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  • Why is C++ backward compatibility important / necessary?

    - by Giorgio
    As far as understand it is a well-established opinion within the C++ community that C is an obsolete language that was useful 20 years ago but cannot support many modern good programming practices, or even encourages bad practices; certain features that were typical of C++ (C with classes) during the nineties are also obsolete and considered bad practice in modern C++ (e.g., new and delete should be replaced by smart pointer primitives). In view of this, I often wonder why backward compatibility with C and obsolete C++ features is still considered important: to my knowledge there is no 100% compatibility, but most of C and C++ are contained in C++11 as a subset. Of course, there is a lot of legacy code and libraries (possibly containing templates) that are written using a previous standard of the language and which still need to be maintained or used in connection with new code. Nevertheless, maybe it would still be possible to drop obsolete C and C++ features (e.g. the mentioned new / delete) from a future C++ standard so that it is impossible to use them in new code. In this way, old and dangerous programming practices would be quickly banned from new code, and modern, better programming practices would be enforced by the compiler. Legacy code could still be maintained using separate compilation (having C alongside C++ source files is already a common practice). Developers would have to choose between one compiler supporting the old-style C++ that was common during the nineties and a compiler supporting the modern C++? style (the question mark indicates a future, hypothetical revision). Only mixing the two styles would be forbidden. Would this be a viable strategy for encouraging the adoption of modern C++ practices? Are there conceptual reasons or technical problems (e.g. compiling existing templates) that make such a change undesirable or even impossible? Has such a development been proposed in the C++ community. If there has been some extended discussion on the topic, is there any material on-line?

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  • Functional programming constructs in non-functional programming languages

    - by Giorgio
    This question has been going through my mind quite a lot lately and since I haven't found a convincing answer to it I would like to know if other users of this site have thought about it as well. In the recent years, even though OOP is still the most popular programming paradigm, functional programming is getting a lot of attention. I have only used OOP languages for my work (C++ and Java) but I am trying to learn some FP in my free time because I find it very interesting. So, I started learning Haskell three years ago and Scala last summer. I plan to learn some SML and Caml as well, and to brush up my (little) knowledge of Scheme. Well, a lot of plans (too ambitious?) but I hope I will find the time to learn at least the basics of FP during the next few years. What is important for me is how functional programming works and how / whether I can use it for some real projects. I have already developed small tools in Haskell. In spite of my strong interest for FP, I find it difficult to understand why functional programming constructs are being added to languages like C#, Java, C++, and so on. As a developer interested in FP, I find it more natural to use, say, Scala or Haskell, instead of waiting for the next FP feature to be added to my favourite non-FP language. In other words, why would I want to have only some FP in my originally non-FP language instead of looking for a language that has a better support for FP? For example, why should I be interested to have lambdas in Java if I can switch to Scala where I have much more FP concepts and access all the Java libraries anyway? Similarly: why do some FP in C# instead of using F# (to my knowledge, C# and F# can work together)? Java was designed to be OO. Fine. I can do OOP in Java (and I would like to keep using Java in that way). Scala was designed to support OOP + FP. Fine: I can use a mix of OOP and FP in Scala. Haskell was designed for FP: I can do FP in Haskell. If I need to tune the performance of a particular module, I can interface Haskell with some external routines in C. But why would I want to do OOP with just some basic FP in Java? So, my main point is: why are non-functional programming languages being extended with some functional concept? Shouldn't it be more comfortable (interesting, exciting, productive) to program in a language that has been designed from the very beginning to be functional or multi-paradigm? Don't different programming paradigms integrate better in a language that was designed for it than in a language in which one paradigm was only added later? The first explanation I could think of is that, since FP is a new concept (it isn't new at all, but it is new for many developers), it needs to be introduced gradually. However, I remember my switch from imperative to OOP: when I started to program in C++ (coming from Pascal and C) I really had to rethink the way in which I was coding, and to do it pretty fast. It was not gradual. So, this does not seem to be a good explanation to me. Also, I asked myself if my impression is just plainly wrong due to lack of knowledge. E.g., do C# and C++11 support FP as extensively as, say, Scala or Caml do? In this case, my question would be simply non-existent. Or can it be that many non-FP programmers are not really interested in using functional programming, but they find it practically convenient to adopt certain FP-idioms in their non-FP language? IMPORTANT NOTE Just in case (because I have seen several language wars on this site): I mentioned the languages I know better, this question is in no way meant to start comparisons between different programming languages to decide which is better / worse. Also, I am not interested in a comparison of OOP versus FP (pros and cons). The point I am interested in is to understand why FP is being introduced one bit at a time into existing languages that were not designed for it even though there exist languages that were / are specifically designed to support FP.

