The majority of languages I have come across utilise a VM, or virtual machine. Languages such as Java (the JVM), Python, Ruby, PHP (the HHVM), etc.
Then there are languages such as C, C++, Haskell, etc. which compile directly to native.
My question is, what is the advantage of using a VM (outside of OS-independence)? Isn't using a VM just creating an extra interpretation step, by going [source code -> bytecode -> native] instead of just [source code -> native]?
Why use a VM when you can compile directly?
EDIT
My understanding is that Python, Ruby, et al. use something akin to a VM, if not exactly fitting under such a definition, where scripts are compiled to an intermediate representation (for Python, e.g. .pyc files).
EDIT 2
Yep. Looked it up. Python, Ruby and PHP all use intermediate representations, but are simply not stored in seperate files but executed by the VM directly. See question : Java "Virtual Machine" vs. Python "Interpreter" parlance?
" Even though Python uses a virtual machine under the covers, from a
user's perspective, one can ignore this detail most of the time. "
An advantage of VM is that, it is much easier to modify some parts of the code on runtime, which is called Reflection. It brings some elegance capabilities. For example, you can ask the user which function/class he want to call, and call the function/class by its STRING name. In Java programs (and maybe some other VM-based languages) users can add additional library to the program in runtime, and the library can be run immediately!
Another advantage is the ability to use advanced garbage collection, because the bytecode's structure is easier to analyze.
Let me note that a virtual machine does not always interpret the code, and therefore it is not always slower than machine code. For example, Java has a component named hotspot which searches for code blocks that are frequently called, and replaces their bytecode with native code (machine code). For instance, if a for loop is called for, say , 100+ times, hotspot converts it to machine-code, so that in the next calls it will run natively! This insures that just the bottlenecks of your code are running natively, while the rest part allows for the above advantages.
P.S. It is not impossible to compile the code directly to native code. Many VM-based languages have compiler versions (e.g. there is a compiler for PHP: http://www.phpcompiler.org). However, remember that you are disabling some of the above features by compiling the whole program to native code.
P.S. The [source-code -> byte-code] part is not a problem, it is compiled once and does not relate to execution time. I presumed you are asking why they do not execute the machine code while it is possible.
Python, Ruby, and PhP do not utilize VMs. They are, however, interpreted.
To answer your actual question: Java utilizes a VM in order to add some distance between the operating system/hardware and the code being executed. The goal there was security and hardiness (hardiness meaning there was a lower likelihood of code having an averse effect on other processes in the system.)
All the languages you listed are interpreted so I think what you may have actually meant to ask was the difference between interpreted and compiled languages. Interpreted languages are cross-platform. That is the biggest, and main, advantage. You need not compile them for each different set of hardware or operating system they operate on, and instead they will simply work everywhere.
The advantage of a compiled language, traditionally, is speed and efficiency.
Because a VM allows for the same set of instructions to be run on my different operating systems (provided they have the interperetor)
Let's take Java as an example. Java gets compiled into bytecode, which is basically a set of operations for a computer to follow. However, not all processors in computers understand the same set of instructions the same way - meaning, what one set of native instruction means on computer A could be something different on computer B.
As a result, a VM is run, with one specific to each computer. This way, the Java bytecode that is written is standardized, and only the interpreter has to work to convert it to machine language.
OS independence is a big part of it but you also get abstractions from other things like CPUs... the same Java code can execute on ARM, x86, whatever without modification so long as there is a JVM in place.
I'm looking to play around with the Rubinius VM to create a langauage, but just reading the documentation, I'm still quite lost on how to get started. Even looking at the projects, I still can't seem to figure out where the parsing and using the vm comes into place. Does anyone have any resources for this?
Hey I'm a contributor to the Fancy language that runs on rubinius. If you're interested in parsing take a look at boot/rbx-compiler there you'll find a Parser (implemented with KPEG) that basically constructs a tree of AST nodes, each of those nodes has a bytecode method that produces the rubinius vm instructions for everything to work. Fancy share a lot of semantics with ruby, so I guess starting with it would be easy if you're already familiar with ruby. You'll just need to checkout the examples/ dir to geet a feeling on the language and then the kpeg parser, ast nodes, loader, as you progress exploring the compiler. These days Fancy is bootstrapped (that means that the compiler has been written in fancy itself - at lib/compiler) but rbx-compiler is the first step in that process.
