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What is the key difference between natural languages (such as English and French) and programming languages like C++ and Perl?
I am familiar with the ambiguity problem, but can't it be solved using an interactive compiler or using a subset of the natural language using a strict grammar but all the time still retaining the essence of the language?
Another issue is context. But lawyers have ways to solve this issue. (This question is not about reducing the programming complexity, it's simply about concise reasons and roadblock in using natural languages for instructing computer.)
Is there any other significant problem besides these two? Or do these two have greater consequences than I mentioned above? Is the interactive solution and lawyers language technically not feasible for programming?
This is an extremely interesting question and in short, yes, there are some very good reasons why we don't use English to write programs.
It's been said before that the greatest gift that computer science has given us is not the ability to talk to computers but now that formal languages exist for describing algorithms we now have even better tools for communicating these ideas to other people. Even if a computer is not involved. Indeed the best software engineers see their jobs primarily as writing software that is readable to other people so as to make maintenance and addition of new features as easy as possible. This is not possible in a language as big and as free form as any natural, spoken language.
Ambiguity
One reason is that of ambiguity. Have you ever looked a menu in a restaurant and seen that with your burger you can get "Coleslaw and fries or salad"? What does this mean? Can I get both coleslaw and fries or the other option is a salad alone? Or do I always get coleslaw and I have to chose between the fries or a salad? English is full of these things.
I used to teach a class on this and an example I liked to use to explain ambiguity was as follows. I asked the students to write a one paragraph story ending with the sentence "Tom asked Chris if he could help him". About half the time the stories written indicated that the student interpreted the sentence as Tom asking for assistance from Chris. The other half of the time people thought Tom was offering to lend Chris a hand.
If you think about it, there are a lot of people who do write programs in English. They're called product managers and the compiler they use is software engineers. The problem here is that a software engineer has to inject a lot of his own understanding of the problem to understand what the description really means. And trust me, there is a lot of back and forth. Even on very simple business requirements I must clarify ambiguities.
Context
I would not agree that lawyers have ways to solve the context problem. We continually have ongoing arguments in courts among, in some cases, some of the most educated people in the country, about the meanings of various laws. Sometimes this involves arguing about the context in which the laws were written long ago. Sometimes in involves applying it to a new, previously non-existent context like the Internet. The fact that we have thousands of lawyers working on disambiguating these issues is proof that it cannot be handled by a simple computer program like a compiler. It's just too hard of a problem.
Conciseness
Another issue is just the ability to be concise. Mathematics long ago invented notations for many different concepts in the maths because it's just easier to read if there's a special syntax that is concise and has a well defined meaning. A mathematician knows what it means when I say "f(x) = 3x+1". It means the same thing as "There is a function called f and it has one argument. The value of applying f to a number is the number that is one more than three times the number given." But the former is a lot easier to read once you've learned the syntax. The same is true for programming languages. Programming languages are specialized to describe computations.
Implementation
Creators of programming languages deliberately create very small languages. These are, in fact, subsets of English also with some extra syntax. The idea of understanding all of English in all of its free form ways and, worse yet, increasing vocabulary is a job for Natural Language Processing (NPL). A very hard job. If you want to be able to assign unambiguous meaning to a program and have the program's behavior never change, you need a well defined syntax and semantics.
The take-away point here is that English is a very big, very flexible language with no formal specification. Programming languages need to have a well-defined syntax and semantics in order for algorithms to have unambiguous and unchanging meaning. Someone could, in fact, write a formal syntax for a subset of English and give it unambiguous meaning. But this would be a huge, huge job.
Its been done
Check out BabelBuster. The idea here was to take C and convert it to and from a very small subset of very rigorous English such that one could write a program in C and then convert it to English. During the DeCSS DVD decryption arguments, the MPAA was trying to get programs that could decrypt their DVDs declared illegal. BabelBuster fought back with a very interesting idea. Create a way to convert English, which is protected under the freedom of speech into working code in C and thus make the point that C code is also just a language which should be protected as such. Therefor one should be able to publish code that cracks DeCSS. It's an interesting piece of work relevant to your question regardless of which side you're on.
