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  • A deque based on binary trees

    - by Greg Ros
    This is a simple immutable deque based on binary trees. What do you think about it? Does this kind of data structure, or possibly an improvement thereof, seem useful? How could I improve it, preferably without getting rid of its strengths? (Not in the sense of more operations, in the sense of different design) Does this sort of thing have a name? Red nodes are newly instantiated; blue ones are reused. Nodes aren't actually red or anything, it's just for emphasis.

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  • Critique of the IO monad being viewed as a state monad operating on the world

    - by Petr Pudlák
    The IO monad in Haskell is often explained as a state monad where the state is the world. So a value of type IO a monad is viewed as something like worldState -> (a, worldState). Some time ago I read an article (or a blog/mailing list post) that criticized this view and gave several reasons why it's not correct. But I cannot remember neither the article nor the reasons. Anybody knows? Edit: The article seems lost, so let's start gathering various arguments here. I'm starting a bounty to make things more interesting.

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  • Link between tests and user stories

    - by Sardathrion
    I have not see these links explicitly stated in the Agile literature I have read. So, I was wondering if this approach was correct: Let a story be defined as "In order to [RESULT], [ROLE] needs to [ACTION]" then RESULT generates system tests. ROLE generates acceptance tests. ACTION generates component and unit tests. Where the definitions are the ones used in xUnit Patterns which to be fair are fairly standard. Is this a correct interpretation or did I misunderstand something?

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  • Resources for improving your comprehension of recursion?

    - by Andrew M
    I know what recursion is (when a patten reoccurs within itself, typically a function that calls itself on one of its lines, after a breakout conditional... right?), and I can understand recursive functions if I study them closely. My problem is, when I see new examples, I'm always initially confused. If I see a loop, or a mapping, zipping, nesting, polymorphic calling, and so on, I know what's going just by looking at it. When I see recursive code, my thought process is usually 'wtf is this?' followed by 'oh it's recursive' followed by 'I guess it must work, if they say it does.' So do you have any tips/plans/resources for building up your skills in this area? Recursion is kind of a wierd concept so I'm thinking the way to tackle it may be equally wierd and inobvious.

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  • Internal Mutation of Persistent Data Structures

    - by Greg Ros
    To clarify, when I mean use the terms persistent and immutable on a data structure, I mean that: The state of the data structure remains unchanged for its lifetime. It always holds the same data, and the same operations always produce the same results. The data structure allows Add, Remove, and similar methods that return new objects of its kind, modified as instructed, that may or may not share some of the data of the original object. However, while a data structure may seem to the user as persistent, it may do other things under the hood. To be sure, all data structures are, internally, at least somewhere, based on mutable storage. If I were to base a persistent vector on an array, and copy it whenever Add is invoked, it would still be persistent, as long as I modify only locally created arrays. However, sometimes, you can greatly increase performance by mutating a data structure under the hood. In more, say, insidious, dangerous, and destructive ways. Ways that might leave the abstraction untouched, not letting the user know anything has changed about the data structure, but being critical in the implementation level. For example, let's say that we have a class called ArrayVector implemented using an array. Whenever you invoke Add, you get a ArrayVector build on top of a newly allocated array that has an additional item. A sequence of such updates will involve n array copies and allocations. Here is an illustration: However, let's say we implement a lazy mechanism that stores all sorts of updates -- such as Add, Set, and others in a queue. In this case, each update requires constant time (adding an item to a queue), and no array allocation is involved. When a user tries to get an item in the array, all the queued modifications are applied under the hood, requiring a single array allocation and copy (since we know exactly what data the final array will hold, and how big it will be). Future get operations will be performed on an empty cache, so they will take a single operation. But in order to implement this, we need to 'switch' or mutate the internal array to the new one, and empty the cache -- a very dangerous action. However, considering that in many circumstances (most updates are going to occur in sequence, after all), this can save a lot of time and memory, it might be worth it -- you will need to ensure exclusive access to the internal state, of course. This isn't a question about the efficacy of such a data structure. It's a more general question. Is it ever acceptable to mutate the internal state of a supposedly persistent or immutable object in destructive and dangerous ways? Does performance justify it? Would you still be able to call it immutable? Oh, and could you implement this sort of laziness without mutating the data structure in the specified fashion?

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  • Compute if a function is pure

    - by Oni
    As per Wikipedia: In computer programming, a function may be described as pure if both these statements about the function hold: The function always evaluates the same result value given the same argument value(s). The function result value cannot depend on any hidden information or state that may change as program execution proceeds or between different executions of the program, nor can it depend on any external input from I/O devices. Evaluation of the result does not cause any semantically observable side effect or output, such as mutation of mutable objects or output to I/O devices. I am wondering if it is possible to write a function that compute if a function is pure or not. Example code in Javascript: function sum(a,b) { return a+b; } function say(x){ console.log(x); } isPure(sum) // True isPure(say) // False

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  • Object-Oriented equivalent of LISP's progn function?

    - by Archer
    I'm currently writing a LISP parser that iterates through some AutoLISP code and does its best to make it a little easier to read (changing prefix notation to infix notation, changing setq assignments to "=" assignments, etc.) for those that aren't used to LISP code/only learned object oriented programming. While writing commands that LISP uses to add to a "library" of LISP commands, I came across the LISP command "progn". The only problem is that it looks like progn is simply executing code in a specific order and sometimes (not usually) assigning the last value to a variable. Am I incorrect in assuming that for translating progn to object-oriented understanding that I can simply forgo the progn function and print the statements that it contains? If not, what would be a good equivalent for progn in an object-oriented language?

