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  • Pointer inside a struct / thread

    - by bruno
    Hi! I have this warning "warning: assignment from incompatible pointer type " in this line: data1->transformed_block[l] = &transformed_block[l]; - void print_message_function ( void *ptr ) { dt *data; data = (dt *) ptr; printf("Dentro da thread Numero0: %ld\n", data->L_norm_NewBlock); pthread_exit(0); } typedef struct data_thread { long L_norm_NewBlock; int Bsize_X; int Bsize_Y; int *transformed_block[MAX_LEVEL]; long L_norm_OrigBlock; } dt; void function() { int *transformed_block[MAX_LEVEL]; pthread_t thread1; dt *data1; pthread_attr_t attr; pthread_attr_init(&attr); //Fills structure data1 = (dt *) malloc(sizeof(dt)); data1->transformed_block[l] = &transformed_block[l]; data1->L_norm_NewBlock=0; data1->Bsize_Y = Bsize_Y; data1->Bsize_X = Bsize_X; pthread_create(&thread1, &attr, (void *) &print_message_function, (void *) &data1); } I want to get rid of that warning, and the values i get inside the thread are wrong. For example data1-L_norm_NewBlock=0; in the thread guives me a differente value (not 0 like it should be).

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  • Get the first and last posts in a thread

    - by Grampa
    I am trying to code a forum website and I want to display a list of threads. Each thread should be accompanied by info about the first post (the "head" of the thread) as well as the last. My current database structure is the following: threads table: id - int, PK, not NULL, auto-increment name - varchar(255) posts table: id - int, PK, not NULL, auto-increment thread_id - FK for threads The tables have other fields as well, but they are not relevant for the query. I am interested in querying threads and somehow JOINing with posts so that I obtain both the first and last post for each thread in a single query (with no subqueries). So far I am able to do it using multiple queries, and I have defined the first post as being: SELECT * FROM threads t LEFT JOIN posts p ON t.id = p.thread_id ORDER BY p.id LIMIT 0, 1 The last post is pretty much the same except for ORDER BY id DESC. Now, I could select multiple threads with their first or last posts, by doing: SELECT * FROM threads t LEFT JOIN posts p ON t.id = p.thread_id ORDER BY p.id GROUP BY t.id But of course I can't get both at once, since I would need to sort both ASC and DESC at the same time. What is the solution here? Is it even possible to use a single query? Is there any way I could change the structure of my tables to facilitate this? If this is not doable, then what tips could you give me to improve the query performance in this particular situation?

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  • how to edit controls in a system::thread

    - by Ian Lundberg
    I need to be able to add items to a listbox inside of a thread. Code is below. 1. ref class Work 2. { 3. public: 4. static void RecieveThread() 5. { 6. while (true) 7. { 8. ZeroMemory(cID, 64); 9. ZeroMemory(message, 256); 10. if(recv(sConnect, message, 256, NULL) != SOCKET_ERROR && recv(sConnect, cID, 64, NULL) != SOCKET_ERROR) 11. { 12. ID = atoi(cID); 13. String^ meep = gcnew String(message); 14. lbxMessages->Items->Add(meep); 15. check = 1; 16. } 17. } 18. } 19. }; I get the error Error: a nonstatic member reference must be relative to a specific object on line 14. Is there any way to get it to let me do that? Because if I try to use String^ meep; outside of that Thread it doesn't contain anything. It works PERFECT when I use it within the thread but not outside of it. I need to be able to add that message to the listbox. If anyone can help I would appreciate it.

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  • sqlobject: No connection has been defined for this thread or process

    - by Claudiu
    I'm using sqlobject in Python. I connect to the database with conn = connectionForURI(connStr) conn.makeConnection() This succeeds, and I can do queries on the connection: g_conn = conn.getConnection() cur = g_conn.cursor() cur.execute(query) res = cur.fetchall() This works as intended. However, I also defined some classes, e.g: class User(SQLObject): class sqlmeta: table = "gui_user" username = StringCol(length=16, alternateID=True) password = StringCol(length=16) balance = FloatCol(default=0) When I try to do a query using the class: User.selectBy(username="foo") I get an exception: ... File "c:\python25\lib\site-packages\SQLObject-0.12.4-py2.5.egg\sqlobject\main.py", line 1371, in selectBy conn = connection or cls._connection File "c:\python25\lib\site-packages\SQLObject-0.12.4-py2.5.egg\sqlobject\dbconnection.py", line 837, in __get__ return self.getConnection() File "c:\python25\lib\site-packages\SQLObject-0.12.4-py2.5.egg\sqlobject\dbconnection.py", line 850, in getConnection "No connection has been defined for this thread " AttributeError: No connection has been defined for this thread or process How do I define a connection for a thread? I just realized I can pass in a connection keyword which I can give conn to to make it work, but how do I get it to work if I weren't to do that?

