tail call optimization in php 5.3

*note this has been revised: (http://commonphp.blogspot.com/2010/12/revision-to-php-tco-trampoline.html) *

So I have been holding off on releasing my php lisp for the past couple of months for a few reasons. The first was I wanted to get quasiquotes/macros working correctly and the other was I wanted to get TCO working correctly. I checked quasiquotation and macro expansion off a few weeks ago and have been wrestling with TCO since. There are many work arounds such as only using higher order functions like map etc. which "underneath" use php foreach/while etc. but this always felt like a hack. Today I will show how I am accomplishing TCO. It is actually "pretty" enough that I would even use this technique in "normal" php if the need arose i.e. when implementing an algorithm where mutual recursion is the cleanest way of dealing with a given problem.

I had heard "tail" of a magical device called a "trampoline" but had yet to see one implemented in php. First lets set the scene for why such a thing is necessary.

Regular mutually recursive functions:

function even($n) { 
    return $n == 0 ? true : odd($n - 1);
}

function odd($n) { 
    return $n == 0 ? false: even($n - 1);
}

echo even(15000000) ? 'Yep' : 'Nope'; 

Result?
Fatal error: Allowed memory size of 335544320 bytes exhausted (tried to allocate 261900 bytes)

As you can imagine this would blow the stack pretty quickly. The solution is to return closures which close over the next function to be executed by the trampoline:

function trampoline($func, $args) { 
    $return = call_user_func_array($func, $args);
    while(is_callable($return)) {
        $return = $return();
    }
    return $return;
}

Ready for trampoline version of functions, notice the closures being returned:

function even($n) { 
    return $n == 0 ? true : function() use($n) { return odd($n - 1); };
}

function odd($n) { 
    return $n == 0 ? false : function() use($n) { return even($n - 1); };
}

echo trampoline('even', array(15000000)) ? 'Yep' : 'Nope';

No more blowing the stack!

For another example here's a tail call version of factorial:

function fact($n) {
    $product = function($min, $max) use($n, &$product) { 
    return $min == $n ? 
        $max : 
        function() use(&$product, $min, $max) {
            return $product(bcadd($min, 1), bcmul($min, $max));
        };
    };
    return $product(1, $n);
}

echo trampoline('fact', array(5050));

Result?
48424893876442346868149955534231630311152316586879831768663469768542209496025625521335165836597244986775871359731653944863304801472363186993340977071916545094098397335673370033050095301846673812319519184368943848940208083286841710046697935471447379350531958006231636490517114754236601801475241025829782559539886360417931118962453657701734674670226076209971728976092053885317753609072349073735823517244188123450674446015646408575152721563438720528695200000950089076496921144117143216432888312645825602479454709341393797835166357888263581672355452853869124012486552503932416596193377435247315025743200744002301394585228500268455104719958806656594340592125681513366273489623936710098205543483870278942617415439360839831622538877203695372405175195820803498878614463408507559071691975942963055323977023975579962356660525443011519924497192606177308403911494610046020125224506026847906849303464211582255596044461695809931170735992902861015153948973924760246045409447614027579732580231716105662183345892311470363765765619375852044952237375592745997667408717094345683918382540158972633294341661753292428842087402347931146455707829443369742629955406096433554614672210416043568566713315721328986242009438119615170550083892805000693555351590233960368408610557369747063249171375287938142229602269575258888078115537273321885243640816116313033760859934727694805934472650247398590268106132632051710011975357435411073973400518009267243861234604382480972044372345571394804592046625365990943593155118312928026092849203020843936635478814979107242813498013904401479307173527813374488927846868388843629960975688405420793866362748625568963809336537735196635482323869611480442317616213118017936592673957745954854119905353862738630397953491904711075413351939493239103430359228666385849114266162025489907918597974910444817388693635468572983832473160429177887474388606642754513228627277514269543214565586593047881932646980529181738704933529003783674019143726269073240250043051103116046759758599556492457493255007087656022414606222859948126380313826837273496022239059049197478197470692778763575742323649675518937743998020384086092356484327490394949151360442183700474945898304321062066355143800014715430781407860862440970162255814321544952627961522489938257459652704978558232366966762259286202122152536234765500508950380085381022529769410646695981455932497274422145956223070614311782386939393660505071468075609946312266961804079078124972138751570252042991908322662337184841517195535818561858155408260788044667462475133702274158651640293943156955741104099223849587202623924618562507307459878359309762592535803298228381823574279830281362305301068394370171580376856819398786773168976316243886413852653396055497826310062540076987247412010421678922894559582379982459315131410353844686814801084428315428961052501286596189198414322404856396902069457672137306587727429077418184644185029820725981473585275798688633870939149445242039540994144936753560145261967645980942905198036844963065770953567278843744815101688364553167567561127581285151504170857163788476814410430818485342212602471021116122232536945437600193563274350613219002205135771961855617387540043201574995936521078443417355808962389471634447182986942493573328833479027327904592963332830948909056606878114712587614797355021919494836468834407159424831076501904939674180550971070685974826026825572462514492992573071756584471446779582538562871889253777491503761958039422843336111308820256625552392100765429247159402181090198939792727134760027285582388085697688779314720476653237742989447851866203189262136310101262960823946227816914290542573075004229606139191853412465867836753036378462157616915664130017838805074756547841197620513628237540972492294319305705734478032962949128924196666202655682340324787692515590130551301935088659828410687356942341790891345277677382788831208787364801395111486357308798265828146035781558094135067110230536901270297547242166950881473214274739101845553727897045531119954725288893493225046788944932736466460550726919323972814208640861318915500012832311138673106892856437774916302054198800992266101283582978474791023088963610289841529824634218025633260218581419143652195694797922926870213605428040752771533690783079334348985159752481190866305108822791928707079718613729022578623134006395058165941963834815415900225540158365512962747234118295298072392721688699472738462833608355509413369430747483292086244416854740060940250464784905488189338770945263660330117646978971876652613557429461658723058825915031208985735140615497693407650641375234834094995218262573623896322220855235269130698919365626682758374382064483940359436309642974808514506879129036989811006578121058886468214783989745311306991381502598738427646690529026148074559926029141700545754931041231062108024348298640252182862477489626497023992723452959298403206063199087668949604632790351230972033067574210447769545912131839504735995523543564698434762288745349217285435045940022877153117114056361355516568793184887831573689961226955484323093901470710251285589637162339096014594590326637778408537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So in other words PHLISP should be officially released by christmas eve.