The problem with BabelBuster is that you need to write your program in a very, very limited subset of English. But it is possible to do this.
Conclusion
English, like all natural languages, allows us to describe a computation or algorithm but the language is verbose, offers many ways to say the same thing, is dependent on the context of the speaker, and not formally specified. If your goal is to describe computations, you should take English, chose a minimal workable subset in which you can say everything you need. Formally specify what each word in this subset will mean. Then create a few special notations to make it concise to say the things you say just like mathematics did. If you do this, you do this you'll end up with a typical programming language, or something like it.
There are three principal reasons.
First, as Gabe says - people have figured out through trial and error that programming in things that are close to English sentences only forces programmers to type more useless cruft. (And yes, COBOL was explicitly designed to read more "naturally".)
To a programmer,
windows++
is more readable than
You should now increment the number of windows by one.
For example, Tetris is a rather easy game to code. I would be terribly surprised if you managed to make an English explanation that is detailed enough for a computer (remember, computers are dumb, so you have to spell it all out) in less pages than a short novel.
The second reason is that the range of things a computer knows how to do is rather small, so the number of language constructs that are needed for that is also limited. In contrast, natural languages need to be able to express the entirety of human experience, which does require many language constructs to pull off. For example, "According to his wife, John would have caught the fish yesterday if it hadn't rained" is not expressible in C - and does not need to be.
And third is, indeed, ambiguity, as you yourself note. There are a lot of places where a software error is simply not permissible. People do enough bugs in unambiguous languages; allowing ambiguity would be a disaster waiting to happen. And on the same subject, we are still unable to parse human language sufficiently well - state of the art parsers still have unacceptably high error rates.
It is possible to automatically translate structured English into code, as long as a restricted subset of the English language is used.
As a proof-of-concept, I have developed an programming language called EngScript, which translates English sentences sentences into Python source code.
Arithmetic operations can be written in plain English:
#print{3 to the power of 2}
#print{3 raised to the power of 2}
#Both of these statements print "9".
print{3 plus (the sum of 1 and 2)}
#This prints "5".
Variables can be initialized in plain English, too:
let x be (x plus 1)
if (x is not equal to 7) :
print x
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A pure function is one that has no side effects -- it cannot do any kind of I/O and it cannot modify the state of anything -- and it is referentially transparent -- when called multiple times with the same inputs, it always gives the same outputs.
Why is the word "pure" used to describe functions with those properties? Who first used the word "pure" in that way, and when? Are there other words that mean roughly the same thing?
To answer your first question, mathematical functions have often been described as "pure" in terms of some specified variables. e.g.:
the first term is a pure function of x and the second term is a pure function of y
Because of this, I don't think you'll find a true "first" occurrence.
For programming languages, a little searching shows that Ada 95 (pragma Pure), High Performance Fortran (1993) (PURE) and VHDL-93 (pure) all contain formal notions of 'pure functions'.
Haskell (1990) is fairly obvious, but purity isn't explicit. GCC's C has various function attributes for various differing levels of 'pure'.
A couple of books: Rationale for the C programming language (1990) uses the term, as does Programming Languages and their Definitions (1984). However, both apparently only use it once! Programming the IBM Personal Computer, Pascal (also 1984) uses the term, but it isn't clear from Google's restricted view whether or not the Pascal compiler had support for it. (I suspect not.)
An interesting note is that Green, the predecessor to Ada, actually had a fairly strict 'function' definition - even memory allocation was disallowed. However, this was dropped before it became Ada, where functions can have side-effects (I/O or global variables), but can't modify their arguments.