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  • Is this function thread-safe?

    - by kiddo
    Hello all,I am learning multi-threading and for the sake of understanding I have wriiten a small function using multithreading...it works fine.But I just want to know if that thread is safe to use,did I followed the correct rule. void CThreadingEx4Dlg::OnBnClickedOk() { //in thread1 100 elements are copied to myShiftArray(which is a CStringArray) thread1 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction1,this); WaitForSingleObject(thread1->m_hThread,INFINITE); //thread2 waits for thread1 to finish because thread2 is going to make use of myShiftArray(in which thread1 processes it first) thread2 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction2,this); thread3 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction3,this); } UINT MyThreadFunction1(LPARAM lparam) { CThreadingEx4Dlg* pthis = (CThreadingEx4Dlg*)lparam; pthis->MyFunction(0,100); return 0; } UINT MyThreadFunction2(LPARAM lparam) { CThreadingEx4Dlg* pthis = (CThreadingEx4Dlg*)lparam; pthis->MyCommonFunction(0,20); return 0; } UINT MyThreadFunction3(LPARAM lparam) { CThreadingEx4Dlg* pthis = (CThreadingEx4Dlg*)lparam; WaitForSingleObject(pthis->thread3->m_hThread,INFINITE); //here thread3 waits for thread 2 to finish so that thread can continue pthis->MyCommonFunction(21,40); return 0; } void CThreadingEx4Dlg::MyFunction(int minCount,int maxCount) { for(int i=minCount;i<maxCount;i++) { //assume myArray is a CStringArray and it has 100 elemnts added to it. //myShiftArray is a CStringArray -public to the class CString temp; temp = myArray.GetAt(i); myShiftArray.Add(temp); } } void CThreadingEx4Dlg::MyCommonFunction(int min,int max) { for(int i = min;i < max;i++) { CSingleLock myLock(&myCS,TRUE); CString temp; temp = myShiftArray.GetAt(i); //threadArray is CStringArray-public to the class threadArray.Add(temp); } myEvent.PulseEvent(); }

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  • Is the below thread pool implementation correct(C#3.0)

    - by Newbie
    Hi Experts, For the first time ever I have implemented thread pooling and I found it to be working. But I am not very sure about the way I have done is the appropriate way it is supposed to be. Would you people mind in spending some valuable time to check and let me know if my approach is correct or not? If you people find that the approach is incorrect , could you please help me out in writing the correct version. I have basicaly read How to use thread pool and based on what ever I have understood I have developed the below program as per my need public class Calculation { #region Private variable declaration ManualResetEvent[] factorManualResetEvent = null; #endregion public void Compute() { factorManualResetEvent = new ManualResetEvent[2]; for (int i = 0; i < 2; i++){ factorManualResetEvent[i] = new ManualResetEvent(false); ThreadPool.QueueUserWorkItem(ThreadPoolCallback, i);} //Wait for all the threads to complete WaitHandle.WaitAll(factorManualResetEvent); //Proceed with the next task(s) NEXT_TASK_TO_BE_EXECUTED(); } #region Private Methods // Wrapper method for use with thread pool. public void ThreadPoolCallback(Object threadContext) { int threadIndex = (int)threadContext; Method1(); Method2(); factorManualResetEvent[threadIndex].Set(); } private void Method1 () { //Code of method 1} private void Method2 () { //Code of method 2 } #endregion }

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  • KVM network bridge and public static IP for both host and guests