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  • Introducing functional programming constructs in non-functional programming languages

    - by Giorgio
    This question has been going through my mind quite a lot lately and since I haven't found a convincing answer to it I would like to know if other users of this site have thought about it as well. In the recent years, even though OOP is still the most popular programming paradigm, functional programming is getting a lot of attention. I have only used OOP languages for my work (C++ and Java) but I am trying to learn some FP in my free time because I find it very interesting. So, I started learning Haskell three years ago and Scala last summer. I plan to learn some SML and Caml as well, and to brush up my (little) knowledge of Scheme. Well, a lot of plans (too ambitious?) but I hope I will find the time to learn at least the basics of FP during the next few years. What is important for me is how functional programming works and how / whether I can use it for some real projects. I have already developed small tools in Haskell. In spite of my strong interest for FP, I find it difficult to understand why functional programming constructs are being added to languages like C#, Java, C++, and so on. As a developer interested in FP, I find it more natural to use, say, Scala or Haskell, instead of waiting for the next FP feature to be added to my favourite non-FP language. In other words, why would I want to have only some FP in my originally non-FP language instead of looking for a language that has a better support for FP? For example, why should I be interested to have lambdas in Java if I can switch to Scala where I have much more FP concepts and access all the Java libraries anyway? Similarly: why do some FP in C# instead of using F# (to my knowledge, C# and F# can work together)? Java was designed to be OO. Fine. I can do OOP in Java (and I would like to keep using Java in that way). Scala was designed to support OOP + FP. Fine: I can use a mix of OOP and FP in Scala. Haskell was designed for FP: I can do FP in Haskell. If I need to tune the performance of a particular module, I can interface Haskell with some external routines in C. But why would I want to do OOP with just some basic FP in Java? So, my main point is: why are non-functional programming languages being extended with some functional concept? Shouldn't it be more comfortable (interesting, exciting, productive) to program in a language that has been designed from the very beginning to be functional or multi-paradigm? Don't different programming paradigms integrate better in a language that was designed for it than in a language in which one paradigm was only added later? The first explanation I could think of is that, since FP is a new concept (it isn't new at all, but it is new for many developers), it needs to be introduced gradually. However, I remember my switch from imperative to OOP: when I started to program in C++ (coming from Pascal and C) I really had to rethink the way in which I was coding, and to do it pretty fast. It was not gradual. So, this does not seem to be a good explanation to me. Or can it be that many non-FP programmers are not really interested in understanding and using functional programming, but they find it practically convenient to adopt certain FP-idioms in their non-FP language? IMPORTANT NOTE Just in case (because I have seen several language wars on this site): I mentioned the languages I know better, this question is in no way meant to start comparisons between different programming languages to decide which is better / worse. Also, I am not interested in a comparison of OOP versus FP (pros and cons). The point I am interested in is to understand why FP is being introduced one bit at a time into existing languages that were not designed for it even though there exist languages that were / are specifically designed to support FP.

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  • Merge sort versus quick sort performance