Hope exploring Fancy's source code can be of help to you.
In case you hadn't seen it, check out Evan's keynote from 2011 LA Ruby Conf. He shows how to build a simple language, which might be helpful.
There are several packages out there that help in automating the task of writing bindings between C\C++ and other languages.
In my case, I'd like to bind Python, some options for such packages are: SWIG, Boost.Python and Robin.
It seems that the straight forward process is to use these packages to create C\C++ linkable libraries (with mostly static functions) and have the higher language be extended using them.
However, my situation is that I already have a developed working system in C++ therefore plan to embed Python into it so that future development will be in Python.
It's not clear to me how, and if at all possible, to use these packages in helping to extend embedded Python in such a way that the Python code would be able to interact with the various Singleton instances already running in the system, and instantiate C++ classes and interact with them.
What I'm looking for is an insight regarding the design best fitted for this situation.
Boost.python lets you do a lot of those things right out of the box, especially if you use smart pointers. You can even inherit from C++ classes in Python, then pass instances of those back to your C++ code and have everything still work. My favorite resource on how to do various stuff is this (especially check out the "How To" section): http://wiki.python.org/moin/boost.python/ .
Boost.python is especially good if you're using smart pointers or intrusive pointers, as those translate transparently into PyObject reference counting. Also, it's very good at making factory functions look like Python constructors, which makes for very clean Python APIs.
If you're not using smart pointers, it's still possible to do all the things you want, but you have to mess with various return and lifetime policies, which can give you a headache.
To make it short: There is the modern alternative pybind11.
Long version: I also had to embed python. The C++ Python interface is small so I decided to use the C Api. That turned out to be a nightmare. Exposing classes lets you write tons of complicated boilerplate code. Boost::Python greatly avoids this by using readable interface definitions. However I found that boost lacks a sophisticated documentation and dor some things you still have to call the Python api. Further their build system seems to give people troubles. I cant tell since i use packages provided by the system. Finally I tried the boost python fork pybind11 and have to say that it is really convenient and fixes some shortcomings of boost like the necessity of the use of the Python Api, ability to use lambdas, the lack of an easy comprehensible documentation and automatic exception translation. Further it is header only and does not pull the huge boost dependency on deployment, so I can definitively recommend it.
I'm interested in making a language to run on the AVM2 and I'm looking for advice on where to start. I do realize that this is by no means a trivial task, but I would like to give it a try and at the very least learn more about implementing a language along the way.
I have messed around with ANTLR and have been reading up on syntax issues for language development. What I'm looking for is advice on a path to take or useful references/books.
For instance I would like to generate (script/manually) some very simple AVM2 bytecode and get that to run on the VM as a start.
Thanks
If you are not interested in Haxe, you will basically need to write your own compiler that compiles objects down to ABC (Actionscript Byte Code). The AVM2 Overview document available from Adobe on ABC and the AVM2 which should help you get started. It's a fairly thorough document but stay alert for a few typo's in the bytecode instructions.
You will also need to wrap the bytecode in a doABC tag as part of a SWF container. You can get more information from the SWF File Format documentation.
If you'd like a headstart on writing the data structures (optimised int formats, etc), feel free to checkout the code at asmock, a dynamic mocking project I've been working on. The SWF/ByteCode generation stuff is a bit messy but there are IDataOutput wrappers (SWF, ByteCode) that might come in handy.
Project Alchemy by Adobe can be a good reference
http://labs.adobe.com/technologies/alchemy/
How did it go?
I'm also interested in doing a Java to AVM2 compiler...
Do you have any published code?
Take a look at Haxe: it is an open source language that can target different platforms, including the AVM. You can dig into the SWF compiler source code to get some inspiration.
I'm referring to distinctions such as in this answer:
...bash isn't for writing applications it's for, well, scripting. So sure, your application might have some housekeeping scripts but don't go writing critical-business-logic.sh because another language is probably better for stuff like that.
As programmer who's worked in many languages, this seems to be C, Java and other compiled language snobbery. I'm not looking for reenforcement of my opinion or hand-wavy answers. Rather, I genuinely want to know what technical differences are being referred to.
(And I use C in my day job, so I'm not just being defensive.)