    - by Javier Martinez
    I have a Debian Server with 4 public static addresses. There is a KVM guest (also Debian) installed and running. What I want is to give the guest an IP of the host, so that both machines have public IPs. IP 1: 188.165.A.B IP 2: 178.33.CCC.D IP 3: 178.33.CCC.E IP 4: 178.33.CCC.F What should I do to have connection for host and guest ? This is network conf: # ifconfig br0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:188.165.A.B Bcast:188.165.255.255 Mask:255.255.255.0 inet6 addr: fe80::ea40:f2ff:fe0a:cc28/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:3618 errors:0 dropped:4 overruns:0 frame:0 TX packets:4853 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:599562 (585.5 KiB) TX bytes:1693443 (1.6 MiB) eth0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet6 addr: fe80::ea40:f2ff:fe0a:cc28/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:4274 errors:0 dropped:0 overruns:0 frame:0 TX packets:4879 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:720045 (703.1 KiB) TX bytes:1715641 (1.6 MiB) Interrupt:20 Memory:fe500000-fe520000 eth0:0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.D Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 eth0:1 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.E Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 eth0:2 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.F Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:27932 errors:0 dropped:0 overruns:0 frame:0 TX packets:27932 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1820862 (1.7 MiB) TX bytes:1820862 (1.7 MiB) vnet0 Link encap:Ethernet HWaddr fe:54:00:87:40:ec inet6 addr: fe80::fc54:ff:fe87:40ec/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:18 errors:0 dropped:0 overruns:0 frame:0 TX packets:204 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:1452 (1.4 KiB) TX bytes:16958 (16.5 KiB) #route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default aa.bb.cc.eu 0.0.0.0 UG 0 0 0 br0 188.165.255.0 * 255.255.255.0 U 0 0 0 br0 # brctl show bridge name bridge id STP enabled interfaces br0 8000.e840f20acc28 no eth0 vnet0 There is no firewall enabled and DNS is configured properly. What I want to achieve: | | | +----+-------------------------+-+------+ | | Host | | | | | | | | | | +------------+------+ | | eth0 | eth0:0-1 | | | 188.165.A.B | | | | | | | | | br0 vnet0 | | | +------------+------+ | | | | | | | | +------------+------+ | | | | | | | | eth0:2-+ | | | | 178.33.CCC.F | | | | | | | | Guest | | | +-------------------+ | +---------------------------------------+ Thanks you

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  • KVM network bridge and public static IP for both host and guests

    - by Javier Martinez
    I have a Debian Server with 4 public static addresses. There is a KVM guest (also Debian) installed and running. What I want is to give the guest an IP of the host, so that both machines have public IPs. IP 1: 188.165.A.B IP 2: 178.33.CCC.D IP 3: 178.33.CCC.E IP 4: 178.33.CCC.F What should I do to have connection for host and guest ? This is network conf: # ifconfig br0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:188.165.A.B Bcast:188.165.255.255 Mask:255.255.255.0 inet6 addr: fe80::ea40:f2ff:fe0a:cc28/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:3618 errors:0 dropped:4 overruns:0 frame:0 TX packets:4853 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:599562 (585.5 KiB) TX bytes:1693443 (1.6 MiB) eth0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet6 addr: fe80::ea40:f2ff:fe0a:cc28/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:4274 errors:0 dropped:0 overruns:0 frame:0 TX packets:4879 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:720045 (703.1 KiB) TX bytes:1715641 (1.6 MiB) Interrupt:20 Memory:fe500000-fe520000 eth0:0 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.D Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 eth0:1 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.E Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 eth0:2 Link encap:Ethernet HWaddr e8:40:f2:0a:cc:28 inet addr:178.33.CCC.F Bcast:178.33.255.255 Mask:255.255.255.255 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:20 Memory:fe500000-fe520000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:27932 errors:0 dropped:0 overruns:0 frame:0 TX packets:27932 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1820862 (1.7 MiB) TX bytes:1820862 (1.7 MiB) vnet0 Link encap:Ethernet HWaddr fe:54:00:87:40:ec inet6 addr: fe80::fc54:ff:fe87:40ec/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:18 errors:0 dropped:0 overruns:0 frame:0 TX packets:204 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:1452 (1.4 KiB) TX bytes:16958 (16.5 KiB) #route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default aa.bb.cc.eu 0.0.0.0 UG 0 0 0 br0 188.165.255.0 * 255.255.255.0 U 0 0 0 br0 # brctl show bridge name bridge id STP enabled interfaces br0 8000.e840f20acc28 no eth0 vnet0 There is no firewall enabled and DNS is configured properly. What I want to achieve: | | | +----+-------------------------+-+------+ | | Host | | | | | | | | | | +------------+------+ | | eth0 | eth0:0-1 | | | 188.165.A.B | | | | | | | | | br0 vnet0 | | | +------------+------+ | | | | | | | | +------------+------+ | | | | | | | | eth0:2-+ | | | | 178.33.CCC.F | | | | | | | | Guest | | | +-------------------+ | +---------------------------------------+ Thanks you

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  • Pxe net install Centos with Static IP

