perl6 - Why isn't my object attribute populated? -


given oversimplified xml file:

<foo>bar</foo>

and code extracts value foo element:

use xml::rabbit; use data::dump::tree;  class runinfo xml::rabbit::node {     has $.foo xpath("/foo"); }  sub main ( $file! ) {      $xml = runinfo.new( file => $file );      dump $xml;      put "-----------------------";     put "foo $xml.foo()"; }

you'll see value foo nil, though output shows foo bar:

.runinfo @0 ├ $.foo = nil ├ $.context rw = .xml::document @1 │ ├ $.version = 1.0.str │ ├ $.encoding = nil │ ├ %.doctype = {0} @2 │ ├ $.root = .xml::element @3 │ │ ├ $.name rw = foo.str │ │ ├ @.nodes rw = [1] @4 │ │ │ └ 0 = .xml::text @5 │ │ │   ├ $.text = bar.str │ │ │   └ $.parent rw = .xml::element §3 │ │ ├ %.attribs rw = {0} @7 │ │ ├ $.idattr rw = id.str │ │ └ $.parent rw = .xml::document §1 │ ├ $.filename = example.xml.str │ └ $.parent rw = nil └ $.xpath rw = .xml::xpath @9   ├ $.document = .xml::document §1   └ %.registered-namespaces rw = {0} @11 ----------------------- foo bar

(disclaimer: came across behavior today in code, wrote q & style. other answers welcome.).

by way, here links xml::rabbit , data::dump::tree.

this not result of built-in perl 6 feature, rather xml::rabbit module does.

that module provides is xpath trait, , makes sure @ class composition time, attribute has trait applied gets accessor method overridden custom one.

the custom accessor method calculates , sets value attribute first time called, , on subsequent calls returns value that's stored in attribute.

the custom accessor method implemented follows (taken the module's source code parts elided):

method (mu:d:) {     $val = $attr.get_value( self );     unless $val.defined {         ...         $val = ...;         ...         $attr.set_value( self, $val );     }     return $val; }

here, $attr attribute object corresponding attribute, , retrieved prior installing method using meta-object protocol (mop).

the data::dump::tree module, in turn, doesn't use accessor method fetch attributes's value, rather reads directly using mop.

therefore sees attribute's value nil if has not yet been set because accessor not yet called.


Comments

Post a Comment

Popular posts from this blog

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

sql server - Why does Linq-to-SQL add unnecessary COUNT()? -

here tables: create table customer ( cid int primary key, name varchar(32) ) create table ord ( oid int primary key, cid int foreign key references customer, address varchar(20) ) here's linq-to-sql statement: var aaa = c in db.customer select new { c, o = c.ord.tolist() }; and here's inexplicable query generated linq-to-sql: select [t0].[cid] [cid], [t0].[name] [name], [t1].[oid] [oid], [t1].[cid] [cid2], [t1].[address] [address], (select count(*) [dbo].[ord] [t2] [t2].[cid] = [t0].[cid]) [value] [dbo].[customer] [t0] left outer join [dbo].[ord] [t1] on [t1].[cid] = [t0].[cid] order [t0].[cid], [t1].[oid] i want understand how rid of count(*) part. it's entirely uncalled for! linq needs count(*) in order determine number of entries returned o = c.ord.tolist() .
Read more