    - by Giorgio
    I have implemented merge sort and quick sort using C (GCC 4.4.3 on Ubuntu 10.04 running on a 4 GB RAM laptop with an Intel DUO CPU at 2GHz) and I wanted to compare the performance of the two algorithms. The prototypes of the sorting functions are: void merge_sort(const char **lines, int start, int end); void quick_sort(const char **lines, int start, int end); i.e. both take an array of pointers to strings and sort the elements with index i : start <= i <= end. I have produced some files containing random strings with length on average 4.5 characters. The test files range from 100 lines to 10000000 lines. I was a bit surprised by the results because, even though I know that merge sort has complexity O(n log(n)) while quick sort is O(n^2), I have often read that on average quick sort should be as fast as merge sort. However, my results are the following. Up to 10000 strings, both algorithms perform equally well. For 10000 strings, both require about 0.007 seconds. For 100000 strings, merge sort is slightly faster with 0.095 s against 0.121 s. For 1000000 strings merge sort takes 1.287 s against 5.233 s of quick sort. For 5000000 strings merge sort takes 7.582 s against 118.240 s of quick sort. For 10000000 strings merge sort takes 16.305 s against 1202.918 s of quick sort. So my question is: are my results as expected, meaning that quick sort is comparable in speed to merge sort for small inputs but, as the size of the input data grows, the fact that its complexity is quadratic will become evident? Here is a sketch of what I did. In the merge sort implementation, the partitioning consists in calling merge sort recursively, i.e. merge_sort(lines, start, (start + end) / 2); merge_sort(lines, 1 + (start + end) / 2, end); Merging of the two sorted sub-array is performed by reading the data from the array lines and writing it to a global temporary array of pointers (this global array is allocate only once). After each merge the pointers are copied back to the original array. So the strings are stored once but I need twice as much memory for the pointers. For quick sort, the partition function chooses the last element of the array to sort as the pivot and scans the previous elements in one loop. After it has produced a partition of the type start ... {elements <= pivot} ... pivotIndex ... {elements > pivot} ... end it calls itself recursively: quick_sort(lines, start, pivotIndex - 1); quick_sort(lines, pivotIndex + 1, end); Note that this quick sort implementation sorts the array in-place and does not require additional memory, therefore it is more memory efficient than the merge sort implementation. So my question is: is there a better way to implement quick sort that is worthwhile trying out? If I improve the quick sort implementation and perform more tests on different data sets (computing the average of the running times on different data sets) can I expect a better performance of quick sort wrt merge sort? EDIT Thank you for your answers. My implementation is in-place and is based on the pseudo-code I have found on wikipedia in Section In-place version: function partition(array, 'left', 'right', 'pivotIndex') where I choose the last element in the range to be sorted as a pivot, i.e. pivotIndex := right. I have checked the code over and over again and it seems correct to me. In order to rule out the case that I am using the wrong implementation I have uploaded the source code on github (in case you would like to take a look at it). Your answers seem to suggest that I am using the wrong test data. I will look into it and try out different test data sets. I will report as soon as I have some results.

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  • How to implement early exit / return in Haskell?

    - by Giorgio
    I am porting a Java application to Haskell. The main method of the Java application follows the pattern: public static void main(String [] args) { if (args.length == 0) { System.out.println("Invalid number of arguments."); System.exit(1); } SomeDataType d = getData(arg[0]); if (!dataOk(d)) { System.out.println("Could not read input data."); System.exit(1); } SomeDataType r = processData(d); if (!resultOk(r)) { System.out.println("Processing failed."); System.exit(1); } ... } So I have different steps and after each step I can either exit with an error code, or continue to the following step. My attempt at porting this to Haskell goes as follows: main :: IO () main = do a <- getArgs if ((length args) == 0) then do putStrLn "Invalid number of arguments." exitWith (ExitFailure 1) else do -- The rest of the main function goes here. With this solution, I will have lots of nested if-then-else (one for each exit point of the original Java code). Is there a more elegant / idiomatic way of implementing this pattern in Haskell? In general, what is a Haskell idiomatic way to implement an early exit / return as used in an imperative language like Java?

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  • Why do some programmers think there is a contrast between theory and practice?

    - by Giorgio
    Comparing software engineering with civil engineering, I was surprised to observe a different way of thinking: any civil engineer knows that if you want to build a small hut in the garden you can just get the materials and go build it whereas if you want to build a 10-storey house you need to do quite some maths to be sure that it won't fall apart. In contrast, speaking with some programmers or reading blogs or forums I often find a wide-spread opinion that can be formulated more or less as follows: theory and formal methods are for mathematicians / scientists while programming is more about getting things done. What is normally implied here is that programming is something very practical and that even though formal methods, mathematics, algorithm theory, clean / coherent programming languages, etc, may be interesting topics, they are often not needed if all one wants is to get things done. According to my experience, I would say that while you do not need much theory to put together a 100-line script (the hut), in order to develop a complex application (the 10-storey building) you need a structured design, well-defined methods, a good programming language, good text books where you can look up algorithms, etc. So IMO (the right amount of) theory is one of the tools for getting things done. So my question is why do some programmers think that there is a contrast between theory (formal methods) and practice (getting things done)? Is software engineering (building software) perceived by many as easy compared to, say, civil engineering (building houses)? Or are these two disciplines really different (apart from mission-critical software, software failure is much more acceptable than building failure)?

