CSCE 314 Lecture 5

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Haskell (cont'd)

Type Class

a collection of function signatures that a type must satisfy in order to be a member of the class (similar to a Java interface

class Eq a where
  (==), (/=) :: a -> a -> Bool

class Eq a => Ord a where
  (<), (<=), (>), (>=) :: a -> a -> Bool
  min, max :: a -> a -> a

Examples of usage in function type declarations

elem :: Eq a => a -> [a] -> Bool
min :: Ord a => a -> a -> a

Where Eq and Ord are type classes

Show and Read

class Show a where
  show :: a -> String

class Read a where
  read :: String -> a

Read needs type annotations in some uses so that Haskell knows what to read into:

> read "10" :: Int
10
> read ("[1, 2, 3]") :: [Int]
[1,2,3]
> map (* 2.0) (read ("[1, 2]")
[2.0,4.0]

In map's case, map expects a list of types acceptable to a function. The * operator expects two arguments of the same type. Since one type is given in the curried function (* 2.0), Haskell can infer the type that the read function is supposed to return.