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I am an autodidact, teaching myself how to program and script in different languages (novice in: Java, C++, Javascript/Node.js, HTML/CSS), design projects and schematics, adding electronics and peripherals.

What I have seen a lot of while researching is the use of multiple languages to achieve a set of goals (such as building a Web server in Javascript/Node to handle HTTP requests and responses, responding with a Web page written in HTML and customized/stylized with CSS and embedded with Javascript mannerisms; or instead of Node, you write it in PHP or Python).

I'm having a hard time wrapping my mind around WHY multiple languages are used instead of just one (some high-level languages are capable of performing a large portion, if not all, of the required tasks) and HOW information is passed in between the different languages. Could one program call another (I know that an HTML file can make "calls" to CSS and Javascript files, so, I understand that instance)?

I think the reason why I am hung up on this is because of my inexperience and lack of knowledge of other common languages. Does that mean that certain languages are meant to handle only specific tasks in a specific manner?

I feel like some languages, such as Java and C++, for example, can be used in various ways and in various instances to handle a myriad of different tasks. Is that not true of some of the others (PHP & Python, for instance)?

I'm digging into the wealth of knowledge and the collective experience of some of the most brilliant minds this world has to offer but remember that I am new to this and I don't have the advantage of doing this in a classroom but I have read and own many books on programming in specific languages and the like. Please answer in a way that I and the others that may follow can understand.

Thank you for your time and I look forward to the responses.

Cheers.

Fantastic answers!

I'm curious though; when working towards a solution for a specific problem, when does the programmer know when to stop in one language and continue a segment in another language?

That's where I am confused. Is it typically up to the software developer and his/her own particular and artistic preference on how something is done or are certain things just not possible without using multiple languages?

I do understand scripting and when it's beneficial to use rather than a program or application and I know runtime execution/compiled code, environments and frameworks and virtual machines but none of that clearly lays out a defined perimeter or a limit in functionality/ability for any particular language. Why call a C++ function in Python? Could Python not accomplish what was needed in the first place and could choosing a more appropriate language have mitigated the need for adding another level of complexity to the solution? I may be overthinking it but knowing this will guide me in my learning and help me map out better solutions as a programmer.

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    Ultimately there are instructions that work on data. If language A has some data structure Foo, and language B knows that Foo is 8 bytes of ASCII, then it can translate that into a data structure in language B and operate on it itself. The same goes for any arbitrary piece of data. You need to know the mappings. I'm specifically thinking of using C and C# together on the same data. As for the web languages working in concert, that's a discussion for another commenter. Commented Jan 6, 2015 at 4:39
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    In regards to one language doing everything, the answer is as much historical as is practical. HTML was first. We can teach a piece of software (browser) to interpret a text file (HTML) as a configurable way to display data (web page). Then Javascript (whoa, now we can alter that text file on the fly!) Then CSS (now we can separate the appearance of the elements from their functionality and enforce uniformity across pages instead of mixing the display in with the logic) etc.. Things built on each other. Certainly some frameworks have come together to try to be an all-in-one package Commented Jan 6, 2015 at 4:51
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    but the web is a historical beast. To answer the question about programs talking to each other, yeah they can! The most simple way could be thought of as signalling. Most OS's have a Control Panel of sorts that allows you to kill a process. Isn't this one program talking to another? They use signals, and these are pretty simple. Sockets are another way to accomplish this, as are pipes, semaphores, and sometimes via shared memory (a really simple example would be XML! One writes and another reads) Commented Jan 6, 2015 at 4:55

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Basically the different technologies (browsers, operating systems etc) and with it programming languages evolved over time so that there are many different languages used in practice. For the same reason that there are multiple real languages. You could design a web browser that supports Python instead of JavaScript for front-end programming, but this would involve designing the APIs the scripts use to access the pages (DOM HTML model), it would need to be supported by all major web browsers, standardized, and web developers would need to use it.

Yes in many cases it is possible for programs written in one language to call programs written in another languages. There needs to be some kind of interface connecting the two parts, depending on the context. For example:

  • C and C++ are both compiled languages. That is, they get translated into machine code to be executed by the processor. The positions in the machine code where code for one function is located are stored with it. The linker of the operating system is responsible for linking two modules (.c files) such that function calls made in one module towards a function defined in the other result in the right machine code being loaded. For a C++ program to call a C program, one problem (many others) is that functions are named differently (name mangling). In practice the functions from the C program would need to be declared extern "C" in the C++ source code for the linker to set this up correctly.

  • JavaScript, CSS and HTML are interpreted and executed (for JavaScript) by the browser, but not necessarily translated into machine code. (JavaScript engined may use Just-in-time compilation). So the browser provides possibilities for the JavaScript code to access the CSS definition, for example. .style.color = ....

  • For scripting languages like Perl, PHP, Python etc to call each other, different libraries exist that handle the necessary intermediate steps ("glue code"). There are many possibilities, for instance the PHP code could invoke the Python interpreter to execute a Python program, or it could pass data to a running Python program through the operating system's mechanisms etc.

  • Wrappers such as SWIG allow C/C++ code to be called from scripting languages. They add the necessary symbols (functions) to the code that Python would call internally. Than the C++ program is compiled as a Python extension, which is loaded by the Python interpreter, itself a compiled program, and the operating system's linker is used. The Python interpreter then interprets Python code in such a way that calling a given Python function results in the machine code of the extension's wrapper function to be executed.

There are many ways to classify programming languages into categories. For example from low level (machine code) to higher level (more abstraction, translation to machine code handled automatically):

  • Assembly (for expressing machine code instructions)
  • Compiled languages for system-level programming. (C, C++, Pascal, ...)
  • Compiled languages running in a VM (Java, C#, ...)
  • Scripting languages (Python, Perl, PHP, ...) Less focused on efficiency, but more flexible.
  • Higher domain-speficic level languages (MATLAB, AppleScript)
  • Shell scripting (bash, sh)
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Programming is all about creating solutions to problems. People think differently. People see the world from different perspectives. People like to tweak solutions and play with tools. Languages are created by people in order to solve different problems and in some cases just for play. My response is more along the lines of 'Why would there only be one language?'.

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