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  • Can Scala be considered a functional superset of Java?

    - by Giorgio
    Apart from the differences in syntax, can Scala be considered a superset of Java that adds the functional paradigm to the object-oriented paradigm? Or are there any major features in Java for which there is no direct Scala equivalent? With major features I mean program constructs that would force me to heavily rewrite / restructure my code, e.g., if I had to port a Java program to Scala. Or can I expect that, given a Java program, I can port it to Scala almost line-by-line?

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  • KISS principle applied to programming language design?

    - by Giorgio
    KISS ("keep it simple stupid", see e.g. here) is an important principle in software development, even though it apparently originated in engineering. Citing from the wikipedia article: The principle is best exemplified by the story of Johnson handing a team of design engineers a handful of tools, with the challenge that the jet aircraft they were designing must be repairable by an average mechanic in the field under combat conditions with only these tools. Hence, the 'stupid' refers to the relationship between the way things break and the sophistication available to fix them. If I wanted to apply this to the field of software development I would replace "jet aircraft" with "piece of software", "average mechanic" with "average developer" and "under combat conditions" with "under the expected software development / maintenance conditions" (deadlines, time constraints, meetings / interruptions, available tools, and so on). So it is a commonly accepted idea that one should try to keep a piece of software simple stupid so that it easy to work on it later. But can the KISS principle be applied also to programming language design? Do you know of any programming languages that have been designed specifically with this principle in mind, i.e. to "allow an average programmer under average working conditions to write and maintain as much code as possible with the least cognitive effort"? If you cite any specific language it would be great if you could add a link to some document in which this intent is clearly expressed by the language designers. In any case, I would be interested to learn about the designers' (documented) intentions rather than your personal opinion about a particular programming language.

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  • Sort algorithms that work on large amount of data

    - by Giorgio
    I am looking for sorting algorithms that can work on a large amount of data, i.e. that can work even when the whole data set cannot be held in main memory at once. The only candidate that I have found up to now is merge sort: you can implement the algorithm in such a way that it scans your data set at each merge without holding all the data in main memory at once. The variation of merge sort I have in mind is described in this article in section Use with tape drives. I think this is a good solution (with complexity O(n x log(n)) but I am curious to know if there are other (possibly faster) sorting algorithms that can work on large data sets that do not fit in main memory. EDIT Here are some more details, as required by the answers: The data needs to be sorted periodically, e.g. once in a month. I do not need to insert a few records and have the data sorted incrementally. My example text file is about 1 GB UTF-8 text, but I wanted to solve the problem in general, even if the file were, say, 20 GB. It is not in a database and, due to other constraints, it cannot be. The data is dumped by others as a text file, I have my own code to read this text file. The format of the data is a text file: new line characters are record separators. One possible improvement I had in mind was to split the file into files that are small enough to be sorted in memory, and finally merge all these files using the algorithm I have described above.

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  • C++11 support for higher-order list functions

    - by Giorgio
    Most functional programming languages (e.g. Common Lisp, Scheme / Racket, Clojure, Haskell, Scala, Ocaml, SML) support some common higher-order functions on lists, such as map, filter, takeWhile, dropWhile, foldl, foldr (see e.g. Common Lisp, Scheme / Racket, Clojure side-by-side reference sheet, the Haskell, Scala, OCaml, and the SML documentation.) Does C++11 have equivalent standard methods or functions on lists? For example, consider the following Haskell snippet: let xs = [1, 2, 3, 4, 5] let ys = map (\x -> x * x) xs How can I express the second expression in modern standard C++? std::list<int> xs = ... // Initialize the list in some way. std::list<int> ys = ??? // How to translate the Haskell expression? What about the other higher-order functions mentioned above? Can they be directly expressed in C++?

