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I know there are different ways to combine programming languages (Haskell's FFI, Boost with C++ and Python, etc...). I have an odd interest in combining programming languages; however, I have only found it "necessary" once (I didn't want to rewrite some older code). Also, I notice that this interest is shared (there are an abundance of questions about integrating languages on SO).

My question is, simply, are there any other benefits in combining programming languages? Is there value in mixing different programming paradigms (e.g. functional+OO, procedural+aspect-oriented)?

Any from-the-field examples would be much appreciated.

UPDATE

When I say "combine two languages" I am talking about using them in conjunction, in ways not necessarily originally intended. For example, suppose I use Boost to incorporate Python code in C++.

mjgpy3
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8 Answers8

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A typical example shows up in the Computer games, particularly AAA titles where a C++ backend is the norm. The interface section will often be designed in a scripting language such as Python or Lua. This allows for easy modification by both the developers so they can test out new interface designs without messing with the highly complex physics and graphics engines underneath and at the same time allows for easy modification by players who may not have the coding chops to handle a full game engine but can competently do a few interface tweaks.

Another widespread use case is the web itself. Javascript, CSS and HTML combine to form a front end with whatever you want as a backend on the server. Windows 8 apps use a similar approach with the declarative XAML defining interfaces and providing class outlines while C# fills in the details and runs the thing.

Thus, the typical divide will be a fast, statically typed language running a solidly tested codebase handling heavily numerical work with a scripting language running on top of it to provide easily changeable frontends handling visual and presentation functions as well as input.

Other use cases exist as well. Data processing languages like R can be bolted on to other codebases to provide analytic and presentation functions more easily than the native codebase might be able to. As far as combinations of paradigms, a combination of Prolog for database work and some other language to handle the standard program functions is a known case. Another case would be having Fortran or Assembly routines for fast numerical computation within some scripting language like Python. Which is exactly what Numpy and Scipy do.

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Programming languages are fundamentally abstractions that assist in helping you tell a computer what to do. As they are abstractions, this means that they hide some of the details and so simplify the overall task, enabling you to comprehend how to do some particular piece of programming without having to understand the whole thing at once. But that hiding also means that they restrict what you can do: going outside the abstraction is hard, and so also is having to build a lot of new abstractions on top of the base level provided by the language.

The net effect of this is to mean that a particular programming language has a range of programming tasks that it is well suited to, but it is not going to be good for everything. A low-level programming language allows you to do low-level work, often that happens to be performance critical (such as numeric-heavy processing). That's OK, but doing high-level work in the same language is nothing like as easy as using a language that specializes in higher-level abstractions. What's more, there's no perfect set of abstractions to choose: some problems are better described in one group (e.g., functional programming), and others require a different approach (e.g., logic programming).

The best way out of this situation where a programming language that can do one task isn't good for others is to use multiple programming languages. Let each language focus on doing the tasks that it is good at, and then transfer control to code written in another language for other parts. It's specialization (and also componentization) and it is a good thing indeed. Of course, there are languages that happen to work particularly well together since they have strongly compatible ABIs (e.g., Java and Scala, C# and VB, C++ and Lua) but they're most certainly not the only levels that you can plug things together at. Indeed, when you're accessing a web service you're probably using the same ideas: the languages that clients are written in are probably not those that servers are written in because, even if there's no theoretical reason for a language to be unable to do both sides, some languages have better abstractions for one side or the other.

Donal Fellows
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Another example that was not mentioned is the use of a new programming paradigm with existing legacy code and / or libraries.

For example, Java has tons of existing libraries and frameworks that can be used directly by new languages like Scala and Clojure. In this way, one has the advantage of using a clean implementation of a different programming paradigm that is not supported by Java, while retaining backward compatibility with legacy code and existing libraries. (I chose Java, Scala and Clojure but there are probably other examples I do not know of).

Giorgio
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There is no such thing as a "general purpose" programming language. Every language is strong in one area and useless in the others. Therefore, in a complex project which covers many different problem domains it is most efficient to use multiple languages. As far as I know, there are no benefits whatsoever in using a single language for a project where you can use multiple languages.