    - by Stu2000
    I seem to be unable to perform a kickstart installation of centos5.8 with a netinstall. It correctly gets into the text installer, but keeps sending out a request for the dhcp server and failing. I have tried to manually set the IP everywhere. Here is my pxelinux.cfg file DEFAULT menu PROMPT 0 MENU TITLE Ubuntu MAAS TIMEOUT 200 TOTALTIMEOUT 6000 ONTIMEOUT local LABEL centos5.8-net kernel /images/centos5.8-net/vmlinuz MENU LABEL centos5.8-net append initrd=/images/centos5.8-net/initrd.img ip=192.168.1.163 netmask=255.255.255.0 hostname=client101 gateway=192.168.1.1 ksdevice=eth0 dns=8.8.8.8 ks=http://192.168.1.125/cblr/svc/op/ks/profile/centos5.8-net MENU end and here is my kickstart file: # Kickstart file for a very basic Centos 5.8 system # Assigns the server ip: 192.211.48.163 # DNS 8.8.8.8, 8.8.4.4 # London TZ install url --url http://mirror.centos.org/centos-5/5.8/os/i386 lang en_US.UTF-8 keyboard us network --device=eth0 --bootproto=static --ip=192.168.1.163 --netmask=255.255.255.0 --gateway=192.168.1.1 --nameserver=8.8.8.8,8.8.4.4 --hostname=client1-server --onboot=on rootpw --iscrypted $1$Snrd2bB6$CuD/07AX2r/lHgVTPZyAz/ firewall --enabled --port=22:tcp authconfig --enableshadow --enablemd5 selinux --enforcing timezone --utc Europe/London bootloader --location=mbr --driveorder=xvda --append="console=xvc0" # The following is the partition information you requested # Note that any partitions you deleted are not expressed # here so unless you clear all partitions first, this is # not guaranteed to work part /boot --fstype ext3 --size=100 --ondisk=xvda part pv.2 --size=0 --grow --ondisk=xvda volgroup VolGroup00 --pesize=32768 pv.2 logvol swap --fstype swap --name=LogVol01 --vgname=VolGroup00 --size=528 --grow --maxsize=1056 logvol / --fstype ext3 --name=LogVol00 --vgname=VolGroup00 --size=1024 --grow %packages @base @core @dialup @editors @text-internet keyutils iscsi-initiator-utils trousers bridge-utils fipscheck device-mapper-multipath sgpio emacs Here is my dhcp file: ddns-update-style interim; allow booting; allow bootp; ignore client-updates; set vendorclass = option vendor-class-identifier; subnet 192.168.1.0 netmask 255.255.255.0 { host tower { hardware ethernet 50:E5:49:18:D5:C6; fixed-address 192.168.1.163; option routers 192.168.1.1; option domain-name-servers 8.8.8.8,8.8.4.4; option subnet-mask 255.255.255.0; filename "/pxelinux.0"; default-lease-time 21600; max-lease-time 43200; next-server 192.168.1.125; } } Is it impossible to prevent it asking for a dynamic ip before trying to install from the net? Perhaps there is an error in of my files? My dhcp server is set to ignore client-updates, and is set to only works with one mac address whilst testing.

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  • Nginx + Wordpress Multisite 3.4.2 + subdirectories + static pages and permalinks

    - by UrkoM
    I am trying to setup Wordpress Multisite, using subdirectories, with Nginx, php5-fpm, APC, and Batcache. As many other people, I am getting stuck in the rewrite rules for permalinks. I have followed these two guides, which seem to be as official as you can get: http://evansolomon.me/notes/faster-wordpress-multisite-nginx-batcache/ http://codex.wordpress.org/Nginx#WordPress_Multisite_Subdirectory_rules It is partially working: http://blog.ssis.edu.vn works. http://blog.ssis.edu.vn/umasse/ works. But other permalinks, like these two to a post or to a static page, don't work: http://blog.ssis.edu.vn/umasse/2008/12/12/hello-world-2/ http://blog.ssis.edu.vn/umasse/sample-page/ They either take you to a 404 error, or to some other blog! Here is my configuration: server { listen 80 default_server; server_name blog.ssis.edu.vn; root /var/www; access_log /var/log/nginx/blog-access.log; error_log /var/log/nginx/blog-error.log; location / { index index.php; try_files $uri $uri/ /index.php?$args; } # Add trailing slash to */wp-admin requests. rewrite /wp-admin$ $scheme://$host$uri/ permanent; # Add trailing slash to */username requests rewrite ^/[_0-9a-zA-Z-]+$ $scheme://$host$uri/ permanent; # Directives to send expires headers and turn off 404 error logging. location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ { expires 24h; log_not_found off; } # this prevents hidden files (beginning with a period) from being served location ~ /\. { access_log off; log_not_found off; deny all; } # Pass uploaded files to wp-includes/ms-files.php. rewrite /files/$ /index.php last; if ($uri !~ wp-content/plugins) { rewrite /files/(.+)$ /wp-includes/ms-files.php?file=$1 last; } # Rewrite multisite '.../wp-.*' and '.../*.php'. if (!-e $request_filename) { rewrite ^/[_0-9a-zA-Z-]+(/wp-.*) $1 last; rewrite ^/[_0-9a-zA-Z-]+.*(/wp-admin/.*\.php)$ $1 last; rewrite ^/[_0-9a-zA-Z-]+(/.*\.php)$ $1 last; } location ~ \.php$ { # Forbid PHP on upload dirs if ($uri ~ "uploads") { return 403; } client_max_body_size 25M; try_files $uri =404; fastcgi_pass unix:/var/run/php5-fpm.sock; fastcgi_index index.php; fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name; include /etc/nginx/fastcgi_params; } } Any ideas are welcome! Have I done something wrong? I have disabled Batcache to see if it makes any difference, but still no go.