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  • Performance of concurrent software on multicore processors

    - by Giorgio
    Recently I have often read that, since the trend is to build processors with multiple cores, it will be increasingly important to have programming languages that support concurrent programming in order to better exploit the parallelism offered by these processors. In this respect, certain programming paradigms or models are considered well-suited for writing robust concurrent software: Functional programming languages, e.g. Haskell, Scala, etc. The actor model: Erlang, but also available for Scala / Java (Akka), C++ (Theron, Casablanca, ...), and other programming languages. My questions: What is the state of the art regarding the development of concurrent applications (e.g. using multi-threading) using the above languages / models? Is this area still being explored or are there well-established practices already? Will it be more complex to program applications with a higher level of concurrency, or is it just a matter of learning new paradigms and practices? How does the performance of highly concurrent software compare to the performance of more traditional software when executed on multiple core processors? For example, has anyone implemented a desktop application using C++ / Theron, or Java / Akka? Was there a boost in performance on a multiple core processor due to higher parallelism?

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  • How often is seq used in Haskell production code?

    - by Giorgio
    I have some experience writing small tools in Haskell and I find it very intuitive to use, especially for writing filters (using interact) that process their standard input and pipe it to standard output. Recently I tried to use one such filter on a file that was about 10 times larger than usual and I got a Stack space overflow error. After doing some reading (e.g. here and here) I have identified two guidelines to save stack space (experienced Haskellers, please correct me if I write something that is not correct): Avoid recursive function calls that are not tail-recursive (this is valid for all functional languages that support tail-call optimization). Introduce seq to force early evaluation of sub-expressions so that expressions do not grow to large before they are reduced (this is specific to Haskell, or at least to languages using lazy evaluation). After introducing five or six seq calls in my code my tool runs smoothly again (also on the larger data). However, I find the original code was a bit more readable. Since I am not an experienced Haskell programmer I wanted to ask if introducing seq in this way is a common practice, and how often one will normally see seq in Haskell production code. Or are there any techniques that allow to avoid using seq too often and still use little stack space?

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  • What triggered the popularity of lambda functions in modern mainstream programming languages?

    - by Giorgio
    In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon? IMPORTANT NOTE The focus of this question is the introduction of anonymous functions in modern, main-stream (and therefore, maybe with a few exceptions, non functional) languages. Also, note that anonymous functions (blocks) are present in Smalltalk, which is not a functional language, and that normal named functions have been present even in procedural languages like C and Pascal for a long time. Please do not overgeneralize your answers by speaking about "the adoption of the functional paradigm and its benefits", because this is not the topic of the question.

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  • Evolution of mainstream programming languages: simplicity versus complexity.

    - by Giorgio
    I had posted this question on http://stackoverflow.com but I was suggested that it may be more appropriate to post it on this forum. I did a quick search on this site and it seems to me that this question has not been asked yet. Please give me a hint if the topic has been raised already by someone else. Update I have rephrased this question, removed personal opinions and made it shorter. I hope in this way it is better suited for this forum. By looking at the recent development of Java (Java 7) and C++ (C++0x) I see that new features are added to these languages. For sure this makes it easier to use certain programming idioms, adding to the productivity of developers. On the other hand, there might be the following risks A language becomes too big, complex, and difficult to understand. It lacks coherence in the design, e.g. if it mixes different paradigms like object-orientation and functional programming, which might not fit well together. Questions: what is more important to you as a developer: to have a rich language that captures a large collection of programming idioms or to have a small language that aims at coherence and simplicity (of course, with a good deal of libraries and tools accompanying it)? Or is it possible to have both? With respect to these issues: How do you judge the current evolutions of main-stream programming languages like Java or C++? Are they becoming too complex, less intuitive? Do they have enough features? Do they need more? Are they still easy enough to understand and use?

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  • What kinds of projects is SCRUM considered to be suitable for?

    - by Giorgio
    Is SCRUM considered by its proponents a general-purpose software development methodology or is it considered especially suited for certain categories of projects or application areas? For example, I recently looked at the website of a company producing software for the aerospatial industry and noticed that they are using the V-model. Would a SCRUM proponent say that SCRUM is not suited for this kind of projects or rather suggest that this company should try switching to SCRUM? Notice that I am not asking for the opinion of the readers of this forum, but I want to know what is the established opinion among SCRUM proposers: is SCRUM considered general-purpose or rather suitable for certain classes of projects only? In the latter cases, for what kinds of projects?

