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• Why is writing a compiler in a functional language so efficient and easier?

  - by wvd

  Hello all, I've been thinking of this question very long, but really couldn't find the answer on Google as well a similar question on Stackoverflow. If there is a duplicate, I'm sorry for that. A lot of people seem to say that writing compilers and other language tools in functional languages such as OCaml and Haskell is much more efficient and easier then writing them in imperative languages. Is this true? And if so -- why is so efficient and easy to write them in functional languages instead of in an imperative language, like C? Also -- isn't a language tool in a functional language slower then in some low-level language like C? Thanks in advance, William v. Doorn

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• Why I sould not develop an opensource runtime UI Autogeneration from domain objects?

  - by Marco Bettiolo

  I'm using for my projects a rather complete UI auto-generation tool from database entities for windows forms and asp.net I wrote. Now I've built a working prototype UI auto-generation tool from domain objects. Right now it is in early stage of development and by reflection it generates user interface for creating and updating domain objects. I searched a bit and I didn't find other opensource projects that have the same goal. Why? This type of tool is not useful? Is this idea fundamentally flawed? Thanks.

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• Why is writing a compiler in a functional language easier?

  - by wvd

  Hello all, I've been thinking of this question very long, but really couldn't find the answer on Google as well a similar question on Stackoverflow. If there is a duplicate, I'm sorry for that. A lot of people seem to say that writing compilers and other language tools in functional languages such as OCaml and Haskell is much more efficient and easier then writing them in imperative languages. Is this true? And if so -- why is it so efficient and easy to write them in functional languages instead of in an imperative language, like C? Also -- isn't a language tool in a functional language slower then in some low-level language like C? Thanks in advance, William v. Doorn

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• Why do I get strange output from Perl using SQL?

  - by benjamin button

  Here is my Perl code: foreach my $line (@tmp_field_validation) { chomp $line; my ($cycle_code,$cycle_month,$cycle_year)= split /\s*\|\s*/, $line; $cycle_code=~ s/^\s*(.*)\s*$/$1/; $cycle_month=~ s/^\s*(.*)\s*$/$1/; $cycle_year=~ s/^\s*(.*)\s*$/$1/; print "$line\n"; print "$cycle_code|$cycle_month|$cycle_year"; } Here is the output: 1 10 2009 1 10 2009|| What's wrong over here? I expected the pipes to be between the variables. Why are the pipes getting printed after all the three variables?

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• Why do I need to give my options a value attribute in my dropdown? JQuery.

  - by Alex

  So far in my web developing experiences, I've noticed that almost all web developers/designers choose to give their options in a select a value like so: <select name="foo"> <option value="bar">BarCheese</option> // etc. // etc. </select> Is this because it is best practice to do so? I ask this because I have done a lot of work with jQuery and dropdown's lately, and sometimes I get really annoyed when I have to check something like: $('select[name=foo]').val() == "bar"); To me, many times that seems less clear than just being able to check the val() against BarCheese. So why is it that most web developers/designers specify a value paramater instead of just letting the options actual value be its value?

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• Why does s ++ t not lead to a stack overflow for large s?

  - by martingw

  I'm wondering why Prelude> head $ reverse $ [1..10000000] ++ [99] 99 does not lead to a stack overflow error. The ++ in the prelude seems straight forward and non-tail-recursive: (++) :: [a] -> [a] -> [a] (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys So just with this, it should run into a stack overflow, right? So I figure it probably has something to do with the ghc magic that follows the definition of ++: {-# RULES "++" [~1] forall xs ys. xs ++ ys = augment (\c n -> foldr c n xs) ys #-} Is that what helps avoiding the stack overflow? Could someone provide some hint for what's going on in this piece of code?

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• Why has Foundation 4 made its grid classes less natural and readable?

  - by Brenden

  The Background I love responsive CSS grids. I hate Bootstrap's complex class names. I fell in love with Foundations human readable class names. The Problem With Foundation 4, they have changed four columns to large-4 small-4 columns and in my opinion this makes the HTML markup less clear. This style of CSS class names is exactly why I switched from Bootstrap to Foundation. The Question What advantage is gained by Foundation 4's Grid in making this change? It seems that you can have a different grid layout on smaller screens via media queries, but I can't think of a design that would require this. Note: I've been focused on native mobile development and therefore I may be missing out on recent best practices.

