CS 457/557 Homework 6 – due 2pm, Tuesday, November 15, 2005
the solutions should be placed in a single .hs file, which should be an attachment.
1. Do Hudak exercise 12.2, using the 7-color version of Color on p. 160. Hint: You can save
yourself a quadratic amount of typing by looking carefully at the full definitions of the Ord and
Enum classes given in Ch. 24. Warning: You do not get a static error message if you fail to define
all the needed members in a class instance declaration – though you will get a politely-phrased
runtime error if you try to use an undefined member.
2. Suppose we define a new class, inspired by the Java Serialiazable interface, describing
types that can be “flattened” into a compact sequence of bytes (suitable for storing or transmitting
over a network connection) and subsequently can be recovered.
import Word -- unsigned integers of various sizes
type Bytestream = [Word8]
class Serializable a where
serialize :: a -> Bytestream
deserialize :: Bytestream -> (a,Bytestream)
Here serialize xproduces a Bytestream containing an encoding of x, and
deserialize does the inverse: it converts a prefix of its argument to a value of type aand
returns that value together with the remainder of the bytestream. We might use them as follows:
a :: Char
a = ’a’
b :: [Int]
b = [101,104]
c :: Maybe String
c = Just "pdq"
bytes :: Bytestream
bytes = serialize a ++ serialize b ++ serialize c
-- and later on...
a’:: Char
(a’,rest1) = deserialize bytes
b’ :: [Int]
(b’,rest2) = deserialize rest1
c’ :: Maybe String
(c’,_) = deserialize rest2
-- now should have a == a’, b == b’, c == c’
Complete the following instance declarations. Assume that Int is 32 bits wide. You’ll find the
Bits library useful.
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