I want to write a Web application for which I think Prolog would be interesting to apply rules and solve constraints. Let's say that this is an application about recommending movies to a user based on the user's preferences + a set of global relations/facts about movies. For instance a global movie_genre(terminator, action). and for one user likes_genres(alice, [action, comedy]). and for another user likes_genres(bob, [thriller, drama])., so maybe suggest_movie(alice, Movie). would return Movie = terminator.
I was thinking of having a main back-end in another language using a framework (like PHP or Node.js) and calling the Prolog back-end when needed. However, I'm not sure how to proceed. How would you:
store data: usually, I would store data in a relational database. But from what I understand, I could also store facts with asserta/z and remove them with retract? But then, is it scalable to load all facts for all users? Or would I load facts for a given user?
call the Prolog back-end: Run a Prolog HTTP server? Start the interpreter when needed?
do request/get responses, in particular when there are several responses: using a higher-level protocol, or instrumenting the Prolog interpreter?
Any help or advice is appreciated, thanks!
Additional notes:
I know there was a similar question, but it was asked literally a decade ago. Times have changed, and perhaps the answers could be different today.
I know there are libraries implementing Prolog, and I prefer using a full (ISO compliant) Prolog interpreter.
Performance is not a primary concern at this point, development speed is!
Relatively new to ML and h2o. Is there a way to do collaborative learning/training with h2o? Would prefer a way that uses the flow UI, else woud be using python.
My use case is that there would be new feature samples x=[a, b, c, d] periodically coming into a system where an h2o algorithm (say, running from a java program using a MOJO) assigns a binary class that users should be able to manually reclassify as either good(0) or bad(1), at which point these samples (with their newly assigned responses) get sent back to theh h2o algorithm to be used to further train it.
Thanks
FLOW UI is great for prototyping something very quick with H2O without writing a single like of code. You can ingest the data, build desired model and the evaluate the results. Unfortunately FLOW UI is can not be extended for the reason you asked, and FLOW is limited for that reason.
For collaborative learning you can write your whole application directly in python or R and it will work as expected.
Are there some tutorials or practical lessons on how to write an extension for Ruby in Go?
Go 1.5 added support for building shared libraries that are callable from C (and thus from Ruby via FFI). This makes the process easier than in pre-1.5 releases (when it was necessary to write the C glue layer), and the Go runtime is now usable, making this actually useful in real life (goroutines and memory allocations were not possible before, as they require the Go runtime, which was not useable if Go was not the main entry point).
goFuncs.go:
package main
import "C"
//export GoAdd
func GoAdd(a, b C.int) C.int {
return a + b
}
func main() {} // Required but ignored
Note that the //export GoAdd comment is required for each exported function; the symbol after export is how the function will be exported.
goFromRuby.rb:
require 'ffi'
module GoFuncs
extend FFI::Library
ffi_lib './goFuncs.so'
attach_function :GoAdd, [:int, :int], :int
end
puts GoFuncs.GoAdd(41, 1)
The library is built with:
go build -buildmode=c-shared -o goFuncs.so goFuncs.go
Running the Ruby script produces:
42
Normally I'd try to give you a straight answer but the comments so far show there might not be one. So, hopefully this answer with a generic solution and some other possibilities will be acceptable.
One generic solution: compile high level language program into library callable from C. Wrap that for Ruby. One has to be extremely careful about integration at this point. This trick was a nice kludge to integrate many languages in the past, usually for legacy reasons. Thing is, I'm not a Go developer and I don't know that you can compile Go into something callable from C. Moving on.
Create two standalone programs: Ruby and Go program. In the programs, use a very efficient way of passing data back and forth. The extension will simply establish a connection to the Go program, send the data, wait for the result, and pass the result back into Ruby. The communication channel might be OS IPC, sockets, etc. Whatever each supports. The data format can be extremely simple if there's no security issues and you're using predefined message formats. That further boosts speed. Some of my older programs used XDR for binary format. These days, people seem to use things like JSON, Protocol Buffers and ZeroMQ style wire protocols.
Variation of second suggestion: use ZeroMQ! Or something similar. ZeroMQ is fast, robust and has bindings for both languages. It manages the whole above paragraph for you. Drawbacks are that it's less flexible wrt performance tuning and has extra stuff you don't need.
The tricky part of using two processes and passing data between them is a speed penalty. The overhead might not justify leaving Ruby. However, Go has great native performance and concurrency features that might justify coding part of an application in it versus a scripting language like Ruby. (Probably one of your justifications for your question.) So, try each of these strategies. If you get a working program that's also faster, use it. Otherwise, stick with Ruby.