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• Why doesn't Python's `re.split()` split on zero-length matches?

  - by Tim Pietzcker

  One particular quirk of the (otherwise quite powerful) re module in Python is that re.split() will never split a string on a zero-length match, for example if I want to split a string along word boundaries: >>> re.split(r"\s+|\b", "Split along words, preserve punctuation!") ['Split', 'along', 'words,', 'preserve', 'punctuation!'] instead of ['', 'Split', 'along', 'words', ',', 'preserve', 'punctuation', '!'] Why does it have this limitation? Is it by design? Do other regex flavors behave like this?

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• Why will this for loop not return one field from list rather than the list?

  - by Dick Eshelman

  import csv """sample row = 10/6/2010,73.42,74.43,72.9,74.15,2993500""" filename_in = 'c:/python27/scripts/fiverows.csv' reader = csv.reader(open(filename_in, "rb"), dialect="excel", delimiter="\t", quoting =csv.QUOTE_MINIMAL) for row in reader: for item in row: print 'row = ',row print 'item = ', item When you run this script and print the row you get the sample row returned in [] as a list. When you print the item you get the sample row as an unquoted string. Why do I not get each field ie, (10/6/2010), (73.42), etc. returned as an item? How do I return a single item?

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• Why is new showat attribute required when using code generation?

  - by Patrick Karcher

  When I generate code using T4 templates in Visual Studio 2010, I get the following error for each of my asp controls when I try to compile: Control "ddState" is missing required attribute "showat". I have never gotten this error in previous versions of .NET. Further, I don't get this error when I manually construct my pages either by dragging/dropping, nor do I get it when I type out the control text myself. When I generate code, I have to manually add showat="client" to my tag for the compiler to be happy. It was my understanding that I never had to explicitly specify this tag. The following: <asp:dropdownlist id="ddState" runat="server" showat="client" /> solves the problem. Why do I have to add this to generated code but not other times? (It's a VS-2010 webforms project, using VB, in case that makes a difference.)

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• In Scala 2.8 collections, why was the Traversable type added above Iterable?

  - by Seth Tisue

  I know that to be Traversable, you need only have a foreach method. Iterable requires an iterator method. Both the Scala 2.8 collections SID and the "Fighting Bitrot with Types" paper are basically silent on the subject of why Traversable was added. The SID only says "David McIver... proposed Traversable as a generalization of Iterable." I have vaguely gathered from discussions on IRC that it has to do with reclaiming resources when traversal of a collection terminates? The following is probably related to my question. There are some odd-looking function definitions in TraversableLike.scala, for example: def isEmpty: Boolean = { var result = true breakable { for (x <- this) { result = false break } } result } I assume there's a good reason that wasn't just written as: def isEmpty: Boolean = { for (x <- this) return false true }

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• Why has to be size of dynamically-allocated array a static field?

  - by Ondrej Slinták

  I have a dummy class where I am testing arrays. I've noticed that when I want to dynamically allocate size of array at runtime, fields that indicate this size have to be static. I know I should probably use collections for this kind of code, but I'm more interested why do these fields have to be static? Is there any particular reason behind this? class Foo { private static int x; private static int y; private int[,] bar = new int[ x, y ]; public Foo( int a, int b ) { x = a; y = b; } }

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• Why does Cache.Add return an object that represents the cached item?

  - by Pure.Krome

  From MSDN about the differences between Adding or Inserting an item the ASP.NET Cache: Note: The Add and Insert methods have the same signature, but there are subtle differences between them. First, calling the Add method returns an object that represents the cached item, while calling Insert does not. Second, their behavior is different if you call these methods and add an item to the Cache that is already stored there. The Insert method replaces the item, while the Add method fails. [emphasis mine] The second part is easy. No question about that. But with the first part, why would it want to return an object that represents the cached item? If I'm trying to Add an item to the cache, I already have/know what that item is? I don't get it. What is the reasoning behind this?

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• Why is Postgres doing a Hash in this query?

