What is a computer programming language?

Question

At the risk of sounding naive, I ask this question in search of a deeper understanding of the concept of programming languages in general. I write this question for my own edification and the edification of others.

What is a useful definition of a computer programming language and what are its basic and necessary components? What are the key features that differentiate languages (functional, imperative, declarative, object oriented, scripting, etc...)?

One way to think about this question. Imagine you are looking at the hardware of a modern desktop or laptop computer. Assume, that the C language or any of its variants do not exist. How would you describe to others all the things needed to make the computer expressive and functional in terms of what we expect of personal computers today?

Tangentially related, what is it about computer languages that allow other languages to exist? For example take a scripting language like Javascript, Perl, or PHP. I assume part of the definition of these is that there is an interpreter most likely implemented in C or C++ at some level. Is it possible to write an interpreter for Javascript in Javascript? Is this a requirement for a complete language? Same for Perl, PHP, etc?

I would be satisfied with a list of concepts that can be looked up or researched further.

Answer 1

Like any language, programming languages are simply a communication tool for expressing and conveying ideas. In this case, we're translating our ideas of how software should work into a structured and methodical form that computers (as well as other humans who know the language, in most cases) can read and understand.

What is a useful definition of a computer programming language and what are its basic and necessary components?

I would say the defining characteristic of a programming language is as follows: things written in that language are intended to eventually be transformed into something that is executed. Thus, pseudocode, while perhaps having the structure and rigor of a programming language, is not actually a programming language. Likewise, UML can express many powerful ideas in an abstract manner just like a programming language can, but it falls short because people don't generally write UML to be executed.

How would you describe to others all the things needed to make the computer expressive and functional in terms of what we expect of personal computers today?

Even if the word "programming language" wasn't part of the shared vocabulary of the group I was talking to, I think it would be obvious to the others that we'd need a way to communicate with the computer. Just as no one expects a car to drive itself (yet!) without external instructions in the form of interaction with the steering wheel and pedals, no one could expect the hardware to function without being told what to do. As noted above, a programming language is the conduit through which we can make that communication happen.

Tangentially related, what is it about computer languages that allow other languages to exist?

All useful programming languages have a property called Turing completeness. If one language in the Turing-complete set can do something, then any of them can; they are said to be computationally equivalent.

However, just because they're equally "powerful" doesn't mean they're equally nice to work with for humans. This is why many people are willing to sacrifice the unparalleled micromanagement you get from writing assembly code in exchange for the expressiveness and power you get with higher-level languages, like Ruby, Python, or C#.

Is it possible to write an interpreter for Javascript in Javascript? Is this a requirement for a complete language? Same for Perl, PHP, etc?

Since there is a Javascript interpreter written in C, it follows that it must be possible to write a Javascript interpreter in Javascript, since both are Turing-complete. However, again, note that Turing-completeness says nothing about how hard it is to do something in one language versus another -- only whether it is possible to begin with. Your Javascript-interpreter-inside-Javascript might well be horrendously inefficient, consume absurd amounts of memory, require enormous processing power, and be a hideously ugly hack. But Turing-completeness guarantees it can be done!

Answer 2

While this doesn't directly answer your question, I am reminded of the Revenge of the Nerds essay by Paul Graham about the evolution of programming languages. It's certainly an interesting place to start your investigation.