Lecture 26-2
Lazy Lists
Lists are never evaluated until they are needed. This means that take 1 [1..10] is just as efficient as take 1 [1..1000].
Infinite Lists
The reason why we are able to make infinite lists is a result of lazy evaluation. As evaluation only happens right at the end, the program won’t hang by trying to evaluate it in the background. You can make simple infinite list like so:
[1,1..]
Or like this:
all1s :: [Int]
all1s = 1 : all1s
As a result of this you are able to do infinite computations on infinite lists provided that you can resolve to a finite value.
all2s = zipwith (+) all1s all1s
This will produce an infinite sum of infinite lists.
Example
sieve (x:xs) = x : filter (\y -> y `mod` x /= 0) (sieve xs)
You can then use this function to define other useful infinite lists:
primes = sieve [2..]
> take 10 primes
[2,3,]
> primes !! 1000
7927
The $ Operator
The $ function evaluates a function.
Take this function and apply it to this argument
Strict Evaluation
The $! operator makes Haskell evaluate strictly:
> let f x = 1
> f $ undefined
1
> f $! undefined
Error
For most code you won’t notice the difference but it can change the error outputs.