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  • Timer in java ,time difference problem

    - by javatechi
    I want to create a timer for my app. The sample code is shown below. When the method datetwo() is called the same time in milliseconds is shown as there in the main method. Please help me out with this import java.util.Date; import java.util.Timer; public class TimerChe { Timer timer; static Date date = new Date(); static Date date2 = new Date(); public static void timerMethod(){ new Thread() { public void run() { try { while (true) { sleep(10000); datetwo(); } } catch (InterruptedException ex) { } } }.start(); } public static void datetwo() { System.out.println ("OK, It's time to do something!") ; System.out.println("The Time is " + date2.getTime() + " milliseconds since 1970/01/01"); } public static void main(String args[]) throws Exception { System.out.println("The Time is " + date.getTime() + " milliseconds since 1970/01/01" ); System.out.println ("Schedule something to do in the mean time.") ; timerMethod(); } }

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  • What can Haskell's type system do that Java's can't?

    - by Matt Fenwick
    I was talking to a friend about the differences between the type systems of Haskell and Java. He asked me what Haskell's could do that Java's couldn't, and I realized that I didn't know. After thinking for a while, I came up with a very short list of minor differences. Not being heavy into type theory, I'm left wondering whether they're formally equivalent. To try and keep this from becoming a subjective question, I'm asking: what are the major, non-syntactical differences between their type systems? I realize some things are easier/harder in one than in the other, and I'm not interested in talking about those. And to make it more specific, let's ignore Haskell type extensions since there's so many out there that do all kinds of crazy/cool stuff.

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  • What can Haskell's type system do that Java's can't and vice versa?

    - by Matt Fenwick
    I was talking to a friend about the differences between the type systems of Haskell and Java. He asked me what Haskell's could do that Java's couldn't, and I realized that I didn't know. After thinking for a while, I came up with a very short list of minor differences. Not being heavy into type theory, I'm left wondering whether they're formally equivalent. To try and keep this from becoming a subjective question, I'm asking: what are the major, non-syntactical differences between their type systems? I realize some things are easier/harder in one than in the other, and I'm not interested in talking about those. And to make it more specific, let's ignore Haskell type extensions since there's so many out there that do all kinds of crazy/cool stuff.

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  • Type checking and recursive types (Writing the Y combinator in Haskell/Ocaml)

    - by beta
    When explaining the Y combinator in the context of Haskell, it's usually noted that the straight-forward implementation won't type-check in Haskell because of its recursive type. For example, from Rosettacode [1]: The obvious definition of the Y combinator in Haskell canot be used because it contains an infinite recursive type (a = a -> b). Defining a data type (Mu) allows this recursion to be broken. newtype Mu a = Roll { unroll :: Mu a -> a } fix :: (a -> a) -> a fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x)) And indeed, the “obvious” definition does not type check: ?> let fix f g = (\x -> \a -> f (x x) a) (\x -> \a -> f (x x) a) g <interactive>:10:33: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 Expected type: t2 -> t0 -> t1 Actual type: (t2 -> t0 -> t1) -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a <interactive>:10:57: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a (0.01 secs, 1033328 bytes) The same limitation exists in Ocaml: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; Error: This expression has type 'a -> 'b but an expression was expected of type 'a The type variable 'a occurs inside 'a -> 'b However, in Ocaml, one can allow recursive types by passing in the -rectypes switch: -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. By using -rectypes, everything works: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> utop # let fact_improver partial n = if n = 0 then 1 else n*partial (n-1);; val fact_improver : (int -> int) -> int -> int = <fun> utop # (fix fact_improver) 5;; - : int = 120 Being curious about type systems and type inference, this raises some questions I'm still not able to answer. First, how does the type checker come up with the type t2 = t2 -> t0 -> t1? Having come up with that type, I guess the problem is that the type (t2) refers to itself on the right side? Second, and perhaps most interesting, what is the reason for the Haskell/Ocaml type systems to disallow this? I guess there is a good reason since Ocaml also will not allow it by default even if it can deal with recursive types if given the -rectypes switch. If these are really big topics, I'd appreciate pointers to relevant literature. [1] http://rosettacode.org/wiki/Y_combinator#Haskell