Of course, the most efficient language for any given problem domain is a language tailored specifically for this narrow domain. This approach is known as Language Oriented Programming, and such languages are called Domain Specific Languages. But even if you, for some weird reason, do not want to implement you own languages, there are multiple ready to use languages out there, and picking one most suitable for a task still will be much more efficient than using a language which does not fit well into the problem domain constraints.

One of the most efficient and convenient way of mixing different languages together is using embedded DSLs hosted inside a single meta-language. This way you won't need any FFI, you can reuse features of your host language library from within your DSLs, you can reuse your host language optimising compiler.

Another common approach is to host different languages within a runtime of a single common VM, like Java+Clojure+Scala or C#+VB.NET+F#.

SK-logic
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    I can't agree with that first statement. Of course there are general purpose programming languages. Most scripting languages such as Python and Ruby fall into this category. You can do desktop GUIs, web apps, text processing, networking, interactive shells, games, etc. They are very general purpose, useful for a very wide range of programs. – Bryan Oakley Mar 26 '13 at 11:27
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    "if you, for some weird reason, do not want to implement you own languages" - I'd suggest only very very few people want to and can implement their own languages. Most programmers simply want to write business logic in an existing language. That's not weird at all. – deceze Mar 26 '13 at 11:31
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    @BryanOakley, What? Python? General? Purpose? It does not even run on my PIC16, how could you call it "general purpose"? Needless to mention that I won't dream of coding distributed HPC stuff in python (and NumPy won't change my mind), I won't use it as an agent exchange language, I won't use it in my build system (because Scons is awful). – SK-logic Mar 26 '13 at 11:32
  • @deceze, most of the programmers simply do not know that implementing compilers and designing languages is very, very easy. Much simpler then most of the "business logic" stuff they're coding. – SK-logic Mar 26 '13 at 11:32
  • @BryanOakley, and, for each and every applications for Python you've listed, there is always a much better fitting language. Of course, you can code in Brainfuck if you want, and call it "general purpose" because it's Turing-complete, but it would be equally laughable as proposing to use a single language in an area where a better choice is available. – SK-logic Mar 26 '13 at 11:39
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    @SK-logic: yes, for each application I listed there's a better fitting language. That's why python is a good general purpose language -- it will do fine for a large number of tasks if you don't have a highly tuned special purpose language. You seem to think that because there's a better language than python for each specific task, that python can't be a good general purpose language? That's the very definition of a general purpose language -- useful for a wide variety of programming tests. The fact that there may be better languages doesn't change the fact that, often, python is Good Enough. – Bryan Oakley Mar 26 '13 at 12:04
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    @sk-logic: "[Python] does not even run on my PIC16". I'm not sure I see your point. A language doesn't have to run on every known platform to be general purpose. You seem to think that "general purpose" is defined as "perfect for every task". To me it means "good enough for most tasks". – Bryan Oakley Mar 26 '13 at 12:07
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    @BryanOakley: Every language has an application area or domain. Python is definitely very good in certain areas (e.g. web applications, prototyping, as a scripting language to be used on top of some existing engine) while it is unusable in other areas (like writing a device driver or avionic software). I would not say that "Python is not perfect for writing device drivers" but rather that "Python is unusable for writing device drivers". – Giorgio Mar 26 '13 at 12:22
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    @BryanOakley I think "general purpose" has been abused in the past to mean that a language can do everything and do it equally good, by various authors. I've seen and used "multi purpose" as a better term for languages that are good enough for a variety of distinct tasks. – yannis Mar 26 '13 at 12:35