Traditionally a program is compiled and a script is interpreted, but that is not really important anymore. You can generate a compiled version of most scripts if you really want to, and other 'compiled' languages like Java are in fact interpreted (at the byte code level.)
A more modern definition might be that a program is intended to be used by a customer (perhaps an internal one) and thus should include documentation and support, while a script is primarily intended for the use of the author.
The web is an interesting counter example. We all enjoy looking things up with the Google search engine. The bulk of the code that goes into creating the 'database' it references is used only by its authors and maintainers. Does that make it a script?
I would say that an application tends to be used interactively, where a script would run its course, suitable for batch work. I don't think it's a concrete distinction.
Usually, it is "script" versus "program".
I am with you that this distinction is mostly "compiled language snobbery", or to quote Larry Wall and take the other side of the fence, "a script is what the actors have, a programme is given to the audience".
This is an interesting topic, and I don't think there are very good guidelines for the differentiating a "script" and a "application."
Let's take a look at some Wikipedia articles to get a feel of the distinction.
Script (Wikipedia -> Scripting language):
A scripting language, script language or extension language, is a programming language that controls a software application. "Scripts" are often treated as distinct from "programs", which execute independently from any other application. At the same time they are distinct from the core code of the application, which is usually written in a different language, and by being accessible to the end user they enable the behavior of the application to be adapted to the user's needs.
Application (Wikipedia -> Application software -> Terminology)
In computer science, an application is a computer program designed to help people perform a certain type of work. An application thus differs from an operating system (which runs a computer), a utility (which performs maintenance or general-purpose chores), and a programming language (with which computer programs are created). Depending on the work for which it was designed, an application can manipulate text, numbers, graphics, or a combination of these elements.
Reading the above entries seems to suggest that the distinction is that a script is "hosted" by another piece of software, while an application is not. I suppose that can be argued, such as shell scripts controlling the behavior of the shell, and perl scripts controlling the behavior of the interpreter to perform desired operations. (I feel this may be a little bit of a stretch, so I may not completely agree with it.)
When it comes down to it, it is in my opinion that the colloquial distinction can be made in terms of the scale of the program. Scripts are generally smaller in scale when compared to applications.
Also, in terms of the purpose, a script generally performs tasks that needs taken care of, say for example, build scripts that produce multiple release versions for a certain piece of software. On the otherhand, applications are geared toward providing functionality that is more refined and geared toward an end user. For example, Notepad or Firefox.
John Ousterhout (the inventor of TCL) has a good article at http://www.tcl.tk/doc/scripting.html where he proposes a distinction between system programming languages (for implementing building blocks, emphasis on correctness, type safety) vs scripting languages (for combining building blocks, emphasis on responsiveness to changing environments and requirements, easy conversion in and out of textual representations). If you go with that categorisation system, then 99% of programmers are doing jobs that are more appropriate to scripting languages than to system programming languages.
A script tends to be a series of commands that starts, runs, and terminates. It often requires no/little human interaction. An application is a "program"... it often requires human interaction, it tends to be larger.
Script to me implies line-by-line interpretation of the code. You can open a script and view its programmer-readable contents. An application implies a stand-alone compiled executable.
It's often just a semantic argument, or even a way of denigrating certain programming languages. As far as I'm concerned, a "script" is a type of program, and the exact definition is somewhat vague and varies with context.
I might use the term "script" to mean a program that primarily executes linearly, rather than with lots of sequential logic or subroutines, much like a "script" in Hollywood is a linear sequence of instructions for an actor to execute. I might use it to mean a program that is written in a language embedded inside a larger program, for the purpose of driving that program. For example, automating tasks under the old Mac OS with AppleScript, or driving a program that exposes itself in some way with an embedded TCL interface.
But in all those cases, a script is a type of program.
The term "scripting language" has been used for dynamically interpreted (sometimes compiled) languages, usually these have a lot of common features such as very high level instructions, built in hashes and arbitrary-length lists and other high level data structures, etc. But those languages are capable of very large, complicated, modular, well-designed programs, so if you think of a "script" as something other than a program, that term might confuse you.
See also Is it a Perl program or a Perl script? in perlfaq1.
A script generally runs as part of a larger application inside a scripting engine
eg. JavaScript -> Browser
This is in contrast to both traditional static typed compiled languages and to dynamic languages, where the code is intended to form the main part of the application.