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  • How to solve 404 for static files with Django and Nginx?

    - by Lucio
    I setup a Trusty VM with Django + Nginx (other stuffs too). The server is working, I get the "Welcome to Django" page. But when I enter to servername/admin it loads the HTML page but fails to load the static content. And the admin page have this links to static content: <link rel="stylesheet" type="text/css" href="/static/admin/css/base.css" /> <link rel="stylesheet" type="text/css" href="/static/admin/css/login.css" /> Both of the CSS files give me 404, as the Nginx log shows: 192.168.56.1 - - [05/Jun/2014:12:04:09 -0300] "GET /admin HTTP/1.1" 301 5 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:09 -0300] "GET /admin/ HTTP/1.1" 200 833 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:10 -0300] "GET /static/admin/css/base.css HTTP/1.1" 404 142 "http://ubuntu-server/admin/" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:10 -0300] "GET /static/admin/css/login.css HTTP/1.1" 404 142 "http://ubuntu-server/admin/" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" I think that the error is on my nginx.conf file, but do not know how to solve it.

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  • Dynamic vs Statically typed languages for websites

    - by Bradford
    Wanted to hear what others thought about this statement: I’ll contrast that with building a website. When rendering web pages, often you have very many components interacting on a web page. You have buttons over here and little widgets over there and there are dozens of them on a webpage, as well as possibly dozens or hundreds of web pages on your website that are all dynamic. With a system with a really large surface area like that, using a statically typed language is actually quite inflexible. I would find it painful probably to program in Scala and render a web page with it, when I want to interactively push around buttons and what-not. If the whole system has to be coherent, like the whole system has to type check just to be able to move a button around, I think that can be really inflexible. Source: http://www.infoq.com/interviews/kallen-scala-twitter

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  • Types of semantic bugs, logic errors [closed]

    - by C-Otto
    I am a PhD student and currently focus on automatically finding instances of new types of bugs in (Java) programs that cannot be found by existing tools like FindBugs. The existing tool currently is used to prove/disprove termination of (Java) programs. I have some ideas (see below), but I could need more input from you (experienced programmers, potential users of my tool). What kind of bugs do you wish to find? What types of bugs exist and might be suitable for my analysis? One strength of the approach I use is detailled information about the heap. So in contrast to FindBugs, I can work with knowledge of the form "variable x and variable y are disjoint on the heap" or "variable z is not cyclic". It is also possible to see if a method might have side effects (and if so, which variables may/may not be affected by it). Example 1: Vacuous call: Graph graphOne = createGraph(); Graph graphTwo = createGraph(); Node source = graphTwo.getRootNode(); for (Node n : graphOne.getNodes()) { if (areConnected(source, n)) { graphTwo.addNode(n); } } Imagine createGraph() creates a fresh graph, so that graphOne and graphTwo are disjoint on the heap. Then, because source is taken from graphTwo instead of graphOne, the call to areConnected always returns false. In this situation I could find out that the call areConnected is useless (because it does not have any side effect and the return value always is false) which helps finding the real bug (taking source from the wrong graph). For this the information that x and y are disjoint (because graphOne and graphTwo are disjoint) is crucial. This bug is related to calling x.equals(y) where x and y are objects of different classes. In this scenario, most implementations of equals() always return false, which most likely is not the intended result. FindBugs already finds this bug (hardcoded to equals(), semantics of implementation is not checked). Example 2: Useless code: someCode(); while (something()) { yetMoreSomething(); } moreCode(); In the case that the loop (so the code in something() and yetMoreSomething()) does not modify anything visible outside the loop, it does not make sense to run this code - the program has the same behaviour as someCode(); moreCode() (i.e., without the loop). To find this out, one needs detailled information about the side effects of the (possibly useless) code. If I can prove that the code does not have any side effect that can be observed afterwards (in the example: in moreCode() or later), then the code indeed is useless. Of course, here Input/Output of any form must be seen as a side effect, so that a System.out.println(...) is not considered useless. Example 3: Ignored return value: Instead of x = foo(); and making use of x, the method is called without storing the result: foo();. If the method does not have any side effect, its invocation is useless and can be dropped. Most likely, the bug here is that the returned value should have been used. Here, too, detailled information about side effects are needed. Can you think of similar types of bugs that might be detected (only) with detailled information about the heap, side effects, semantics of called methods, ...? Did you encounter bugs related to the ones shown below in "real life"? By the way, the tool is AProVE and Java related publications can be found on my homepage. Thanks a lot, Carsten