C28-6571-3 (the first PL/I reference manual, written before the compiler) shows that PL/I had support for pure functions, in the form of the REDUCIBLE (= pure) attribute, as far back as 1966 - when the compiler was first released. (This also answers your third question.)
This last document specifically notes that it includes REDUCIBLE as a new change since document C28-6571-2. So REDUCIBLE, which is possibly the first incarnation of formal pure functions in programming languages, appeared somewhere between January and July 1966.
Update: The earliest instance of "pure function" on Google Groups in this sense is from 1988, which easily postdates the book references.
A couple of myths:
The term "pure functional" doesn't come from mathematics, where all functions are by nature "pure" and, so, there was never any need to call anything a "pure function".
The term doesn't come from imperative programming. The early imperative programming languages, Fortran, Algol 60, Pascal etc., always had two kinds of abstractions: "functions" that produced results based on their inputs and "procedures" which took some inputs and did an action. It was considered good programming practice for "functions" not to have side effects. There was no need for them to have side effects because one could always use procedures instead.
So, where else could the term "pure functional" have come from? The answer is - sort of- obvious. It came from impure functional programming languages, the foremost among them being Lisp. Lisp was designed sometime between 1958 and 1960 (between the first and second reports of Algol 60, whose design McCarthy was involved in, but didn't feel satisfied with). Lisp's design was based fundamentally on functional programming. However, it also allowed side-effects as a pragmatic choice. It did not have a notion of a command or a procedure. So, in Lisp, one mostly wrote "pure functions", but occasionally, one wrote "impure functions," i.e., functions with side-effects, to get something done. The terms "pure Lisp" or "purely functional subset of Lisp" have been in use for a long time. Slowly, by osmosis, this idea of "purity" has come to invade all our space.
The imperative programming languages could have resisted the trend. But, once C decided to abolish the idea of "procedures" and call them "void functions" instead, they didn't have much of a leg to stand on.
It comes from the mathematical definition of "function", where it is not possible for functions to have side effects.
Why is the word "pure" used to describe functions with those properties?
From Wiktionary > pure # adjective
free of flaws or imperfections; unsullied
free of foreign material or pollutants
free of immoral behavior or qualities; clean
of a branch of science, done for its own sake instead of serving another branch of science.
It should be obvious that the behavior of interacting functions is easiest to reason about when they are influenced only by their inputs, and they themselves influence only their outputs. Therefore it is inevitable that these kinds of functions will be noticed and classified. Now what word could we use to describe a function with such properties? "free of foreign material or pollutants" and "free of immoral behavior or qualities" seem to describe this rather well.
Who first used the word "pure" in that way, and when?
I am much too young to answer this with any degree of confidence. I argue, however, that it was inevitable that the word pure (or some very close synonym) would be used to describe functions that behave in this way.
Are there other words that mean roughly the same thing?
You said it yourself: "referentially transparent". However, you seem to suggest that "referential transparency" encompasses only part of the meaning of the phrase "pure function". I disagree; I feel it is entirely synonymous. From Wikipedia > Referential Transparency:
An expression is said to be referentially transparent if it can be replaced with its value without changing the behavior of a program. (emphasis mine)
The Haskell community sometimes uses the adjective "safe" in a similar manner. (See the Safe library, made to avoid throwing exceptions. Contrast with unsafePerformIO)
I can't think of any other synonyms right now.
The concept of a function originated in mathematics. The mathematical concept of a function is more-or-less a mapping from one set onto another. In this sense it's impossible for functions to have side effects; not because they're "better" that way or because they're specifically defined as to not have side effects, but because the concept of "having side effects" doesn't make any sense with this definition of a function. Mathematical functions aren't a series of steps that execute, so how could any of those steps somehow "affect" other mathematical objects you're talking about?
When people started studying computation, they became interested in machine-implementable algorithms for computing the values of mathematical functions given their inputs. People started talking about computable functions. But functions as implemented in a computer (in imperative languages at least, which are what programmers first worked with) are a series of executable steps, which obviously can have side effects.
So it became natural for programmers to think about functions as algorithms, not as mathematical functions. So then a pure function is one that is purely a mathematical function, to which all the hundreds of years of theory about functions applies, as opposed to the generalised programmer's function, which can't be reasoned about that way.
I see a lot of the word 'expressiveness' when people want to stress one language is better than the other. But I don't see exactly what they mean by it.
Is it the verboseness/succinctness? I mean, if one language can write down something shorter than the other, does that mean expressiveness? Please refer to my other question - Article about code density as a measure of programming language power
Is it the power of the language? Paul Graham says that one language is more powerful than the other language in a sense that one language can do that the other language can't do (for example, LISP can do something with macro that the other language can't do).
Is it just something that makes life easier? Regular expression can be one of the examples.
Is it a different way of solving the same problem: something like SQL to solve the search problem?
What do you think about the expressiveness of a programming language? Can you show the expressiveness using some code?
What's the relationship with the expressiveness and DSL? Do people come up with DSL to get the expressiveness?
Personally, I feel that the "expressiveness" of a language really comes down to how clearly the language constructs can "express" the developer's intentions.
For example, I feel that C# (especially LINQ via C# 3+) is becoming much more expressive. This LINQ statement is a great example:
var results = collection.Where(item => item > 5);
Without knowing the details of the language or the implementation being used, the developer intent is (in my opinion) very clear in the above statement.
I do not think that the verboseness of the language is equal to its expressiveness, however, there is some correlation in place. If a language requires a lot of code in order to express an abstraction, it is less expressive. These are two related, but different, concepts.
The same is true with power - although here a language's features (ie: power) must be complete enough to express the abstraction clearly. Without this, expressiveness will suffer. That being said, a language can be very "powerful" in terms of features, but not necessarily be expressive, if the feature set is difficult to understand.
"Expressiveness" means the ability to say only what you want done:
bad_event = events.find(&:bad)
rather than how you want it done:
i = 0
bad_event = nil
while i < events.size && bad_event.nil?
event = events[i]
if event.bad?
bad_event = event
end
i += 1
end
Among the things that contribute to expressiveness are:
A lack of required syntactic sugar
First-class functions
Garbage collection
Either dynamic typing or type inference
The language core not being slavishly minimalistic
Good functionality in the standard library
To some degree, the expressiveness of any language can be increased by shoving as much "how to do it" off into subroutines/objects as possible so that most of the remaining code is "what to do." The amount of "how to do it" code needed in the most abstract code is one measure of a language's expressiveness: The more the code looks like pseudocode, the more expressive it is of the programmer's intent.
One can also think about the "meta-expressiveness" of a language: How expressive is the language at constructing Domain Specific Languages?
I like Matthias Felleisen's notion of expressive power, which is comparative:
Language A is strictly more expressive than language B if both of the following are true:
Any program written in language B can be rewritten in language A while keeping the essential structure of the program intact.
Some programs written in language A have to be violently restructured in order to be written in language B.
Usually we want to make these comparisons by looking at some kind of "essential core" of a language—for example, maybe we want to consider a dialect of C with only while and not also for and do...while. Or maybe we want to consider a dialect of Perl with only a prefix if form and no unless form. But sometimes these superficial syntactic distinctions are exactly what we mean by "expressive power"; to some programmers it's important to say
die ("found no solutions") unless length(solutions) > 0;
instead of
if (length(solutions) == 0) { die("found no solutions"); }
So you have to establish whether you're asking about expressive power of surface syntax or deeper structure.
The other thing I like about Felleisen's idea is that it admits of the notion of two languages which are definitely different, but neither is more expressive than the other.
You can read a more detailed exposition in the first two pages of his paper On the Expressive Power of Programming Languages. After that comes a lot of pointy-headed theory :-)
If you want an answer that's somewhat theoretical but more rigorous than most, you might want to look around for Matthias Felleisen's On the Expressive Power of Programming Languages. I'm pretty sure a bit of looking around the net will turn up at least a few copies.
If you want a more practical answer of what most people actually mean when they say it, that's, frankly, rather different. At least in my experience, an "expressive" language usually means: "I like the language, but can't cite much (if any) objective support for doing so." Conversely, things like "less expressive", or "not expressive" generally mean: "I don't like the language (or like it less), but can't cite much (if any) objective support for doing so."
"Not expressive" is often similar to a politician accusing another of being "fascist" -- clearly pejorative, but without any meaningful definition of what's supposedly wrong.
One of the big problems stems from a fundamental difference of opinion. There are at least two fundamentally different general ideas that people seem to have about expressiveness:
the ability to express a wide variety of ideas.
the ability to express some specific ideas clearly (and often succinctly).
To consider some extreme examples, assembly language would qualify as highly expressive by the first criteria--you can do essentially anything in assembly language that you can in a higher level language, and you can do some things in assembly language that you can't in essentially any higher level language.
Obviously, assembly language doesn't look nearly so good by the second measure--it typically requires quite a large amount of fairly opaque code to accomplish much. This measure would tend to favor a language like Haskell or APL, to give only a couple of examples.
These two notions of what "expressive" means are frequently close to diametrically opposed. The first tends to favor the "lowest" level languages, while the second tends to favor the "highest" level. At least from what I've seen, most people really start from the language they like, and their definition of "expressive" is basically whatever balance of the two criteria is required for their preferred language to be the "best".
For me, it is the ability of the language to clearly express my logic and ideas through code, in a way that somebody else reading the code can easily figure out what I was thinking when I wrote it.
Wikipedia has a bit about the concept. I myself take it to mean that a language can accomplish more with less (the so called "informal usage" in the Wikipedia article).
I consider JavaScript expressive (though this could be because Douglas Crockford drilled that idea into my noggin) because it can do so much with just a few keywords. For instance, the function keyword is a function, as well as a method, a class, and a lambda.
Some code illustration (leaving out some details for brevity) in JavaScript. It's an event class I wrote:
SJJS.util.Event = (function() {
var _listeners = [];
var _listenerReturns = [];
return {
addDomListener: function(element, eventName, listener) {
},
trigger: function(element, eventName) {
},
removeListener: function(eventlistener) {
}
}
})();
With just function, var, and some curly braces and parentheses I made a static class with methods and private variables.
Generally speaking, with a programming language which is turing complete you can do anything that another turing complete language can do. That being said, some can do it a lot better than other.
I take expressiveness to mean how much you can say easily, and how well / clearly it can be said. The ability to be terse is part of that ( a very powerful and terse language is one like J ). Generally I find that being concise is a good marker of being expressive. If the language can express a complex operation in a simple manner, it's going in the proper direction.
As to the power, expressiveness isn't all the power of a language. While it may be part of it, speed, security, stability, all of those things factor in as well.
example: summation of a list in Common lisp using the loop operator is concise and expressive
(loop for x in list sum x)
Precision, concision, and readability are the primary components in expressiveness.
I always took it to be roughly equivalent to how high-level a language is. If you wanted to try to quantify expressiveness, the units would be something like "machine code instructions per language statement"
A more expressive language might be very good at doing rather a lot of work without writing a lot of code. However, it would probably be more domain-specific and a wee bit slower for some tasks than a less expressive one.
From Wikipedia: In computer science, the expressive power (also called expressiveness or expressivity) of a language is the breadth of ideas that can be represented and communicated in that language. The more expressive a language is, the greater the variety and quantity of ideas it can be used to represent.
So, I agree. "How easy, comprehensive and composable the language for you to express your intents.": I believe, this is the measure of expressiveness.
QUESTION: Is it the verboseness/succinctness? I mean, if one language can write down something shorter than the other, does that mean expressiveness?