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  • Actor library / framework for C++

    - by Giorgio
    In the C++ project I am working for we would like to use something like Scala actors and remote actors (see e.g. this tutorial). Being able to use remote actors (actors living in different processes, possibly on different machines and communicating via TCP/IP) has higher priority for us because we have an application consisting of several processes deployed on different machines. Being able to use several actors living in the same process (possibly different threads) is also interesting, but has lower priority for the moment. On wikipedia I have found some links to actor libraries for C++ and I have started to look at Theron. Before I dive too deep into the details and build an extended example with Theron, I wanted to ask if anybody has experience with any of these libraries and which one they would recommend.

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  • Change user name

    - by Giorgio
    Recently i've added new accounts in my PC, and for one of those need to change name: I've try to change it by "graphical procedure" from "account manager menu" but change only the name in LogIn screen and near the clock. If I open a Terminal the name remain the old one. I need to have this situation from Old name: [email protected]:~$ ----- I want obtain New name: [email protected]:~$ o How i can do this without create a new one user and transfer all data? Can you explain me step by step the procedure?

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  • How to implement child-parent aggregation link in C++?

    - by Giorgio
    Suppose that I have three classes P, C1, C2, composition (strong aggregation) relations between P <>- C1 and P <>- C2, i.e. every instance of P contains an instance of C1 and an instance of C2, which are destroyed when the parent P instance is destroyed. an association relation between instances of C1 and C2 (not necessarily between children of the same P). To implement this, in C++ I normally define three classes P, C1, C2, define two member variables of P of type boost::shared_ptr<C1>, boost::shared_ptr<C2>, and initialize them with newly created objects in P's constructor, implement the relation between C1 and C2 using a boost::weak_ptr<C2> member variable in C1 and a boost::weak_ptr<C1> member variable in C2 that can be set later via appropriate methods, when the relation is established. Now, I also would like to have a link from each C1 and C2 object to its P parent object. What is a good way to implement this? My current idea is to use a simple constant raw pointer (P * const) that is set from the constructor of P (which, in turn, calls the constructors of C1 and C2), i.e. something like: class C1 { public: C1(P * const p, ...) : paren(p) { ... } private: P * const parent; ... }; class P { public: P(...) : childC1(new C1(this, ...)) ... { ... } private: boost::shared_ptr<C1> childC1; ... }; Honestly I see no risk in using a private constant raw pointer in this way but I know that raw pointers are often frowned upon in C++ so I was wondering if there is an alternative solution.

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  • What are the typical applications of Lisp macros?

    - by Giorgio
    I am trying to learn some LISP and I have read a lot about the importance of LISP macros so I would like to get some working experience with them. Can you suggest a practical application area that would allow me to use macros to solve a real-world problem, and to understand the usefulness of this programming construct? NOTE This is not a generic what project should I do next question. I am interested to understand which kinds of problems are typically solved by means of LISP macros. E.g., are they good for implementing abstract data types? Why was this construct added to the language? What kinds of problems does it solve that cannot be solved by means of simple functions?

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  • Misconceptions about purely functional languages?

    - by Giorgio
    I often encounter the following statements / arguments: Pure functional programming languages do not allow side effects (and are therefore of little use in practice because any useful program does have side effects, e.g. when it interacts with the external world). Pure functional programming languages do not allow to write a program that maintains state (which makes programming very awkward because in many application you do need state). I am not an expert in functional languages but here is what I have understood about these topics until now. Regarding point 1, you can interact with the environment in purely functional languages but you have to explicitly mark the code (functions) that introduces them (e.g. in Haskell by means of monadic types). Also, AFAIK computing by side effects (destructively updating data) should also be possible (using monadic types?) but is not the preferred way of working. Regarding point 2, AFAIK you can represent state by threading values through several computation steps (in Haskell, again, using monadic types) but I have no practical experience doing this and my understanding is rather vague. So, are the two statements above correct in any sense or are they just misconceptions about purely functional languages? If they are misconceptions, how did they come about? Could you write a (possibly small) code snippet illustrating the Haskell idiomatic way to (1) implement side effects and (2) implement a computation with state?

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  • Are closures with side-effects considered "functional style"?