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    I do not understand why there are so many downvotes to this answer that IMO is not bad. I only do not agree with one statement: "As far as I know, there are no benefits whatsoever in using a single language for a project where you can use multiple languages." One benefit is training costs: in order to work with different languages you need a team of programmers that are proficient in these languages. Most programmers I know are proficient in one or two languages and tend to think in these languages. So using multiple languages can be an advantage but in practice it can be difficult. – Giorgio Mar 26 '13 at 12:36
  • @BryanOakley, if you call your language of choice a "general purpose language" it only means your focus is too narrow, because you fail to see all the other possible purposes outside of your narrow field. Your view of the "most tasks" does not even overlap with anything I ever do. I don't give a crap about GUIs or web, for example, and my definition of "text processing" will make Python shy away. There is a huge difference between a "wide-focus" language (i.e., a language which equally sucks in many fields) and a non-existant "general purpose language" which can be used for any purpose. – SK-logic Mar 26 '13 at 12:56
  • @YannisRizos, yes, "multi-purpose" is a much better wording. – SK-logic Mar 26 '13 at 12:58
  • @Giorgio, training costs are always grossly overrated. I've seen juniors, fresh from the university, picking up an ugly internal legacy scripting language in a couple of hours. And a well-established language without tons of historical cruft is even easier to start working with. I honestly do knot understand why people shy away from learning new languages, it should be trivial for any programmer of any possible skill level. – SK-logic Mar 26 '13 at 13:01
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    @SK-logic: I agree with your points but I disagree on the tone of contempt in which you describe Python or other "wide-focus" languages. I do not think that Python "equally sucks in many fields". I think Python is really a good tool in certain areas. On the other hand, I find it narrow minded to describe Python as a general-purpose tool that is good enough for most application areas: I use Python for 10% of my programming and I find other languages more effective for the rest 90%. What should be criticized is the religious attachment that many programmers have towards one particular language. – Giorgio Mar 26 '13 at 13:03
  • @SK-logic: I agree with you and I find learning different languages very interesting (I have been learning at least 5 new languages in the last 3 years) but many programmers prefer to know the one true language extensively and project that one language on any problem they approach. So I do not think the costs are overrated because you always have to consider a workforce of "average" developers who might not be so interested in learning new stuff. I might be wrong, of course. – Giorgio Mar 26 '13 at 13:08
  • @Giorgio, I love Python, honestly (as long as I'm not forced to touch the ugly Scons), and I often use it for my petty scripting. It really shines in several narrow fields, but when put into a wider use, it starts to suck badly, as well as any other possible langauge will do. There is no single language out there which will not suck when used in a too wide domain. What makes it a "multi-purpose" language is the fact that it sucks evenly and predictably when its application domain is extended. The more narrow-focused languages will suck in a more creative manner. – SK-logic Mar 26 '13 at 13:08
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    @Giorgio, most people are not that interested in doing they job at all. They're just doing it. Interest as a motivator is overrated. And a so called "average coder" is perfectly equipped for learning and using multiple languages. Actually, they do it naturally - a typical Java shop will use many languages: ant/maven, Hibernate, Java itself, Spring configuration XML languages, etc. In non-Java world the selection of languages is often much wider. – SK-logic Mar 26 '13 at 13:10
  • let us continue this discussion in chat – Giorgio Mar 26 '13 at 13:13
  • @SK-logic You say that implementing a DSL is easy. Can you point to a good book/tutorial describing how to do this? Can you tell how you usually implement a DSL? And how a DSL is better than a library written in, say, C++? – Max Jul 10 '13 at 08:44
  • @Max, the best C++ libraries are DSLs indeed. As for books: P. Seibel "Practical Common Lisp", A. Alexandrescu "Modern C++ Design", M. Fowler "Domain Specific Languages", D. Grune et al. "Modern Compiler Design", for more see this: https://www.cis.uab.edu/courses/cs693/spring2010/DSLAnnotatedBib.pdf – SK-logic Jul 10 '13 at 09:05
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I would say the benefit is to get the most expressive and best performing solution to the problem at hand. Also, embedding one language in another often lets you move to a higher level of abstraction for parts of your work.