Maybe less appealing option: use something other than Go that has similar advantages, allows call from C, and can be integrated. Althought it's not very popular, Ada is a possibility. It's long been strong in native code, (restricted) concurrency, reliability, low-level support, cross-language development and IDE (GNAT). Also, Julia is a new language for high performance technical and parallel programming that can be compiled into a library callable from C. It has a JIT too. Maybe changing problem statement from Ruby+Go to Ruby+(more suitable language) will solve the problem?
As of Go 1.5, there's a new build mode that tells the Go compiler to output a shared library and a C header file:
-buildmode c-shared
(This is explained in more detail in this helpful tutorial: http://blog.ralch.com/tutorial/golang-sharing-libraries/)
With the new build mode, you no longer have to write a C glue layer yourself (as previously suggested in earlier responses). Once you have the shared-library and the header file, you can proceed to use FFI to call the Go-created shared library (example here: https://www.amberbit.com/blog/2014/6/12/calling-c-cpp-from-ruby/)
We are learning higher-order functions in our class and our professor mentioned that they can be useful for web programming. I'm unsure as to what cases in which that would be true and was wondering if anyone had any experiences with higher-order functions in some common web programming tasks, and in what situations they would be useful.
I read through our book, but web programming wasn't mentioned, I suppose it was just something the professor mentioned off topic, but it sparked my interest a bit.
Thanks!
I'm not exactly sure which cases your professor might have been talking about, but higher order functions are used heavily in JavaScript. A simple example of a higher order function (a function taking a function as a parameter and/or returning a function as a result) would be the jQuery ready() function:
$(document).ready(function(){
alert('The DOM is ready for manipulation!');
});
A simple example, yes...but it demonstrates how useful higher order functions can be.
Play, (Scala) Lift (Scala) , SeaSide (Smalltalk) and Webmachine (Erlang)
are three web frameworks the rely heavily on functional ideoms such as higher-order-functions.
Clojure makes extensive use of higher order functions for web development in the excellent Ring framework. See this excellent video which explains some of the concepts.
The basic idea is:
A web application is really just a function that maps a request to a response
You can therefore compose the entire web application using higher order functions
I heard some people program in multiple languages in one project. I can't imagine how the languages interact with each other.
I mean there is no Java method like
myProgram.callCfunction(parameters);
never happens or am I wrong?
Having multiple languages in one project is actually quite common, however the principles behind are not always simple.
In the simple case, different languages are compiled to the same code. For example, C and C++ code typically is compiled into machine assembler or C# and VB.Net is compiled into IL (the language understood by the .NET runtime).
It gets more difficult if the languages/compilers use a differnt type system. There can be many different ways, basic data types such as integer, float and doubles are represented internally, and there is even more ways to represent strings. When passing types around between the different languages it must be sure that both sides interpret the type the same or - if not - the types are correctly mapped. This sort of type mapping is also known as marshalling.
Classic examples of interoperability between different program languages are (mostly from the Windows world):
The various languages available for the .NET platfrom. This includes C#, VB.Net, J#, IronRuby, F#, XSLT and many other less popular languages.
Native COM components written in C++ or VB can be used with a huge variety of languages: VBScript, VB, all .NET languages, Java
Win32 api functions can be called from .NET or VB
IPC (inter process communication)
Corba, probably the most comprehensive (and most complex) approach
Web services and other service-oriented architectures, probably the most modern approach
Generally, any decently sized web project will use about five languages: HTML, CSS, Javascript, some kind of server-side “getting things done” language (ASP, JSP, CGI scripts with Perl, PHP, etc.), and some variant of SQL for database connectivity.
(This is, of course, hand-waving away the argument about whether or not HTML and CSS count as programming languages – I’m the “they are, but just not Turing-complete languages” camp, but that’s a whole other thread.)
Some examples of how all those work together:
If you’re going the best-practices route, the structure of a web page is in HTML, and the instructions for how to display it are in CSS – which could be in the same file, but don’t have to be. The CSS contains a bunch of classes, which the HTML refers to, and it’s up to the browser to figure out how to click them together.
Taking all that a step further, any javascript scripts on that page can alter any of the HTML/CSS that is present (change contents of HTML entities, swap out one CSS class for another, change the behavior of the CSS, and so on.) It does this via something called the Document Object Model, which is essentially a language and platform-independent API to manipulate HTML pages in an object-like manner (at which point I’ll back away slowly and just provide a link to the relevant wiki article.)