    - by Giorgio
    Many modern programming languages support some concept of closure, i.e. of a piece of code (a block or a function) that Can be treated as a value, and therefore stored in a variable, passed around to different parts of the code, be defined in one part of a program and invoked in a totally different part of the same program. Can capture variables from the context in which it is defined, and access them when it is later invoked (possibly in a totally different context). Here is an example of a closure written in Scala: def filterList(xs: List[Int], lowerBound: Int): List[Int] = xs.filter(x => x >= lowerBound) The function literal x => x >= lowerBound contains the free variable lowerBound, which is closed (bound) by the argument of the function filterList that has the same name. The closure is passed to the library method filter, which can invoke it repeatedly as a normal function. I have been reading a lot of questions and answers on this site and, as far as I understand, the term closure is often automatically associated with functional programming and functional programming style. The definition of function programming on wikipedia reads: In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state. and further on [...] in functional code, the output value of a function depends only on the arguments that are input to the function [...]. Eliminating side effects can make it much easier to understand and predict the behavior of a program, which is one of the key motivations for the development of functional programming. On the other hand, many closure constructs provided by programming languages allow a closure to capture non-local variables and change them when the closure is invoked, thus producing a side effect on the environment in which they were defined. In this case, closures implement the first idea of functional programming (functions are first-class entities that can be moved around like other values) but neglect the second idea (avoiding side-effects). Is this use of closures with side effects considered functional style or are closures considered a more general construct that can be used both for a functional and a non-functional programming style? Is there any literature on this topic? IMPORTANT NOTE I am not questioning the usefulness of side-effects or of having closures with side effects. Also, I am not interested in a discussion about the advantages / disadvantages of closures with or without side effects. I am only interested to know if using such closures is still considered functional style by the proponent of functional programming or if, on the contrary, their use is discouraged when using a functional style.

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  • Can higher-order functions in FP be interpreted as some kind of dependency injection?

    - by Giorgio
    According to this article, in object-oriented programming / design dependency injection involves a dependent consumer, a declaration of a component's dependencies, defined as interface contracts, an injector that creates instances of classes that implement a given dependency interface on request. Let us now consider a higher-order function in a functional programming language, e.g. the Haskell function filter :: (a -> Bool) -> [a] -> [a] from Data.List. This function transforms a list into another list and, in order to perform its job, it uses (consumes) an external predicate function that must be provided by its caller, e.g. the expression filter (\x -> (mod x 2) == 0) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] selects all even numbers from the input list. But isn't this construction very similar to the pattern illustrated above, where the filter function is the dependent consumer, the signature (a -> Bool) of the function argument is the interface contract, the expression that uses the higher-order is the injector that, in this particular case, injects the implementation (\x -> (mod x 2) == 0) of the contract. More in general, can one relate higher-order functions and their usage pattern in functional programming to the dependency injection pattern in object-oriented languages? Or in the inverse direction, can dependency injection be compared to using some kind of higher-order function?

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  • Programming languages with a Lisp-like syntax extension mechanism

    - by Giorgio
    I have only a limited knowledge of Lisp (trying to learn a bit in my free time) but as far as I understand Lisp macros allow to introduce new language constructs and syntax by describing them in Lisp itself. This means that a new construct can be added as a library, without changing the Lisp compiler / interpreter. This approach is very different from that of other programming languages. E.g., if I wanted to extend Pascal with a new kind of loop or some particular idiom I would have to extend the syntax and semantics of the language and then implement that new feature in the compiler. Are there other programming languages outside the Lisp family (i.e. apart from Common Lisp, Scheme, Clojure (?), Racket (?), etc) that offer a similar possibility to extend the language within the language itself? EDIT Please, avoid extended discussion and be specific in your answers. Instead of a long list of programming languages that can be extended in some way or another, I would like to understand from a conceptual point of view what is specific to Lisp macros as an extension mechanism, and which non-Lisp programming languages offer some concept that is close to them.

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  • Motivation and use of move constructors in C++

    - by Giorgio
    I recently have been reading about move constructors in C++ (see e.g. here) and I am trying to understand how they work and when I should use them. As far as I understand, a move constructor is used to alleviate the performance problems caused by copying large objects. The wikipedia page says: "A chronic performance problem with C++03 is the costly and unnecessary deep copies that can happen implicitly when objects are passed by value." I normally address such situations by passing the objects by reference, or by using smart pointers (e.g. boost::shared_ptr) to pass around the object (the smart pointers get copied instead of the object). What are the situations in which the above two techniques are not sufficient and using a move constructor is more convenient?

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  • What triggered the popularity of lambda functions in modern programming languages?

    - by Giorgio
    In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon?

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