I've recently heard several people remark that the differences between programming languages are "just syntax." In a sense, this is true. The Church-Turing thesis tells us that any programming language that can implement the Lambda calculus or a Turing Machine simulator can implement any algorithm.
To understand why this is meaningless, one needs only look at the esoteric language Malbodge: Although some derivatives are Turing-complete, it took two years for anyone to write a working program in the language.
For a less esoteric example, you can look in the programmers' manual for the STANTEC ZEBRA, a machine released in 1958. It included a restricted version of its instruction set that was easier to program, with a caveat that it could be used only for programs that had fewer than 150 instructions. The manual helpfully informed readers that this wasn't a serious limitation because no one could write a working program so complex.
This is obviously not the case for most modern programming languages. In fact, writing a program that compiles to fewer than 150 instructions is something of a challenge. Different programming languages teach very different ways of thinking about problems. Some programmers learn one language and then write in any other language as if they were using their first language. These are the sorts of people who are likely to tell you that different languages are "just syntax."
The Level of Languages
It's traditional to refer to a programming language as being on a spectrum from a high-level language to a low-level language. I still believe that this is important because without a grounding in computer architecture it's very easy to forget that some constructs in a high-level language are easy to generate efficient code for-and others are not.
Today, however, the idea of a single low-level language doesn't make as much sense as it did even five years ago. A modern mobile phone has somewhere between one and eight ARM CPU cores. It has a GPU, with a very different execution and memory model, which executes somewhere up to 256 threads using a very different parallelism model in which groups of threads proceed in lockstep. It typically has a DSP that can do stream processing very quickly. And it may have some other specialized processors.
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There is no single low-level language. A high-level language is one that is close to how a human would express the problem (albeit often for a somewhat peculiar definition of human, which is not representative outside of most university mathematics departments), whereas a low-level language is one that is close to how a machine operates.
I've argued before that everyone should learn at least one low-level programming language so that they will think about efficiency when writing code in a high-level language that has an abstract machine model that cleanly maps individually to all of these, let alone to the entire system.
This means that it's worth learning C, but it's also worth learning some related languages, such as OpenCL C or GLSL. Increasingly, general-purpose compilers are aiming to target GPUs, so being able to structure loops or map operations in a way that is amenable to this kind of autoparallelism can result in huge performance (and power usage) improvements.
At the opposite end of the spectrum, high-level languages are also very different. The definition of a high-level language has always been somewhat fluid. When I was learning to program, a high-level language meant any language that was not an assembly language, and the book from which I learned C spent a lot of the first chapter explaining that a high-level language like C could be as fast to execute as assembly but had advantages in terms of portability and maintainability. Now people would look at you very strangely if you described C as a high-level language.
So what is a high-level language today? Modern object-oriented or functional languages such as Java or Haskell are a long way away from how the computer works, although it's debatable whether they are close to how humans work. Going past them, there are domain-specific languages.
Perhaps the most well-known is the Structured Query Language (SQL), which (as its name would suggest) is designed for composing queries on structured data and would be a wholly inappropriate language for general-purpose programming.
Domain-specific languages are everywhere. Programs like awk provide domain-specific languages for record-oriented text processing. PostScript is a domain-specific language for rendering vector graphics, and although PostScript itself is increasingly rare, a number of its derivatives survive. PDF uses the core of PostScript (without the flow control) as the base for its rendering model, and the OS X drawing APIs (and Cairo on other UNIX-like platforms) provide APIs that have direct mappings to the PostScript drawing state.
Again, although PostScript would make a terrible choice as a general-purpose programming language, that hasn't stopped people from trying. Sun's NeWS system used PostScript for rendering and allowed entire view objects to be written in PostScript. This made it much more responsive for remote display than X11, because when you clicked on a button it would draw the pressed state immediately while asynchronously sending the button-pressed event to the rest of the program.
NeXT also used a variant of PostScript (Display PostScript) for rendering, but developers usually just generated PostScript commands from another language.
