Editing Entry point (section)

===Haskell===
A [[Haskell (programming language)|Haskell]] program must contain a name <code>main</code> bound to a value of type <code>IO t</code>, for some type <code>t</code>;<ref>{{cite web |url=http://www.haskell.org/onlinereport/modules.html |title=The Haskell 98 Report: Modules |publisher=Haskell.org |access-date=2013-08-19 |archive-date=2013-08-19 |archive-url=https://web.archive.org/web/20130819042412/http://www.haskell.org/onlinereport/modules.html |url-status=live }}</ref> which is usually <code>IO ()</code>. <code>IO</code> is a [[monads in functional programming|monad]], which organizes [[Side effect (computer science)|side-effects]] in terms of [[purely functional programming|purely functional]] code.<ref>[http://geekrant.wordpress.com/2008/06/23/misconceptions/ Some Haskell Misconceptions: Idiomatic Code, Purity, Laziness, and IO] {{Webarchive|url=https://web.archive.org/web/20100727150353/http://geekrant.wordpress.com/2008/06/23/misconceptions/ |date=2010-07-27 }} — on Haskell's monadic IO></ref> The <code>main</code> value represents the side-effects-ful computation done by the program. The result of the computation represented by <code>main</code> is discarded; that is why <code>main</code> usually has type <code>IO ()</code>, which indicates that the type of the result of the computation is <code>()</code>, the [[unit type]], which contains no information.

<syntaxhighlight lang="haskell">
main :: IO ()
main = putStrLn "Hello, World!"
</syntaxhighlight>

Command line arguments are not given to <code>main</code>; they must be fetched using another IO action, such as <code>[https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.11.1.0/System-Environment.html#v%3AgetArgs System.Environment.getArgs]</code>.