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  • functional dependencies vs type families

    - by mhwombat
    I'm developing a framework for running experiments with artificial life, and I'm trying to use type families instead of functional dependencies. Type families seems to be the preferred approach among Haskellers, but I've run into a situation where functional dependencies seem like a better fit. Am I missing a trick? Here's the design using type families. (This code compiles OK.) {-# LANGUAGE TypeFamilies, FlexibleContexts #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u => a -> StateT u IO a -- plus other functions class Universe u where type MyAgent u :: * withAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } defaultWithAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () defaultWithAgent = undefined -- stub -- plus default implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- .. and they'll also need to make SimpleUniverse an instance of Universe -- for their agent type. -- instance Universe SimpleUniverse where type MyAgent SimpleUniverse = Bug withAgent = defaultWithAgent -- boilerplate -- plus similar boilerplate for other functions Is there a way to avoid forcing my users to write those last two lines of boilerplate? Compare with the version using fundeps, below, which seems to make things simpler for my users. (The use of UndecideableInstances may be a red flag.) (This code also compiles OK.) {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u a => a -> StateT u IO a -- plus other functions class Universe u a | u -> a where withAgent :: Agent a => (a -> StateT u IO a) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } instance Universe SimpleUniverse a where withAgent = undefined -- stub -- plus implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- And now my users only have to write stuff like... -- u :: SimpleUniverse u = SimpleUniverse "mydir"

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  • Export value from Excel worksheet to Access record

    - by glinch
    Hi there, I have an excel spreadsheet that contains the primarky-key/id value for a record in a table in an access database. I would like to export specific data from certain cells in the spreadsheet to certain fields in the corresponding record in the table. Is this possible, any help would be greatly appreciated. Many thanks Noel

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  • How do I permanently delete /var/log/lastlog?

    - by GregB
    My /var/log/lastlog file is huge. I know it's really only a few kilobytes, but tar isn't smart enough to know that, so when I image a virtual machine, my restore fails because it thinks I'm trying to load more data than I have capacity on my disk. I want to delete /var/log/lastlog and stop any and all logging to the file. I'm aware of the security implications. This logging needs to stop to preserve my backup strategy. I've made a change to /etc/pam.d/login which I was told would disable logging to /var/log/lastlog, but it does not appear to work as /var/log/lastlog keeps growing. # Prints the last login info upon succesful login # (Replaces the `LASTLOG_ENAB' option from login.defs) #session optional pam_lastlog.so Any ideas? EDIT For anyone interested, I use Centrify Express to authenticate my users via LDAP. Centrify Express is "free", but one of the drawbacks is that I can't manage user UIDs via LDAP, so they are given a dynamic UID when they login to a server. Centrify picks some crazy high UID values (so they don't conflict with local users on the server, presumably). /var/log/lastlog is indexed by UID, and grows to accommodate the largest UID on the system. This means that when a Centrify user logs in, they get a UID in the upper-end of the UID range, which causes lastlog to allocate an obscene amount of space, according to the file system. ~$ ll /var/log/lastlog -rw-rw-r-- 1 root root 291487675780 Apr 10 16:37 /var/log/lastlog ~$ du -h /var/log/lastlog 20K /var/log/lastlog More Into --- Sparse Files

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  • how to limit the foreignkey dropdown with constraints?

    - by FurtiveFelon
    Hi all, I have a database which keeps track of interaction between two different teams (represented in the admin interface by two different groups). For some fields, i have a foreignkey to Users database, and i would like to limit the dropdown people to only the specific groups. If anyone have any suggestions, it would be much appreciated! Jason

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  • Working with Database using LINQ to XML

    - by M.H
    Hi,I am Working on a project (C#) in the university and they said that we can't use a DBMS like SQL Server so we decide to use Linq and XML...we learned some basics in Linq to Xml But really we don't know how we can create tables and fields and work with them in Xml.any suggestions ?

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  • database synchronisation

    - by chetan
    hello friends, I have two databases at two different places, both databases are same with table name as well as fields. now I want to synchronise both database. Is there any java code or we can achieve that directly from mysql or sql ? How ?

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  • Control serialization of GWT

    - by Phuong Nguyen de ManCity fan
    I want GWT to not serialize some fields of my object (which implements Serializable interface). Normally, transient keyword would be enough. However, I also need to put the object on memcache. The use of transient keyword would make the field not being stored on memcache also. Is there any GWT-specific technique to tell the serializer to not serialize a field?

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  • Inserting checkbox values

