They are both about classifying which concepts that a language can describe the language can also abstract over.Ī concept is first-class if the usual abstraction mechanisms of a language can abstract over that concept.įor example, the Java programming language can describe integers, and all the usual mechanisms for abstracting over integers (accepting them as method parameters, returning them as function results, storing them in data structures. ![]() No, the word "first" in "first-class" and in "first-order" mean different things.īut yes, the concepts of "first-class" and "first-order" are related. Scott is a leading researcher in Computer Sci, so will understand and be very happy using the formal maths, but like the best researchers he is skill in explaining the application of the research to the rest of us. His terminology is not used by other people, so it not useful for communicating, but it does gives you a good foundation on the questions you should ask when seeing a new programming language for the first time. So Michearl Scott uses informal terminology that has some relationship to formal mathematics, while presenting the subject in a way that most programmers can benefit from. If you read the specification for most programming language, you will notice a dissident lack of Denotational Semantics, however most well designed languages had someone on the team that is an expert in programming language design, hence understands Denotational Semantics well. I would have learned at lot more if the course was based Scott's Programming Language Pragmatics, however the formal maths is needed for someone that is going to do a PHd in programming language design. ![]() You can choose to use a well defined mathematically foundation, or you can use informal terminology like “first-class status”. It was explained so well in my Computer Sci Degree that I got a top mark in the Denotational Semantics exam, then forgot most of it and have never had a need to use it in a 20 year life as a programmer. In the real-world, the important thing to know is how you can pass code around, treating it like data: functors, lambdas/closures, function pointers, etc.ĭenotational Semantics provides a mathematic bases for describing how values and variables works in a programming language. I am not sure knowing the "class" of value is all that important, except maybe in an academic environment. Some of these may be first, second, or third class values depending on the language. Then add in the keywords, symbols that are functions, etc. Think l-value, r-value, and the other whole mess of value types in C++. Typically, each token has a specific type that may or may not be compatible with other types. Mathematically, I think the best way is to think of a program's AST. What is otherwise a lower-class of value can be treated as a first-class value. The idea there is to create a whole object that, on the surface, appears to behave like a function and can be passed around as if it were data. C++ also has the syntactic sugar of functors. Languages such as C and C++ allow you to take the address of functions: while you cannot modify them, you can pass functions as parameters to other functions. Functional languages are famous for this: some of them let you change the code of the program as it is running (the foundation of genetic programming). In some languages you can treat code as data. But this is a really good approximation that holds for real-world languages.) (roughly speaking: I am sure there are exceptions. I agree with DeadMG, the important distinction is between first-class and "everything else." However, there is a more familiar way to classify the difference.įirst-class values are data, the others are code. Why are the specific cases important, while not other cases not It seems to me that the difference between them is which specific The terminology reminds me of first/second order logic, but are they What is the mathematics foundation for first/second/third class Second-class values in most other imperative languages, and Including Fortran, Modula-2 and -3, Ada 95, C, and C++. With some restrictions, in several other imperative languages, They are also first-class values in C# and, They are first-class values in all functional programming languages and Labels are third-class values in most programming languages, but ![]() ![]() Variable, and a “third-class” value cannot even be passed as a Parameter, but not returned from a subroutine or assigned into a By contrast, a “second-class” value can be passed as a Integers and characters are first-class values in most programming first-class status if it can be passed as a parameter, returned from a subroutine, or assigned into a variable. In general, a value in a programming language is said to have In programming languages, from Michael Scott's Programming Language Pragmatics
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |