In my experience, forcing a grammar representation to not be left-recursive tends to be unintuitive or cumbersome in many cases.
I am surveying various parser generator frameworks and I am currently reading about Antlr and Boost.Spirit.
Do either of them support left-recursive grammars? In general, which one supports more powerful grammars?
Thanks!
I'll google that for you. Yep, ANTLR 4 supports directly left recursive rules. https://theantlrguy.atlassian.net/wiki/display/ANTLR4/Left-recursive+rules
I am looking for implementation of Prolog extension which handles temporal logic operators. Is there any info about this ?
As temporal logic has been a significant part of logic, I am sure that there must have been discussions about this with respect to prototype or implementation.
I suggest to take a look at Etalis. If it turns out to be overkill (I'm sorry I never really delved inside too much), and you're using SWI-Prolog, see if pack Julian could be a better fit. It's nicely integrated with CLP(FD) library and will leave you full freedom about the semantics of your operators. Of course, it's a 'lower level' approach...
I would start with Carlo's suggestions. But if you're looking only for basic temporal logic operators, the Logtalk library includes an implementation for basic temporal interval relations:
https://logtalk.org/docs/interval_0.html
You can use Logtalk as an extension to most Prolog implementations.
I imagine Scheme (and perhaps Lisp) could be made more `user friendly' by using a different syntax. For example, instead of nested S-expressions with ugly parentheses, one could devise some kind of syntax closer to some of the more widely used languages (e.g. Java-like without needing to define classes).
It's not necessarily a bad thing if it's more verbose. For example, the syntax may require line separators and commas in the places where many people will expect them, and expect explicit return statements. Also, it doesn't seem that difficult to allow some operators to be used infix style (just obey the generally accepted operator preference rules).
And if it doesn't make things too messy, the syntax could even be backwards-compatible, so that in any place where an expression is expected, a normal S-expression between parentheses can be used.
What are your opinions and ideas about this? And does anything like this exist? (I expect it does, but "Scheme" is a worthless google term, I can't find anything!)
Originally, Lisp was planned to use a syntax called M-Expressions, with S-Expressions being only a transitional solution for easier compiler building. When M-Expressions were ready to be introduces, the programmers who had already taken on Lisp just stayed with what they had become accustomed to, and M-Expressions never caught on.
There is an infix notation in Guile, but it's rarely used. A good Lisp programmer doesn't even see the parens anymore, and prefix notation does have its merits...
I think "sweet expressions" might be one of the more thoughtful approaches to getting rid of the parentheses in Lisp. It apparently even supports macros.
http://www.dwheeler.com/readable/sweet-expressions.html
However, I think most people eventually get over the parentheses or use another language.
Take a look at "sweet-expressions", which provides a set of additional abbreviations for traditional s-expressions. They add syntactically-relevant indentation, a way to do infix, and traditional function calls like f(x). Unlike nearly all past efforts to make Lisps readable, sweet-expressions are backwards-compatible (you can freely mix well-formatted s-expressions and sweet-expressions), generic, and homoiconic.
Sweet-expressions were developed on http://readable.sourceforge.net and there is a sample implementation.
For Scheme there is a SRFI for sweet-expresssions: http://srfi.schemers.org/srfi-110/
Try SRFI 49 for size. :-P
(Seriously, though, as Rafe commented, "I don't think anybody wants this".)
Some people consider Python to be a kind of Scheme with infix notation for operators, algebraic notation for functions and which uses a more "java-like" syntax for representing the language. I don't agree with that assessment, but I can see where the idea comes from.
The big problem with changing the notation for Scheme is that macros become very hard to write (to see how hard, take a look at the Nimrod language or Boo). Instead of working directly with the code as lists, you have to parse the input language first. This usually involves constructing an AST (abstract syntax tree) for the language from the input. When working directly with Scheme, this is unnecessary.