    - by rabeea
    hey i have registration form that has checkboxes along with other fields. i cant insert the selected checkbox values into the data base. i have made one field in the database for storing all checked values. this is the code for checkbox part in the form: Websites, IT and Software Writing and Content <pre><input type="checkbox" name="expertise[]" value="Design and Media"> Design and Media <input type="checkbox" name="expertise[]" value="Data entry and Admin"> Data entry and Admin </pre> <pre><input type="checkbox" name="expertise[]" value="Engineering and Skills"> Engineering and Science <input type="checkbox" name="expertise[]" value="Seles and Marketing"> Sales and Marketing </pre> <pre><input type="checkbox" name="expertise[]" value="Business and Accounting"> Business and Accounting <input type="checkbox" name="expertise[]" value="Others"> Others </pre> and this is the corresponding php code for inserting data $checkusername=mysql_query("SELECT * FROM freelancer WHERE fusername='{$_POST['username']}'"); if (mysql_num_rows($checkusername)==1) { echo "username already exist"; } else { $query = "insert into freelancer(ffname,flname,fgender,femail,fusername,fpwd,fphone,fadd,facc,facc_name,fbank_details,fcity,fcountry,fexpertise,fprofile,fskills,fhourly_rate,fresume) values ('".$_POST['first_name']."','".$_POST['last_name']."','".$_POST['gender']."','".$_POST['email']."','".$_POST['username']."','".$_POST['password']."','".$_POST['phone']."','".$_POST['address']."','".$_POST['acc_num']."','".$_POST['acc_name']."','".$_POST['bank']."','".$_POST['city']."','".$_POST['country']."','".implode(',',$_POST['expertise'])."','".$_POST['profile']."','".$_POST['skills']."','".$_POST['rate']."','".$_POST['resume']."')"; $result = ($query) or die (mysql_error()); this code inserts data for all fields but the checkbox value field remains empty???

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  • Finding a handle that persists when relaunching parent

    - by cidtek
    I need to interact with 3'rd party applications by knowing the handle of certain text fields, but the hWnd always changes when that window is recreated. Is there any method that would allow me to identify a window and repeatedly be able to access that window (even after it is closed and then reopened) without having the user manually identify the window? I need the user to be able to identify a window with the usual click, drag and release method but only have to do it once and not every time the target window is recreated.

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  • Best Method For Storing Values

    - by Josh streit
    Basically, I want to have a database that's lightweight and I won't need to install a million other things on my clients computers for them to access this. I just need a simple method of reading and writing values so that they're not hardcoded into the program. I could do MySQL (which is what I'm very familiar with), but it doesn't need to be making calls remotely. I would have less than 10 fields and one table, if that matters. Thanks!

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  • Making "helper" text in a form be a different color then typed text.

    - by aslum
    <input name="phone" type="text" id="phone" value="Phone #" onfocus="value=''"> I've got two problems here. The main one is I would like the helper text (in this case "Phone Number") to be a different color then the inputted text from the user, to make it easier for the user to differentiate between filled and unfilled fields. The second is that with this methodology (onfocus="value''") if you mistype something in a field and come back to it you have to retype the whole thing which isn't really acceptable.

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  • i want to show a panel just like facebook or for captcha validation

    - by Lokesh
    i have a simple form and when user enters all the information and hits the submit botton than a panel should open just a width of 200px and height of 100px inside the same window. which should have two fields one is captcha image and a text box and a check botton and if captcha code is right than panel should automatically close and redirect to another page just like facebook . all the details of the panel is saved on another php file.

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  • Django Piston - how can I create custom methods?

    - by orokusaki
    I put my questions in the code comments for clarity: from piston.handler import AnonymousBaseHandler class AnonymousAPITest(AnonymousBaseHandler): fields = ('update_subscription',) def update_subscription(self, request, months): # Do some stuff here to update a subscription based on the # number of months provided. # How the heck can I call this method? return {'msg': 'Your subscription has been updated!'} def read(self, request): return { 'msg': 'Why would I need a read() method on a fully custom API?' }

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  • time interval in c#

    - by NIks
    I am having start time and end time as two fields in my database of type Datetime, Now i want that start time and end time selected by user should not collapse in the interval of start time and end time that are already present in database. How to do this validation.

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  • Date difference in minutes

    - by zurna
    I have DateFirstStarted and DateEnded fields in the database. Date values are recorded as DateFirstStarted 04/13/2010 07:00:00.000 PM DateEnded 04/13/2010 09:00:00.000 PM How do I print minute difference between two dates. I tried the following code but it returned something like 999343 Clock = DateDiff("m", objLiveCommentary("DateFirstStarted"), objLiveCommentary("DateEnded"))

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  • Linking two models in a multi-model form

    - by Elliot
    Hey Guys, I have a nested multimodel form right now, using Users and Profiles. Users has_one profile, and Profile belongs_to Users. When the form is submitted, a new user is created, and a new profile is created, but they are not linked (this is the first obvious issue). The user's model has a profile_id row, and the profile's model has a user_id row. Here is the code for the form: <%= form_for(@user, :url => teams_path) do |f| %> <p><%= f.label :email %><br /> <%= f.text_field :email %></p> <p><%= f.label :password %><br /> <%= f.password_field :password %></p> <p><%= f.label :password_confirmation %><br /> <%= f.password_field :password_confirmation %></p> <%= f.hidden_field :role_id, :value => @role.id %></p> <%= f.hidden_field :company_id, :value => current_user.company_id %></p> <%= fields_for @user.profile do |profile_fields| %> <div class="field"> <%= profile_fields.label :first_name %><br /> <%= profile_fields.text_field :first_name %> </div> <div class="field"> <%= profile_fields.label :last_name %><br /> <%= profile_fields.text_field :last_name %> </div> <% end %> <p><%= f.submit "Sign up" %></p> <% end %> A second issue, is even though the username, and password are successfully created through the form for the user model, the hidden fields (role_id & company_id - which are also links to other models) are not created (even though they are part of the model) - the values are successfully shown in the HTML for those fields however. Any help would be great!

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