Generator (computer programming): Difference between revisions

Revision as of 14:31, 22 January 2023

Revision as of 16:10, 27 January 2023

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===Haskell===

In [[Haskell (programming language)|Haskell]], with its [[lazy evaluation]] model, everything is a generator - every datum created with a [[Non-strict evaluation|non-strict]] data constructor is generated on demand. For example,

<syntaxhighlight lang="haskell">

countFrom :: Integer -> [Integer]

countfromcountFrom n = n : countfromcountFrom (n + 1)

from10to20 :: [Integer]

-- Example use: printing out the integers from 10 to 20.

test1from10to20 = mapM_ print $ takeWhile (<= 20) $ countfromcountFrom 10

primes = 2 : 3 : nextprime 5 where[Integer]

primes = 2 : 3 : nextPrime 5

nextprime n | b = n : nextprime (n+2)

where

| otherwise = nextprime (n+2)

nextPrime n

where b = all ((/= 0).(rem n)) $ takeWhile ((<= n).(^2)) $ tail primes

| notDivisible n = n : nextPrime (n + 2)

| otherwise = nextprimenextPrime (n + 2)

notDivisible n =

where b = 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,

Line 252 ⟶ 257:

| otherwise = []

</syntaxhighlight>

which re-fetcheswalks valuesdown agreeablethe withlist aand stops on the first element that doesn't satisfy the predicate,. andIf stopsthe requestinglist newhas valuesbeen aswalked soonbefore asuntil athat non-agreeablepoint, oneit is encountered.just Thea sharedstrict storagedata accessstructure, isbut usedif asany apart hadn't been walked through before, it will universalbe mediatorgenerated inon Haskelldemand. List comprehensions can be freely used:

<syntaxhighlight lang="haskell">

test2squaresUnder20 = mapM_ print $ takeWhile (<= 20) [x * x | x <- countfromcountFrom 10]

test3squaresForNumbersUnder20 = mapM_ print [x * x | x <- takeWhile (<= 20) $ countfromcountFrom 10]

</syntaxhighlight>