Laziness

Question 1

What is meant by Haskell is a non-strict, or lazy, language?

Answer 1

This means it evaluates expressions only when it needs their results.

Question 2

What does laziness allow?

Answer 2

Lazy evaluation allows easy handling of infinite data-structures.

Question 3

What are the downsides to laziness

Answer 3

sometimes it requires quite a lot of memory

Question 4

Define an infinite list of consecutive integers

Answer 4

[1..]

Question 5

Repeat a set of identical values

Answer 5

repeat ‘a’

Question 6

Name two functions used for dealing with infinite lists

Answer 6

take

drop

Question 7

Does the following expression terminate? What wouuld be the returned value? (give reasons)

let bot = bot
    bottomList = repeat bot
in length(take 5 bottomList)

Answer 7

yes and returns the integer value 5

bot is never evaluated, or the full extent of the infinite list, due to lazy evaluation.

Question 8

How do we generate an infinite list of integer 1 values?

Answer 8

repeat 1

Question 9

Which one of the following functions will not loop infinitely, if we evaluate it in ghci?

length [1..]

tail [1..]

take 10 [1..]

Answer 9

take 10 [1..]

Question 10

Given a Tree data type as defined earlier in the course:

data Tree = Leaf | Node Int facts = map (\x-> (foldr (*) 1 [1..x])) [1..]Tree Tree deriving Show
with Leaf and Node constructors, then how do we define an infinite tree?

Answer 10

mkInfiniteTree = Node 0 (mkInfiniteTree) (mkInfiniteTree)

Question 11

Give the expression which generates an infinite list of successive factorial numbers? (Recall that the nth factorial is the product of the first n positive integers.)

Answer 11

facts = map (\x-> (foldr (*) 1 [1..x])) [1..]