However, you might check out the SIX expression syntax in Gambit Scheme. There's a nice set of slides here which contains a discussion of this:
http://www.iro.umontreal.ca/~gambit/Gambit-inside-out.pdf
But don't tell anyone about it! (The inside joke is that someone suggests writing a Lisp without parentheses and with infix notation about once a day, and someone announces an implementation about once a month.)
There are some languages that do exactly that. For instance: Dylan.
Do they typically use non functional aspects of the language (including mutable variables).
Are there other strategies? Could you describe them?
Do they typically use non functional aspects of the language (including mutable variables)
Many do, yes. In case of haskell, which doesn't have mutable variables or functions with side-effects, they use the IO monad (which is haskell's way of encapsulating side-effects)-
Are there other strategies? Could you describe them?
Yes, there are. Look at functional reactive programming.
Wikipedia says Ruby is a functional language, but I'm not convinced. Why or why not?
Whether a language is or is not a functional language is unimportant. Functional Programming is a thesis, best explained by Philip Wadler (The Essence of Functional Programming) and John Hughes (Why Functional Programming Matters).
A meaningful question is, 'How amenable is Ruby to achieving the thesis of functional programming?' The answer is 'very poorly'.
I gave a talk on this just recently. Here are the slides.
Ruby does support higher-level functions (see Array#map, inject, & select), but it is still an imperative, Object-Oriented language.
One of the key characteristics of a functional language it that it avoids mutable state. Functional languages do not have the concept of a variable as you would have in Ruby, C, Java, or any other imperative language.
Another key characteristic of a functional language is that it focuses on defining a program in terms of "what", rather than "how". When programming in an OO language, we write classes & methods to hide the implementation (the "how") from the "what" (the class/method name), but in the end these methods are still written using a sequence of statements. In a functional language, you do not specify a sequence of execution, even at the lowest level.
I most definitely think you can use functional style in Ruby.
One of the most critical aspects to be able to program in a functional style is if the language supports higher order functions... which Ruby does.
That said, it's easy to program in Ruby in a non-functional style as well. Another key aspect of functional style is to not have state, and have real mathematical functions that always return the same value for a given set of inputs. This can be done in Ruby, but it is not enforced in the language like something more strictly functional like Haskell.
So, yeah, it supports functional style, but it also will let you program in a non-functional style as well.
I submit that supporting, or having the ability to program in a language in a functional style does not a functional language make.
I can even write Java code in a functional style if I want to hurt my collegues, and myself a few months weeks on.
Having a functional language is not only about what you can do, such as higher-order functions, first-class functions and currying. It is also about what you cannot do, like side-effects in pure functions.
This is important because it is a big part of the reason why functional programs are, or functional code in generel is, easier to reason about. And when code is easier to reason about, bugs become shallower and float to the conceptual surface where they can be fixed, which in turn gives less buggy code.
Ruby is object-oriented at its core, so even though it has reasonably good support for a functional style, it is not itself a functional language.
That's my non-scientific opinion anyway.
Edit:
In retrospect and with consideration for the fine comments I have recieved to this answer thus far, I think the object-oriented versus functional comparison is one of apples and oranges.
The real differentiator is that of being imparative in execution, or not. Functional languages have the expression as their primary linguistic construct and the order of execution is often undefined or defined as being lazy. Strict execution is possible but only used when needed. In an imparative language, strict execution is the default and while lazy execution is possible, it is often kludgy to do and can have unpredictable results in many edge cases.
Now, that's my non-scientific opinion.
Ruby will have to meet the following requirements in order to be "TRUELY" functional.
Immutable values: once a “variable” is set, it cannot be changed. In Ruby, this means you effectively have to treat variables like constants. The is not fully supported in the language, you will have to freeze each variable manually.
No side-effects: when passed a given value, a function must always return the same result. This goes hand in hand with having immutable values; a function can never take a value and change it, as this would be causing a side-effect that is tangential to returning a result.
Higher-order functions: these are functions that allow functions as arguments, or use functions as the return value. This is, arguably, one of the most critical features of any functional language.
Currying: enabled by higher-order functions, currying is transforming a function that takes multiple arguments into a function that takes one argument. This goes hand in hand with partial function application, which is transforming a multi-argument function into a function that takes less arguments then it did originally.