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  • Seeking a free Lint for C which programmers will *want* to use

    - by Mawg
    When I try to persuade others to Lint their code I always get excuses - too difficult to set up, too difficult to understand, false positives, etc (most of which translates to too lazy, too stupid or too afraid of new things). Is there any way that I can make Linting easier? We code in C using Netbeans. Can I incorporate Splint into Netbeans? I did find a Splint GUI which was quite good, but there was no way to lint a directory tree. Any ideas? Thanks in advance

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  • IPv4 private address assignment

    - by helloworld922
    I'm working on a private network which uses static IPv4 addresses as well as DHCP addressing for the physical LAN network. At a previous company I worked at they would assign static addresses in the 10.*.*.* space and all DHCP/LAN addresses were assigned in the 192.168.*.* space. Both of these address spaces are defined in the IPv4 private address space and there were never any internal conflicts. From personal experience at home, school, at work, and pretty much any other machine I've dealt with extensively (Windows and a few Linux distros), the DHCP server would always by default choose an address from the 192.168.*.* address space. Now my question is can I rely on this behavior? Do DHCP servers always by default assign from the 192.168.*.* pool (or any pool other than the 10.*.*.* pool), leaving the 10.*.*.* pool free for private static addressing? If not, under what conditions might a DHCP server choose an address in the 10.*.*.* address space?

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  • How can I use timer to stop another thread? [on hold]

    - by Haoda Fu
    How can we stop another thread based on a timer? I was trying to use timer to stop another thread. But I didn't got a success. To better illustrate my point and for your easy to understand the key issue. I made the following sample example. Your help is really appreciated using System; using System.Collections.Generic; using System.Text; using System.Threading; using System.Timers; namespace TestCodes { public static class Program { private static Thread nT = new Thread(PrintABC); private static System.Timers.Timer aTimer; public static void Main() { aTimer = new System.Timers.Timer(1000); aTimer.Elapsed += TimerCallback; aTimer.Interval = 1000; aTimer.Enabled = true; nT.Start(); Console.ReadLine(); } private static void TimerCallback(Object o, ElapsedEventArgs e) { nT.Join(); Console.WriteLine("Complete the PrintABC"); GC.Collect(); } private static void PrintABC() { for (int iter = 1; iter < 300; iter++) { Console.WriteLine(iter+"abc"); Console.ReadKey(); //Thread.Sleep(100); } } } }

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  • Can I use a 302 redirect to serve up static content from a url with escaped_fragment?

    - by Starfs
    We would like to serve up seo-friendly ajax-driven content. We are following this documentation. Has anyone ever tried to write a 302 redirect into the htaccess file, that takes the '?_escaped_fragment=' string and send that to a static page? For example /snapshot/yourfilename/ How will Google react to this? I've gone through the documentation and it's not very clear. The below quote is from Google's documentation this is what I find. I'm not sure if they are saying that you can redirect the _escaped_fragment_ url to a different static page, or if this is to redirect the hashtag URL to static content? Thoughts? From Google's site: Question: Can I use redirects to point the crawler at my static content? Redirects are okay to use, as long as they eventually get you to a page that's equivalent to what the user would see on the #! version of the page. This may be more convenient for some webmasters than serving up the content directly. If you choose this approach, please keep the following in mind: Compared to serving the content directly, using redirects will result in extra traffic because the crawler has to follow redirects to get the content. This will result in a somewhat higher number of fetches/second in crawl activity. Note that if you use a permanent (301) redirect, the url shown in our search results will typically be the target of the redirect, whereas if a temporary (302) redirect is used, we'll typically show the #! url in search results. Depending on how your site is set up, showing #! may produce a better user experience, because the user will be taken straight into the AJAX experience from the Google search results page. Clicking on a static page will take them to the static content, and they may experience avoidable extra page load time if the site later wants to switch them to the AJAX experience.