  - by Claudiu

  I have two tables: A and P. I want to get information out of all rows in A whose id is in a temporary table I created, tmp_ids. However, there is additional information about A in the P table, foo, and I want to get this info as well. I have the following query: SELECT A.H_id AS hid, A.id AS aid, P.foo, A.pos, A.size FROM tmp_ids, P, A WHERE tmp_ids.id = A.H_id AND P.id = A.P_id I noticed it going slowly, and when I asked Postgres to explain, I noticed that it combines tmp_ids with an index on A I created for H_id with a nested loop. However, it hashes all of P before doing a Hash join with the result of the first merge. P is quite large and I think this is what's taking all the time. Why would it create a hash there? P.id is P's primary key, and A.P_id has an index of its own.

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• Why do debug symbols so adversely affect the performance of threaded applications on Linux?

  - by fluffels

  Hi. I'm writing a ray tracer. Recently, I added threading to the program to exploit the additional cores on my i5 Quad Core. In a weird turn of events the debug version of the application is now running slower, but the optimized build is running faster than before I added threading. I'm passing the "-g -pg" flags to gcc for the debug build and the "-O3" flag for the optimized build. Host system: Ubuntu Linux 10.4 AMD64. I know that debug symbols add significant overhead to the program, but the relative performance has always been maintained. I.e. a faster algorithm will always run faster in both debug and optimization builds. Any idea why I'm seeing this behavior?

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• Why is C++ fwrite() producing larger output in release?

  - by waffleShirt

  I recently wrote an implementation of the Canonical Huffman compression algorithm. I have a 500kb test file that can be compressed to about 250kb when running the debug and release builds from within Visual Studio 2008. However when I run the release build straight from the executeable the test file only compresses to about 330kb. I am assuming that something is going wrong when the file is written using fwrite(). I have tested the program and confirmed that uncompressing the files always produces the correct uncompressed file. Does anyone know why this could possibly be? How could the same executeable file be producing different sized outputs based on where it is launched from?

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• Python subprocess.Popen hangs in 'for l in p.stdout' until p terminates, why?

  - by Albert

  I have that code: #!/usr/bin/python -u localport = 9876 import sys, re, os from subprocess import * tun = Popen(["./newtunnel", "22", str(localport)], stdout=PIPE, stderr=STDOUT) print "** Started tunnel, waiting to be ready ..." for l in tun.stdout: sys.stdout.write(l) if re.search("Waiting for connection", l): print "** Ready for SSH !" break The "./newtunnel" will not exit, it will constantly output more and more data to stdout. However, that code will not give any output and just keeps waiting in the tun.stdout. When I kill the newtunnel process externally, it flushes all the data to tun.stdout. So it seems that I can't get any data from the tun.stdout while it is still running. Why is that? How can I get the information? Note that the default bufsize for Popen is 0 (unbuffered). I can also specify bufsize=0 but that doesn't change anything.

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• Why does PHP 5.2 disallow abstract static class methods?

  - by Artem Russakovskii

  After enabling strict warnings in PHP 5.2, I saw a load of strict standards warnings from a project that was originally written without strict warnings: Strict Standards: Static function Program::getSelectSQL() should not be abstract in Program.class.inc The function in question belongs to an abstract parent class Program and is declared abstract static because it should be implemented in its child classes, such as TVProgram. I did find references to this change here: Dropped abstract static class functions. Due to an oversight, PHP 5.0.x and 5.1.x allowed abstract static functions in classes. As of PHP 5.2.x, only interfaces can have them. My question is: can someone explain in a clear way why there shouldn't be an abstract static function in PHP?

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• Why do i get a 403 error when viewing a Drupal custom menu item via clean url?

  - by Chaulky

  Hi all, I've created a custom menu item in my Drupal 6 website by defining it in a custom module. This is an extremely simple MENU_NORMAL_ITEM menu item. The menu item is defined as /** * Implementation of hook_menu(). */ function menu_test_menu() { $items['menu_test'] = array( 'title' => 'Menu Test', 'page callback' => 'menu_test_hello', 'access callback' => TRUE, 'type' => MENU_NORMAL_ITEM, ); return $items; } Since I have clean URLs on, the path should be www.example.com/menu_test. That URL gives me a 403 error. But, if I enter www.example.com/?q=menu_test, everything works fine. Why am I getting the 403 error? The menu item is useless because it's always trying to go to the clean URL path, which should work but doesn't for some reason. Thanks for the help!