Recursion: looping by calling a function from within itself. When you don’t have access to mutable data, recursion is used to build up and chain data construction. This is because looping is not a functional concept, as it requires variables to be passed around to store the state of the loop at a given time.
Lazy-evaluation, or delayed-evaluation: delaying processing of values until the moment when it is actually needed. If, as an example, you have some code that generated list of Fibonacci numbers with lazy-evaluation enabled, this would not actually be processed and calculated until one of the values in the result was required by another function, such as puts.
Proposal (Just a thought)
I would be of great to have some kind of definition to have a mode directive to declare files with functional paradigm, example
mode 'functional'
Ruby is a multi-paradigm language that supports a functional style of programming.
Ruby is an object-oriented language, that can support other paradigms (functional, imperative, etc). However, since everything in Ruby is an object, it's primarily an OO language.
example:
"hello".reverse() = "olleh", every string is a string object instance and so on and so forth.
Read up here or here
It depends on your definition of a “functional language”. Personally, I think the term is itself quite problematic when used as an absolute. The are more aspects to being a “functional language” than mere language features and most depend on where you're looking from. For instance, the culture surrounding the language is quite important in this regard. Does it encourage a functional style? What about the available libraries? Do they encourage you to use them in a functional way?
Most people would call Scheme a functional language, for example. But what about Common Lisp? Apart from the multiple-/single-namespace issue and guaranteed tail-call elimination (which some CL implementations support as well, depending on the compiler settings), there isn't much that makes Scheme as a language more suited to functional programming than Common Lisp, and still, most Lispers wouldn't call CL a functional language. Why? Because the culture surrounding it heavily depends on CL's imperative features (like the LOOP macro, for example, which most Schemers would probably frown upon).
On the other hand, a C programmer may well consider CL a functional language. Most code written in any Lisp dialect is certainly much more functional in style than your usual block of C code, after all. Likewise, Scheme is very much an imperative language as compared to Haskell. Therefore, I don't think there can ever be a definite yes/no answer. Whether to call a language functional or not heavily depends on your viewpoint.
Ruby isn't really much of a multi-paradigm language either, I think. Multi-paradigm tends to be used by people wanting to label their favorite language as something which is useful in many different areas.
I'd describe Ruby is an object-oriented scripting language. Yes, functions are first-class objects (sort of), but that doesn't really make it a functional language. IMO, I might add.
Recursion is common in functional programming. Almost any language does support recursion, but recursive algorithms are often ineffective if there is no tail call optimization (TCO).
Functional programming languages are capable of optimizing tail recursion and can execute such code in constant space. Some Ruby implementations do optimize tail recursion, the other don't, but in general Ruby implementations are not required to do TCO. See Does Ruby perform Tail Call Optimization?
So, if you write some Ruby functional style and rely on TCO of some particular implementation, your code may be very ineffective in another Ruby interpreter. I think this is why Ruby is not a functional language (neither is Python).
Strictly speaking, it doesn't make sense to describe a language as "functional"; most languages are capable of functional programming. Even C++ is.
Functional style is more or less a subset of imperative language features, supported with syntactic sugar and some compiler optimizations like immutability and tail-recursion flattening,
The latter arguably is a minor implementation-specific technicality and has nothing to do with the actual language. The x64 C# 4.0 compiler does tail-recursion optimization, whereas the x86 one doesn't for whatever stupid reason.
Syntactic sugar can usually be worked around to some extent or another, especially if the language has a programmable precompiler (i.e. C's #define).
It might be slightly more meaningful to ask, "does language __ support imperative programming?", and the answer, for instance with Lisp, is "no".
Please, have a look at the beginning of the book: "A-Great-Ruby-eBook". It discusses the very specific topic you are asking. You can do different types of programming in Ruby. If you want to program like functionally, you can do it. If you want to program like imperatively, you can do it. It is a definition question how functional Ruby in the end is. Please, see the reply by the user camflan.