I am doing IOS development, so naturally I am writing in Objective-C++, which gives the awesome power and complexity of C++ and the run-time binding and introspection of Objective-C. I would think this is perhaps the most popular "hybrid" language/paradigm currently. Often I will write .mm (objective c++) files with mostly C++ and STL and just a little Objective-C. For example, last week I wrote a state machine in C++ that called back into an Objective-C object to perform its actions. Writing the machine in C++ was easier, more expressive, and reusable.

In this particular project, I also wrote a scheme interpreter which is embedded in the program and is used to control a communication channel by writing scheme at runtime based on environmental conditions. Scheme is a good choice for this because it is so small, simple and "code is data." It also raises the level of abstraction, and a functional approach works well in this situation.

I don't find embedding little languages to abstract complex problems unusual at all. Be it a set of tokens in a string that control an interpreter to drive some complex decision tree, or a full-blown specialty language of my own design or a unique problem, or a more off-the-shelf solution like LUA.

PatchyFog
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Tkinter is a graphical library for Python that is built upon Tcl/Tk. It's actually an embedded Tcl interpreter that is called from python. Normally this goes mostly unnoticed, but it's possible to put tcl code in a string and run it directly in the interpreter. This lets you do things that the native tcl/tk can do that Tkinter cannot because of an incomplete implementation.

Bryan Oakley
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Two examples:

  1. Wrote a system that imported CSV and XLS spreadsheets into a database using custom mapping documents. Wrote that system in Java and it worked fine. Then came the requirement to read those documents from an FTP server that would need to be polled at set intervals. Ruby's perfect for that, could write that code far more efficiently in Ruby than Java. So wrote a Ruby script to poll and retrieve the documents, which fed them to the original Java application which still could be called standalone as well for additional inserts or testing
  2. Working on a system written in an old language that's probably going the way of the dodo soon (losing support from toolvendors, training providers, etc.). Porting the entire system to a new environment in one go would be too risky for the business, take too long to please marketing, and deny customer new capabilities they've been asking for. So we create for several releases a hybrid system, with part created in that old technology and part in the new, gradually transiting from one to the other until in a year or two maybe the old will all have been retired.
  3. Every system using web services (or other EDI) to communicate is inherently hybrid at least in potentiality. It doesn't matter for your client whether the web service provider is written in Java, C#, Cobol, or whatever as long as the documents exchanged are in a format that both sides understand. Makes for great flexibility in case you have several teams working side by side that have different skill sets. Each can employ what technology it knows best, the end result will be better as a result.
jwenting
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If there actually is a benefit in mixing two languages, soon enough either one language picks up elements or concepts of the second one, or a new language emerges "between" both languages (one could argue that this happened with Scala, which pretty much sits in the "middle" between Java and Haskell).

Landei
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  • As far as I know Scala mayb have taken many concepts directly from ML, which is a predecessor of Haskell. Also, there are many features and idioms that are commonplace in the functional language community and different languages support a different mix of these features. That's why Scala may seem to be between Java and Haskell whereas both Scala and Haskell took certain features from a common source. – Giorgio Mar 26 '13 at 12:46
  • incorrect. If 2 tools each serve a good purpose, no need to corrupt one or either to become a bland mix of them both, just use them both together. – jwenting Mar 26 '13 at 13:02
  • Some languages are unmixable as their runtime environments are designed for mutually exclusive purposes. – SK-logic Mar 26 '13 at 13:14
  • @jwenting: Most mainstream programming languages strive to be "all-purpose" languages, and even those who don't tend to widen their scope over time (e.g. Erlang). Sometimes the result of an extension are good, sometimes they are bad, but its rare that language designers withstand the pressure for including new features. – Landei Mar 26 '13 at 13:21
  • @Landei and as a result most languages over time become monstrosities that become harder and harder to use and learn, until a new language is constructed that's lightweight, easy to learn, and flexible. That's how we got C, Pascal, Java, Python, Ruby, C#, you name it. – jwenting Mar 26 '13 at 13:28
  • @jwenting: True, but I would still prefer to code in hideous Java 8 than in "simple" Java 1.2 (at least until a Scala job comes along). That's the circle of life... – Landei Mar 26 '13 at 13:40
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    "I would still prefer to code in hideous Java 8 than in "simple" Java 1.2 (at least until a Scala job comes along)": I would not be so sure. Sometimes one prefers a missing features to a poorly designed feature. – Giorgio Mar 26 '13 at 13:54
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    well said Giorgio. The whole "closures" thing in Java 8 is a prime example of how to ruin a programming language for the sake of bolting on something the language was never designed for. – jwenting Mar 27 '13 at 06:38
  • On the other hand closure are an important part of any modern mainstream language. The reason why it was finally pushed into Java was that the very important and "fundamental" fork-join framework was pretty much unusable without. I would have preferred if they had given Java a general syntax overhaul (even with breaking compatibility) and a better type system first, but I still think the advantages and possibilities of closures outweigh the ugliness (closure syntax itself isn't too bad, but it just doesn't "blend in") and complexity. – Landei Mar 27 '13 at 08:01