :: a -> a -> [a] -- [n..m] enumFromThenTo :: a -> a -> a -> [a] -- [n,n'..m] -- Minimal complete definition: toEnum, fromEnum succ = toEnum . (1+) . fromEnum pred = toEnum . subtract 1 . fromEnum enumFrom x = map toEnum [ fromEnum x ..] enumFromTo x y = map toEnum [ fromEnum x .. fromEnum y ] enumFromThen x y = map toEnum [ fromEnum x, fromEnum y ..] enumFromThenTo x y z = map toEnum [ fromEnum x, fromEnum y .. fromEnum z ] Instances: (), Bool, Char, Int, Integer, Float Double

18. Class Bounded class Bounded a where minBound, maxBound :: a -- Minimal complete definition: All Instances: Int, Char, Bool (but not Integer)

19. Class Show type ShowS = String -> String class Show a where show :: a -> String showsPrec :: Int -> a -> ShowS showList :: [a] -> ShowS -- Minimal complete definition: show or showsPrec show x = showsPrec 0 x "" showsPrec _ x s = show x ++ s showList [] = showString "[]" showList (x:xs) = showChar '[' . shows x . showl xs where showl [] = showChar ']' showl (x:xs) = showChar ',' . shows x . showl xs Instances: All except ->

20. Class Read type ReadS a = String -> [(a,String)] class Read a where readsPrec :: Int -> ReadS a readList :: ReadS [a] -- Minimal complete definition: readsPrec readList = readParen False (\r -> [pr | ("[",s) <- lex r, pr <- readl s ]) where readl s = [([],t) | ("]",t) <- lex s] ++ [(x:xs,u) | (x,t) <- reads s, (xs,u) <- readl' t] readl' s = [([],t) | ("]",t) <- lex s] ++ [(x:xs,v) | (",",t) <- lex s, (x,u) <- reads t, (xs,v) <- readl' u] Instances: All except IO, ->