Why does Perl's strict mode allow you to dereference a variable with an undefined value in this foreach context but not in an assignment context? -


this code:

#!/usr/bin/perl use 5.18.0; use strict;  # part 1 $undef = undef; print "1 $undef\n"; foreach $index (@$undef) {   print "unreachable no crash\n"; } print "2 $undef\n";  # part 2 $undef = undef; @array = @$undef; print "unreachable crash\n";

outputs:

1  2 array(0x7faefa803ee8) can't use undefined value array reference @ /tmp/perlfile line 12.

questions part 1:

  1. why dereferencing $undef in part 1 change $undef arrayref empty array?
  2. are there other contexts (other foreach) dereferencing $undef change in same way? terminology describe generic such case?

questions part 2:

  1. why dereferencing $undef in part 2 fall afoul of strict?
  2. are there other contexts (other assignment) dereferencing $undef fall afoul of strict. terminology describe generic such case?

1) for() in perl puts operand "l-value context", therefore $undef being auto-vivified existence array (reference) 0 elements (see this relatively similar question/answer regarding l-value context).

3) because you're trying coercively assign undefined value else in r-value context, , that's illegal under strict (nothing gets auto-vivified in context, you're not magically creating variable nothing in l-value operation).

as far question 2 , 4, there several other context, many think of off top of head. 2, map() comes mind, or other operation treats operand l-value.


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj...
Read more

What is happening when Matlab is starting a "parallel pool"? -

running parallel cpu processes in matlab starts command parpool() according documentation , function: [creates] special job on pool of workers, , [connects] matlab client parallel pool. this function takes bit of time execute, on order of 30 seconds. in other multi-cpu paradigms openmp, parallel execution seems totally transparent -- i've never noticed behavior analogous matlab (granted i'm not experienced parallel programming). so, happening between time parpool() called , when finishes executing? takes long? parallel computing toolbox enables run matlab code in parallel using several different paradigms (e.g. jobs , tasks, parfor , spmd , parfeval , batch processing), , run either locally (parallelised across cores in local machine) or remotely (parallelised across machines in cluster - either 1 own, or 1 in cloud). in of these cases, code run on matlab workers , copies of matlab without interactive desktop. if you're intending run on remo...
Read more

php - Cannot override Laravel Spark authentication with own implementation -

during login process of laravel spark need request token of remote api. api stores exact same users , passwords laravel spark application. therefore need username , non hashed password of user during authentication process. i thought overriding authenticated method solution problem. in routes/web.php override post /login endpoint , pointing endpoint own logincontroller: <?php namespace app\http\controllers\auth; use illuminate\support\facades\log; use illuminate\http\request; use laravel\spark\http\controllers\auth\logincontroller sparklogincontroller; class logincontroller extends sparklogincontroller { /** * create new login controller instance. * * @return void */ public function __construct() { parent::__construct(); } /** * handle successful authentication attempt. * * @param request $request * @param \illuminate\contracts\auth\authenticatable $user * @return response */ public fu...
Read more