An application is a collection of scripts geared toward a common set of problems.
A script is a bit of code for performing one fairly specific task.
IMO, the difference has nothing whatsoever to do with the language that's used. It's possible to write a complex application with bash, and it's possible to write a simple script with C++.
Personally, I think the separation is a step back from the actual implementation.
In my estimation, an application is planned. It has multiple goals, it has multiple deliverables. There are tasks set aside at design time in advance of coding that the application must meet.
A script however, is just thrown together as suits, and little planning is involved.
Lack of proper planning does not however downgrade you to a script. Possibly, it makes your application a poorly organized collection of poorly planned scripts.
Further more, an application can contain scripts that aggregated comprise the whole. But a script can only reference an application.
Taking perl as an example, you can write perl scripts or perl applications.
A script would imply a single file or a single namespace. (e.g. updateFile.pl).
An application would be something made up of a collection of files or namespaces/classes (e.g. an OO-designed perl application with many .pm module files).
An application is big and will be used over and over by people and maybe sold to a customer.
A script starts out small, stays small if you're lucky, is rarely sold to a customer, and might either be run automatically or fall into disuse.
What about:
Script:
A script is text file (or collection of text files) of programming statements written in a language which allows individual statements written in it to be interpreted to machine executable code directly before each is executed and with the intention of this occurring.
Application:
An application is any computer program whose primary functionality involves providing service to a human Actor.
A script-based program written in a scripting language can therefore, theoretically, have its textual statements altered while the script is being executed (at great risk of , of course). The analogous situation for compiled programs is flipping bits in memory.
Any takers? :)
First of all, I would like to make it crystal clear that a script is a program. In other words, a script is a set of instructions.
Program:
A set of instructions which is going to be compiled is known as a Program.
Script:
A set of instructions which is going to be interpreted is known as a Script.
#Jeff's answer is good. My favorite explanation is
Many (most?) scripting languages are interpreted, and few compiled
languages are considered to be scripting languages, but the question
of compiled vs. interpreted is only loosely connected to the question
of "scripting" vs. "serious" languages.
A lot of the problem here is that "scripting" is a pretty vague
designation -- it means a language that's convenient for writing
scripts in, as opposed to writing "full-blown programs" (or
applications). But how does one distinguish a complex script from a
simple application? That's an essentially unanswerable question.
Generally, a script is a series of commands applied to some set of
data, possibly in a user-defined order... but then, one could stretch
that description to apply to Photoshop, which is clearly a major
application. Scripts are generally smaller than applications, do
some well-defined thing and are "simpler" to use, and typically can
be decomposed into a clear series of sub-operations, but all of these
things are subjective.
Referenced from here.
I think that there is no matter at all whether code is compiled or interpreted.
The true difference is in core logic of code:
If code makes new functionality that is not implemented in other programs in system - it's a program. It even can be manipulated by a script.
If code is MAINLY manipulates by actions of other programs and total result is MAINLY the results of work of manipulated programs - it's a script. Literally a script of actions for some programs.
Actually the difference between a script ( or a scripting language) and an application is that a script don't require it to be compiled into machine language.. You run the source of the script with an interpreter.. A application compiles the source into machine code so that you can run it as a stand alone application.
I would say a script is usually a set of commands or instructions written in plain text that are executed by a hosting application (browser, command interpreter or shell,...).
It does not mean it's not powerfull or not compiled in some way when it's actually executed. But a script cannot do anything by itself, it's just plain text.
By nature it can be a fragment only, needing to be combined to build a program or an application, but extended and fully developed scripts or set of scripts can be considered programs or applications when executed by the host, just like a bunch of source files can become an application once compiled.
A scripting language doesn't have a standard library or platform (or not much of one). It's small and light, designed to be embedded into a larger application. Bash and Javascript are great examples of scripting languages because they rely absolutely on other programs for their functionality.
Using this definition, a script is code designed to drive a larger application (suite). A Javascript might call on Firefox to open windows or manipulate the DOM. A Bash script executes existing programs or other scripts and connects them together with pipes.
You also ask why not scripting languages, so:
Are there even any unit-testing tools for scripting languages? That seems a very important tool for "real" applications that is completely missing. And there's rarely any real library bindings for scripting languages.
Most of the times, scripts could be replaced with a real, light language like Python or Ruby anyway.