No. For example, is Brainfuck language expressive? I don't think so. Look at an Hello World example in Brainfuck:
++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.
Or: hq9plus language. Hello World code:
H
QUESTION: Is it the power of the language? Paul Graham says that one language is more powerful than the other language in a sense that one language can do that the other language can't do (for example, LISP can do something with macro that the other language can't do).
I disagree with Paul. As you see in the above examples, hq9plus language is doing Hello World with one letter: H. Whereas, most of the other languages will do it with much more letters. But, you can create composable and easy to read code with other languages. If hq9plus is doing Hello World with H, does it mean that is it powerful? I believe no.
QUESTION: Is it just something that makes life easier? Regular expression can be one of the examples.
Regexes are great, but sometimes they lose their expressive power. Sometimes, it depends on the programmer.
QUESTION: Is it a different way of solving the same problem: something like SQL to solve the search problem?
Half Yes. SQL is a declarative and very expressive language. Because the underlying engines and technologies can advance and change without you to change your SQL queries. This makes it very expressive. There are many queries have been working for decades and the underlying technology of databases change. But, your queries don't need to. I think this is due to the power of its expressiveness.
I also believe that functional languages are very expressive. Because, you only describe your intent, not your hows, and the underlying technologies can always change and optimize, but, it won't hurt your expressive code.
Example:
// top 10 products with rating higher than 5
return products
.sort(p => p.rating)
.filter(p => p.rating > 5)
.map(p => p.title)
.take(10)
Above program is expressive, it conveys your intents. And, most probably it won't change when the underlying mechanisms change.
Take for example LINQ. It allows you to use functional programming.
Functional programming emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state.
LINQ allows your to express what you want done instead of how to do it. This is a clear example of expressiveness.
Many programming languages share generic and even fairly universal features. For example, if you compared Java, VB6, .NET, PHP, Python, then you would find common functions such as control structures, numeric and string manipulation, etc.
What has been done to define these features at a meta-language (or language-agnostic) level?
UML offers a descriptive reference of software in every aspect, but the real-world focus seems to be data processes. Is UML relevant?
I'm not asking "Why we don't have a single language that replaces the current plethora." We need many different tools (at least in this eon).
I'm not asking that all languages fit a template -- assembly vs. compiled languages are different enough to make that unfeasible (and some folks call HTML a language, though I wouldn't). Any attempt would start with a properly narrow scope. In line with this, I wouldn't expect the model to cover even a small selection with full validity.
I would expect however that such a model could be used to transpose from one language to another (with limited goals -- think jist translation).
There have been many attempts at this, but none have been very successful. The earliest I'm aware of is UNCOL more than 50 years ago.
You've given a list of languages that have a lot in common because they're pretty similar -- they're all procedural languages with common roots and some OO extensions thrown in, so that's not too suprising. If you start looking at different languages like LISP, haskell, erlang, prolog, or even SQL you start seeing very different things.
What you're describing sounds like the formal semantics of programming languages. There are a variety of approaches and each will give a way to formally specify the meaning of a program in some programming language. In some cases, this specification is essentially a translation into another language such as lambda calculus, or compilation for a formally specified abstract machine such as SECD.
There is so much work here it's hard to pick a specific reference. But I hope I've given you some useful keywords to continue your search.
UML is typically used to define algorithms/code in simpler terms before moving on to real code.
To answer what I am guessing to be your question, there is already a defined set of required parts of languages while,for,if,else... Will this ever be set as a standard, or made into a base library that is used by all languages: no, this is because the different developers of languages like to do it themselves.
I think the closest you can get to this without loss of generality is a Turing machine, which is not very useful for practical purposes. But if you allow Turing machine languages to be "labeled" and reused, you could build up the concepts you need, working from low- to high-level.
I think that MOF is the universal language.