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• Why exactly is calling the destructor for the second time undefined behavior in C++?

  - by sharptooth

  As mentioned in this answer simply calling the destructor for the second time is already undefined behavior 12.4/14(3.8). For example: class Class { public: ~Class() {} }; // somewhere in code: { Class* object = new Class(); object->~Class(); delete object; // UB because at this point the destructor call is attempted again } In this example the class is designed in such a way that the destructor could be called multiple times - no things like double-deletion can happen. The memory is still allocated at the point where delete is called - the first destructor call doesn't call the ::operator delete() to release memory. For example, in Visual C++ 9 the above code looks working. Even C++ definition of UB doesn't directly prohibit things qualified as UB from working. So for the code above to break some implementation and/or platform specifics are required. Why exactly would the above code break and under what conditions?

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• Why await is not taken in consideration after deploy?

  - by Cristian Boariu

  I have a method which does some sync calls to a specific REST api, something like: WSRequestHolder url = WS.url("rest_api_url"); Promise<WS.Response> promisePerPage = url.get(); promisePerPage.getWrappedPromise().await(3000, TimeUnit.MILLISECONDS); WS.Response responsePerPage = promisePerPage.get(); ProductsWrapper productsWrapper = new Gson().fromJson(responsePerPage.getBody(), ProductsWrapper.class); As you notice, I put 3 seconds between calls so each request can be parsed in time and inserted in DB. All works great locally but after I deploy to cloud, all goes continuously, without any more waiting (3 seconds) between requests... Do you know why?

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• Why does cisco IOS require domain-name to be set before SSH keys can be generated?

  - by Daniel Papasian

  Is there a technical reason why IOS requires the device's domain-name to be set (via ip domain-name) before an SSH key can be generated? Is the domain-name used in any way in the generation of the key? Is there any way to force the generation of a key before the domain name is set? UPDATE: Myself (before I posted this question) and others in your answer seem to think it may be used as either a salt or a source of entropy for the key. Wouldn't the domain-name be very predictable? This doesn't seem like a suitable source of entropy.

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• C++: Why does space always terminate a string when read?

  - by Nullw0rm

  Using type std::string to accept a sentence, for practise (I haven't worked with strings in C++ much) I'm checking if a character is a vowel or not. I got this: for(i = 0; i <= analyse.length(); i++) { if(analyse[i] == 'a' || analyse[i] == 'e' [..etc..]) { ...vowels++; } else { ... ...consenents++; } This works fine if the string is all one word, but the second I add a space (IE: aeio aatest) it will only count the first block and count the space as a consenent, and quit reading the sentence (exiting the for loop or something). Does a space count as no character == null? Or some oddity with std::string?, It would be helpful to know why that is happening!

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• Why do values in the row I insert not match the values in the insert query?

  - by user202411

  I just can't understand why is my database (mysql) behaving like this! My console shows that the record is created properly (please, notice the "remote_id" value): Tweet Create (0.3ms) INSERT INTO `tweets` (`remote_id`, `text`, `user_id`, `twitter_account_id`) VALUES (12325438258, 'jamaica', 1, 1) But when I check the record, it shows that the remote_id is 2147483647 intead of the provided value (12325438258 in the example above)... This table has many entries, but this field is always written with 2147483647... It was supposed to fill this space with an unique id (which I guarantee you is being generated properly).

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• F#: Can't hide a type abbreviation in a signature? Why not?

  - by Nels Beckman

  In F#, I'd like to have what I see as a fairly standard Abstract Datatype: // in ADT.fsi module ADT type my_Type // in ADT.fs module ADT type my_Type = int In other words, code inside the module knows that my_Type is an int, but code outside does not. However, F# seems to have a restriction where type abbreviations specifically cannot be hidden by a signature. This code gives a compiler error, and the restriction is described here. If my_Type were instead a discriminated union, then there is no compiler error. My question is, why the restriction? I seem to remember being able to do this in SML and Ocaml, and furthermore, isn't this a pretty standard thing to do when creating an abstract datatype? Thanks

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