Editing Generator (computer programming) (section)

===Haskell===

In [[Haskell (programming language)|Haskell]], with its [[lazy evaluation]] model, every datum created with a [[Non-strict evaluation|non-strict]] data constructor is generated on demand. For example,
<syntaxhighlight lang="haskell">
countFrom :: Integer -> [Integer]
countFrom n = n : countFrom (n + 1)

from10to20 :: [Integer]
from10to20 = takeWhile (<= 20) $ countFrom 10

primes :: [Integer]
primes = 2 : 3 : nextPrime 5
  where
    nextPrime n
        | notDivisible n = n : nextPrime (n + 2)
        | otherwise = nextPrime (n + 2)
    notDivisible n =
        all ((/= 0) . (rem n)) $ takeWhile ((<= n) . (^ 2)) $ tail primes
</syntaxhighlight>
where <code>(:)</code> is a non-strict list constructor, ''cons'', and <code>$</code> is just a ''"called-with"'' operator, used for parenthesization. This uses the standard adaptor function,
<syntaxhighlight lang="haskell">
takeWhile p [] = []
takeWhile p (x:xs) | p x = x : takeWhile p xs
                   | otherwise = []
</syntaxhighlight>
which walks down the list and stops on the first element that doesn't satisfy the predicate. If the list has been walked before until that point, it is just a strict data structure, but if any part hadn't been walked through before, it will be generated on demand. List comprehensions can be freely used:
<syntaxhighlight lang="haskell">
squaresUnder20 = takeWhile (<= 20) [x * x | x <- countFrom 10]
squaresForNumbersUnder20 = [x * x | x <- takeWhile (<= 20) $ countFrom 10]
</syntaxhighlight>