But then, where does all the HTML / CSS / Javascript come from? That’s what the server-side language does. In the simplest form, the serer-side language is a program that returns a giant string holding an HTML page as its output. This, obviously, can get much more complex: HTML forms and query string parameters can be used as input for our server side program, and then you have the whole AJAX thing where the javascript gets to send data directly to the server language as well. You can also get fancy where the server language can customize the HTML, CSS, and Javascript that gets spit out – essentially, you have a program in one language writing a program in another language.
The Server-side language to SQL connection works much the same. There are a lot of ways to make it both more complex and safer, but the simplest way is for your server language to dynamically build a string with a SQL command in it, hand that to the database via some kind of connector, and get back a result set. (This is a case where you really do have a function that boils down to someValue = database.executeThisSQLCommand( SQLString ). )
So to wrap this up, different languages in this case either communicate by actually writing programs in each other, or by handing data around in very simple easy to parse formats that everybody can understand. (Strings, mainly.)
Multiple languages in use is called "interoperability" or "interop" for short.
Your example is wrong. Java can call C functions.
The language provides a mechanism for interoperability.
In the case of .NET, languages are compiled into IL as part of the CLI. Thus any .NET language can interop (call methods defined by) modules defined in any other .NET language.
As an example:
I can define a method in C#
static void Hello(){ Console.WriteLine("Hello World");}
And I can call it from Python (IronPython)
Hello()
And get the expected output.
Generally speaking, some languages interop better than others, especially if the language authors specifically made interop a feature of the language.
Multiple languages can interact with:
Piped input/output (ANY language can do this
because input and output must by necessity be implemented in every non-toy
language)
Having code in one language compile to a native library
while the other supports calling native code.
Communicating over a loopback network connection. You can
run into difficulties with firewall interference this way.
Databases. These can be thought of as a "universal" data
storage format, and hence can be accessed by most languages
with database extensions. This generally
requires one program to finish operation before the next program
can access the database. In addition, all 'communications' are
generally written to disk.
If the languages involved run on the same runtime (i.e. .NET,
JVM), then you generally can pass object data from one language
directly to the other with little impedence.
In almost every case, you have to convert any communication to a common
format before it can be exchanged (the exception is languages on the
same runtime). This is why multiple languages are rarely used in one
project.
I work on a large enterprise project which is comprised of (at the last count) about 8 languages. The majority of the communication is via an enterprise level message bus which contains bindings for multiple languages to tap into and pass data back and forth. It's called tibco.
There are many different ways you can use different languages in one project
There are two main categories that come to mind:
Using Different languages together to build one application. For example using Java to build the GUI and using JNI to access C API (so answering your question you can call C functions from Java ;))
Using different languages together in the one project if they are not part of the same application. For example. I am currently working on an iPhone app that has uses a large amount of text. I am currently using three languages: Python (to work with the original sources of the text), SQL (to put the results of the python app in a format easily accessible from the iPhone sqlite3 API) and Objective C to build the actual app. Even though the final product will only be Objective C, I've used two other languages to get to the final product.
You could have an app where the bulk of the work is done in Java, but there might be some portion of it, like maybe a data parser or something is written in Python or what have you. Almost two separate apps really, perhaps the parser is just doing some work on files and then your main app in Java is using them for something. If someone were to ask me what I used in this project I'd say "Java and Python."
There are various ways that multiple languages can be used in one project. Some examples:
You can write a DLL in, say, C, and then use that library from, say, a VB program.
You could write a server program in, say C++, and have lots of different language implementations of the client.
A web project often uses lots of languages; for example a server program, written in, say, Java (a programming language), which fetches data from a database using SQL (a query language), sends the result to the browser in HTML (a markup language), which the user can interact with using Javascript (a scripting language)...
Badly. If there is no urgent need, stick to a single language. You're increasing dependencies and complexity. But when you have existing code providing interesting functionality, it can be easier to glue it together than to recreate it.
It depends on the type of project. If you want to experiment, you can set up a web project, in .NET, and change the language on a page by page basis. It does not work as you show in your pseudocode, but it is multiple languages. Of course, the actual code directory must be a single language.
There are a couple of ways in which code in languages can interact directly. As long as the data being passed between the code is in the right format, at the bits and bytes level, then there is no reason why different languages can't interop. This approach is used in traditional windows DLL development. Even on different platforms, if you can get the format correct (look at big/little endian if interested) it will work as long as your linker (not compiler) knows how to join code together.
Beyond that there are numerous other ways in which languages can talk to each other. In the .Net world code is compiled down to IL code, which is the same for every language, in this way C#, VB.Net are all the same under the hood and can call/work with each other seamlessly.
Just to add to the list of examples, it's fairly common to optimize Python code in C or C++ or write a C library to bind another library to Python.