Created
December 29, 2016 03:08
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<?php | |
/** | |
* cons for PHP | |
*/ | |
// constructs a pair | |
function cons($a, $b) { | |
return function($m) use ($a, $b) { | |
if ($m == 0) { | |
return $a; | |
} else if ($m == 1) { | |
return $b; | |
} else { | |
throw new Exception("m is not 0 or 1 CONS"); | |
} | |
}; | |
} | |
// returns the first item in a pair | |
function car($x) { | |
return $x(0); | |
} | |
// returns the second item in a pair | |
function cdr($x) { | |
return $x(1); | |
} | |
// same as car, mostly used on lists | |
function first($x) { | |
return car($x); | |
} | |
// same as cdr, mostly used on lists | |
function rest($x) { | |
return cdr($x); | |
} | |
// constructs a list built from nested pairs | |
// (list 1 2 3 4) | |
function plist(...$items) { | |
if (empty($items)) | |
return null; | |
return cons(array_shift($items), plist(...$items)); | |
} | |
/** | |
* Example Code | |
*/ | |
$pair1 = cons(1, 2); | |
$pair2 = cons(3, 4); | |
// "1" | |
car($pair1); | |
// "2" | |
cdr($pair1); | |
// nested pair | |
$pair = cons($pair1, $pair2); | |
// "1" | |
$one = car(car($pair)); | |
// "3" | |
$two = car(cdr($pair)); | |
// Recursive sum example | |
function sum($list) { | |
if (is_null($list)) | |
return 0; | |
return first($list) + sum(rest($list)); | |
} | |
// "10" | |
sum(plist(1, 2, 3, 4)); |
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