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  • Nginx Cache-Control

    - by optixx
    Iam serving my static content with ngnix. location /static { alias /opt/static/blog/; access_log off; etags on; etag_hash on; etag_hash_method md5; expires 1d; add_header Pragma "public"; add_header Cache-Control "public, must-revalidate, proxy-revalidate"; } The resulting header looks like this: Cache-Control:public, must-revalidate, proxy-revalidate Cache-Control:max-age=86400 Connection:close Content-Encoding:gzip Content-Type:application/x-javascript; charset=utf-8 Date:Tue, 11 Sep 2012 08:39:05 GMT Etag:e2266fb151337fc1996218fafcf3bcee Expires:Wed, 12 Sep 2012 08:39:05 GMT Last-Modified:Tue, 11 Sep 2012 06:22:41 GMT Pragma:public Server:nginx/1.2.2 Transfer-Encoding:chunked Vary:Accept-Encoding Why is nginx sending 2 Cache-Control entries, could this be a problem for the clients?

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  • Are the only types of data "sources" static and dynamic?

    - by blunders
    Thinking that there might be others, but not sure -- but before getting into that, let me explain what I mean by static and dynamic data sources. Static (or datastore) - Meaning that the data's state is non-changing, and if was changed, that would be a new state, and the old data would be considered stateless; meaning it no longer is known to exist, or not exist. Another way of possibly looking at a static data source might be that if read and written back without modification, the checksum for before and after should be exactly the same regardless of the duration of time between the reading and rewriting of the data. Examples: Photos, Files, Database Record, Dynamic (or datastream) - Meaning that the data's state is known to be in flux, and never expected to be the same per input. Example: Live video/audio feed, Stock Market feed, First let me say, the above is a very loose mapping of the concepts, and I'd welcome any feedback. Next, onto the core of the question, that being are these the only two types of data sources. My guess, is that yes, they are -- but that there are hybrid versions of the two. That being, streaming data that has a fixed state. For example, the data being streamed has a checksum given and each unique checksum is known to be a single instance of static data. On the flip side, static data could be chained via say a version control system; when played back, each version might be viewed as a segment of a stream; thing is, the very fact that it can be played back makes the data source static. Another type might be that the data source is being organically discovered, and it's simply unknown what the state is. Questions, feedback, requests -- just comment, thanks!!

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  • Dynamically vs Statically typed languages studies

    - by Winston Ewert
    Do there exist studies done on the effectiveness of statically vs dynamically typed languages? In particular: Measurements of programmer productivity Defect Rate Also including the effects of whether or not unit testing is employed. I've seen lots of discussion of the merits of either side but I'm wondering whether anyone has done a study on it. Edit Sadly, only one of the papers shown is actually a study and it does nothing but conclude that the language matters. This leads me to ponder: what if I proposed doing such a study with volunteers from this site?

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  • Can I use a 302 redirect to serve up static content from an URL with escaped_fragment?

    - by Starfs
    We would like to serve up SEO-friendly Ajax-driven content. We are following this documentation. Has anyone ever tried to write a 302 redirect into the .htaccess file, that takes the ?_escaped_fragment= string and send that to a static page?, for example /snapshot/yourfilename/. How will Google react to this? I've gone through the documentation and it's not very clear. The below quote is from Google's documentation this is what I find. I'm not sure if they are saying that you can redirect the _escaped_fragment_ URL to a different static page, or if this is to redirect the hashtag URL to static content? Thoughts? From Google's site: Question: Can I use redirects to point the crawler at my static content? Redirects are okay to use, as long as they eventually get you to a page that's equivalent to what the user would see on the #! version of the page. This may be more convenient for some webmasters than serving up the content directly. If you choose this approach, please keep the following in mind: Compared to serving the content directly, using redirects will result in extra traffic because the crawler has to follow redirects to get the content. This will result in a somewhat higher number of fetches/second in crawl activity. Note that if you use a permanent (301) redirect, the url shown in our search results will typically be the target of the redirect, whereas if a temporary (302) redirect is used, we'll typically show the #! url in search results. Depending on how your site is set up, showing #! may produce a better user experience, because the user will be taken straight into the AJAX experience from the Google search results page. Clicking on a static page will take them to the static content, and they may experience avoidable extra page load time if the site later wants to switch them to the AJAX experience.

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