Thinking in Abstractions
With such a plethora of high-level and domain-specific languages, it's usually fairly easy to find one that is close to the expression of the problem. Unfortunately, in the real world there are other constraints that often mean that you can't use the language that makes the implementation easier.
These constraints can be related to performance or deployment. For example, a language like Python or Ruby may be quick to write in, but might not run the resulting code quickly enough. If you have the resources of a company like Facebook, you can write your own compiler to make your existing code faster, but that's not usually an option.
Alternatively, your language of choice may not support your target platform. For example, trying to ship a Java application for Microsoft or Apple's mobile platforms is not usually possible.
That doesn't make it a waste of time, however. It's often a good idea to prototype in another language. This has several advantages. If the language that you're prototyping in is not usable for deployment, it prevents the prototype (which may have some poor design decisions and be unmaintainable code) from being pressed into service by management. It also means that you are free to make any experimental choices you want, knowing that you won't have to live with them if they don't work.
More importantly, creating a prototype allows you to structure your thoughts about what the design should look like. For more-experienced programmers, actually creating the prototype is not always an essential part of this process.
Having identified a language in which the problem can be expressed naturally, I often spend some time considering how I would implement the program in that language and then how the constructs that I would use would map to the language that I am using. The end result is code that solves the problem and is easy to restructure when I revisit it a few years later.
It's possible to write C, or even assembly, in a style that mimics an object-oriented language, with fine-grained encapsulation and independent parts accessible only via well-defined interfaces. It's much harder to do this if you've never used a language where these ideas are exposed syntactically and enforced by the language model.
Models of Parallelism
One of the most obvious differences between languages on modern hardware is how they expose parallelism. This is increasingly important, for two reasons. For the purely performance minded, single-threaded execution speeds have not become much faster for quite a while. In the '90s, you could expect your single-threaded program to roughly double in speed if you replaced your year-old computer with a new one. Now, you'd be lucky to get a 20% speedup (unless it's I/O bound and you replace a hard disk with an SSD). Very little code these days is actually CPU-bound, so this isn't a huge problem, but increasingly power consumption matters.
All other things being equal, a single CPU core running at 1GHz will consume more power than two running at 500MHz. If you have perfectly written parallel code without any contention, it will run at about the same speed on both. If it's running on a mobile device, this translates to longer battery life. More importantly, it gives the operating system more opportunity to adapt. A four-threaded program with work distributed evenly among the threads can be scheduled on one, two, or four cores, depending on which makes more sense for the current device conditions.
I've already mentioned GPUs. They have a very different programming model from more traditional SMP systems because there is a significant performance penalty if the threads diverge (or, indeed, need to communicate). On the CPU side, there are a lot of models that are worth learning.
Go and Erlang both provide primitives built into the language for communication. Somewhat confusingly, Erlang uses the syntax from Hoare's Communicating Sequential Processes (CSP) formalism but provides actor-model concurrency, whereas Go provides CSP semantics but its own syntax.
Both the actor model and CSP are shared-nothing models (although Go's implementation relaxes this). The cardinal rule for writing scalable and maintainable parallel code is that no object should be both aliased between threads and mutable. This is enforced by Erlang, which provides a single mutable object per process (the process dictionary) and makes everything else immutable.
Some Π-calculus extensions to Haskell do the same. In Go, you must enforce this yourself, although the language does make it quite easy and provides design patterns such as "share memory by communicating" that make it easy.
In C, you have a shared-everything model of parallelism and no explicit mechanisms for sending messages between thread (or, until recently, of creating threads without extensions such as POSIX or Win32). All that you have is some fundamental building blocks for creating and synchronizing threads. You can build Erlang-style message passing or Go-style channels on top of them.
Learning either of these languages and writing some code in it gives you the mental building blocks to use the same ideas elsewhere. This applies to a great many other aspects of a language. It doesn't matter whether you actually use the language that taught you the concepts; that experience gives you a much wider set of tools to apply to whatever language you happen to find yourself using.