takewhile and dropwhile

[sergiors]

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[haskellcamargo]

The implementation logic is such simple:

takeWhile               :: (a -> Bool) -> [a] -> [a]
takeWhile _ []          =  []
takeWhile p (x:xs)
            | p x       =  x : takeWhile p xs
            | otherwise =  []

A gambiarra and mutable way to do it:

<?php

function takewhile($p, $xss)
{
  if (empty($xss)) {
    return [];
  }

  $head = $xss[0];

  array_shift($xss);

  return $p($head)
    ? array_merge([$head], takewhile($p, $xss))
    : [];
}

$list = [2, 4, 6, 8, 7, 10, 12];
$even = function ($x) { return $x % 2 === 0; };

var_dump(takewhile($even, $list));

Forgive me for the mutable and imperative way to do it. It's a 30 seconds code.