Testing for the definedness of a variable in Scheme -

-

besides regular definitions (define variable value) scheme allows uninitialized definitions (define variable), when variable bound value later on aid of set!.

are there procedures allowing test whether symbol defined (not initialized) variable? (defined? 'variable) should return #t if variable defined , #f otherwise?

the form (define var) non-standard extension.

according r5rs define has 1 of following forms:

(define <variable> <expression>) (define (<variable> <formals>) <body>) (define (<variable> . <formal>) <body>)

as far can tell same goes r6rs , r7rs.

most (define foo) expands (define foo the-undefined-value) in implementation. means can use (if (eq? foo the-undefined-value) ...) test whetherfoo` has been initialized or not.

http://www.schemers.org/documents/standards/r5rs/html/r5rs-z-h-8.html#%_idx_190

update:

i checked r6rs , says:

(define <variable> <unspecified>) <unspecified> side-effect-free expression returning  unspecified value.

consider

(define foo) (define bar)

it implementor whether same unspecified value bound both foo , bar.

try program:

(define unspecified) (define unspecified1) (eq? unspecified unspecified1)

if program evaluates #t can write initialized? predicate this:

(define unspecified) (define (initialized? x) (not (eq? x unspecified)))  (define test) (initialized? test) (set! test 42) (initialized? test)

Comments

Post a Comment

Popular posts from this blog

Fail to load namespace Spring Security http://www.springframework.org/security/tags -

-
i have problem namespace of spring security on xhtml page. don't understand why can't load uri, , have following issue: whitelabel error page application has no explicit mapping /error, seeing fallback. tue jul 21 16:58:12 utc 2015 there unexpected error (type=not found, status=404). please need help! possible uri has been changed? best regards you using incorrect namespace url. correct url http://www.springframework.org/schema/security the error page seeing spring source website since not recognize url. try visiting http://www.springsource.org/security/tags in browser update : spring security tags used in jsp page correct tag lib declaration is: <%@ taglib prefix="sec" uri="http://www.springframework.org/security/tags" %>
Read more

sql - MySQL query optimization using coalesce -

-
can explain me why second query faster first? (first 1 takes around 40 seconds, second 1 less 0.1) select releases.feature_code, releases.platform_id, version, release_date, general_available, uri, filesize, display_name, feature_name.full_name, if ((select exists(select 1 release_exceptions releases.feature_code = release_exceptions.feature_code , releases.platform_id = release_exceptions.platform_id , releases.version = release_exceptions.version) = 1), 1, 0) release_exceptions releases left outer join platform using(platform_id) left outer join feature_name using(feature_code) order version desc; ~~~ query 2 ------------------ ~~~ select feature_code, version, release_date, general_available, platform_id, uri, filesize, display_name, feature_name.full_name, coalesce(release_exceptions, 0) release_exceptions mus.releases left outer join ( select distinct feature_code, version, platform_id, 1 release_exceptions mus.release_exceptions ) release_exceptions using ...
Read more

unity3d - Unity local avoidance in user created world -

-
i'm using unity3d networked multiplayer online game have large complex 3d terrain scene forest, trees, cliff, hills, mountains, bounders, etc. players can build structures sort of minecraft, , put them anywhere in scene, or move them around anytime. aside human controlled players, there automated ai players , objects animals roaming around scene following path. the problem how make these automated ai players , animals, able navigate around static , dynamic player created structures, because path follow can blocked player created structures, or other players , other ai objects, cliffs etc. have find away around them or on track if tumble down off high cliff example. so made navmesh , used navagents, takes care of static, non moving objects, how make ai players navigate around each other , dynamic structures created players can number in hundreds? i thought of adding navmeshobstacle everything, result in being attached hundreds of objects since user created structures b...
Read more