You can for example create UML diagrams from MOF via a UML metamodel. If you save this metamodel information into xmi then you can save what ever information you need and even more than in any language. XMI semantic is so rich that there is no limit to its use. If you map UML to xmi on the top of a metamodel live synchronize with MOF then this is for me the universal language.
The author of Pattern Calculus seems to propose such a universal model. I expect that it will turn out to be just as useful as previous attempts to define a universal model, that is to say, good in parts but not the last word.
People like Alexander Stepanov and Sean Parent vote for a formal and abstract approach on software design.
The idea is to break complex systems down into a directed acyclic graph and hide cyclic behaviour in nodes representing that behaviour.
Parent gave presentations at boost-con and google (sheets from boost-con, p.24 introduces the approach, there is also a video of the google talk).
While i like the approach and think its a neccessary development, i have a problem with imagining how to handle subsystems with amorphous behaviour.
Imagine for example a common pattern for state-machines: using an interface which all states support and having different behaviour in concrete implementations for the states.
How would one solve that?
Note that i am just looking for an abstract approach.
I can think of hiding that behaviour behind a node and defining different sub-DAGs for the states, but that complicates the design considerately if you want to influence the behaviour of the main DAG from a sub-DAG.
Your question is not clear. Define amorphous subsystems.
You are "just looking for an abstract approach" but then you seem to want details about an implementation in a conventional programming language ("common pattern for state-machines"). So, what are you asking for? How to implement nested finite state-machines?
Some more detail will help the conversation.
For a real abstract approach, look at something like Stream X-Machines:
... The X-machine model is structurally the
same as the finite state machine, except
that the symbols used to label the machine's
transitions denote relations of type X→X. ...
The Stream X-Machine differs from Eilenberg's
model, in that the fundamental data type
X = Out* × Mem × In*,
where In* is an input sequence,
Out* is an output sequence, and Mem is the
(rest of the) memory.
The advantage of this model is that it
allows a system to be driven, one step
at a time, through its states and
transitions, while observing the
outputs at each step. These are
witness values, that guarantee that
particular functions were executed on
each step. As a result, complex
software systems may be decomposed
into a hierarchy of Stream
X-Machines, designed in a top-down
way and tested in a bottom-up way.
This divide-and-conquer approach to
design and testing is backed by
Florentin Ipate's proof of correct
integration, which proves how testing
the layered machines independently is
equivalent to testing the composed
system. ...
But I don't see how the presentation is related to this. He seems to speak about a quite mainstream approach to programming, nothing similar to X-Machines. Anyway, the presentation is quite confusing and I have no time to see the video right now.
First impression of the talk, reading the slides only
The author touches haphazardly on numerous fields/problems/solutions, apparently without recognizing it: from Peopleware (for example Psychology of programming), to Software Engineering (for example software product lines), to various programming techniques.
How the various parts are linked and what exactly he is advocating is not clear at all (I'm accustomed to just reading slides and they are usually consequential):
Dataflow programming?
Constraints solving for User Interfaces? For practical implementations, see Garnet for Common Lisp, Amulet/OpenAmulet for C++.
What advantages gives us this "new" concept-based generic programming with respect to well-known approaches (for example, tools based on Hoare logic pre/post conditions and invariants or, better, Hoare's Communicating Sequential Processes (CSP) or Hehner's Practical Theory of Programming or some programming language with a sophisticated type-system like ATS, Qi or Epigram and so on)? It seems to me that introducing "concepts" - which, as-is, are specific to C++ - is not more simple than using the alternatives. Is it just about jargon and "politics"? (Finally formal methods... but disguised).
Why organizing program modules as a DAG and not as a tree, like David Parnas advocated decades ago in Designing software for ease of extension and contraction? (here a directly accessible .pdf and here slides from a lecture). The work on X-Machines probably is an answer to this question (going even beyond DAGs), but, again, the author seems to speak about a quite conventional program development regime in which Parnas' approach is the only sensible.
If/when I will see the video I will update this answer.