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  • How to handle mutiple API calls using javascript/jquery

    - by James Privett
    I need to build a service that will call multiple API's at the same time and then output the results on the page (Think of how a price comparison site works for example). The idea being that as each API call completes the results are sent to the browser immediately and the page would get progressively bigger until all process are complete. Because these API calls may take several seconds each to return I would like to do this via javascript/jquery in order to create a better user experience. I have never done anything like this before using javascript/jquery so I was wondering if there was any frameworks/advice that anyone would be willing to share.

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  • How to have a maintainable and manageable Javascript code base

    - by dade
    I am starting a new job soon as a frontend developer. The App I would be working on is 100% Javascript on the client side. all the server returns is an index page that loads all the Javascript files needed by the app. Now here is the problem: The whole of the application is built around having functions wrapped to different namespaces. And from what I see, a simple function like rendering the HTML of a page can be accomplished by having a call to 2 or more functions across different namespace... My initial thought was "this does not feel like the perfect solution" and I can just envisage a lot of issues with maintaining the code and extending it down the line. Now I would soon start working on taking the project forward and would like to have suggestions on good case practices when it comes to writing and managing a relatively large amount of javascript code.

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  • A programming language that does not allow IO. Haskell is not a pure language

    - by TheIronKnuckle
    (I asked this on Stack Overflow and it got closed as off-topic, I was a bit confused until I read the FAQ, which discouraged subjective theoratical debate style questions. The FAQ here doesn't seem to have a problem with it and it sounds like this is a more appropriate place to post. If this gets closed again, forgive me, I'm not trying to troll) Are there any 100% pure languages (as I describe in the Stack Overflow post) out there already and if so, could they feasibly be used to actually do stuff? i.e. do they have an implementation? I'm not looking for raw maths on paper/Pure lambda calculus. However Pure lambda calculus with a compiler or a runtime system attached is something I'd be interested in hearing about.

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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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  • How to write functionally in a web framework

    - by Kevin Burke
    I love Rich Hickey, Clojure and Haskell and I get it when he talks about functions and the unreliability of side-effecting code. However I work in an environment where nearly all the functions I write have to read from the database, write to the database, make HTTP requests, decrement a user's balance, modify a frontend HTML component based on a click action, return different results based on the URI or the POST body. We also use PHP for the frontend, which is littered with functions like parse_str(), which modifies an object in place. All of these are side-effecting to one degree or another. Given these constraints and the side-effecting nature of the logic I'm coding, what can I do to make my code more reliable and function-able?

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  • How is intermediate data organized in MapReduce?

    - by Pedro Cattori
    From what I understand, each mapper outputs an intermediate file. The intermediate data (data contained in each intermediate file) is then sorted by key. Then, a reducer is assigned a key by the master. The reducer reads from the intermediate file containing the key and then calls reduce using the data it has read. But in detail, how is the intermediate data organized? Can a data corresponding to a key be held in multiple intermediate files? What happens when there is too much data corresponding to one key to be held by a single file? In short, how do intermediate partitions differ from intermediate files and how are these differences dealt with in the implementation?

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  • What is the value of a let expression

    - by Grzegorz Slawecki
    From what I understand, every code in f# is an expression, including let binding. Say we got the following code: let a = 5 printfn "%d" a I've read that this would be seen by the compiler as let a = 5 in ( printfn "%d" a ) And so the value of all this would be value of inner expression, which is value of printf. On the other hand, in f# interactive: > let a = 5;; val a : int = 5 Which clearly indicates that the value of let expression is the value bound to the identifier. Q: Can anyone explain what is the value of a let expression? Can it be different in compiled code than in F# interactive?

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  • Non-trivial functions that operate on any monad

    - by Strilanc
    I'm looking for examples of interesting methods that take an arbitrary monad and do something useful with it. Monads are extremely general, so methods that operate on monads are widely applicable. On the other hand, methods I know of that can apply to any monad tend to be... really, really trivial. Barely worth extracting into a function. Here's a really boring example: joinTwice. It just flattens an m m m t into an m t: join n = n >>= id joinTwice n = (join . join) n main = print (joinTwice [[[1],[2, 3]], [[4]]]) -- prints [1,2,3,4] The only non-trivial method for monads that I know of is bindFold (see my answer below). Are there more?

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  • Immutable design with an ORM: How are sessions managed?

    - by Programmin Tool
    If I were to make a site with a mutable language like C# and use NHibernate, I would normally approach sessions with the idea of making them as create only when needed and dispose at request end. This has helped with keeping a session for multiple transactions by a user but keep it from staying open too long where the state might be corrupted. In an immutable system, like F#, I would think I shouldn't do this because it supposes that a single session could be updated constantly by any number of inserts/updates/deletes/ect... I'm not against the "using" solution since I would think that connecting pooling will help cut down on the cost of connecting every time, but I don't know if all database systems do connection pooling. It just seems like there should be a better way that doesn't compromise the immutability goal. Should I just do a simple "using" block per transaction or is there a better pattern for this?

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  • FP for simulation and modelling

    - by heaptobesquare
    I'm about to start a simulation/modelling project. I already know that OOP is used for this kind of projects. However, studying Haskell made me consider using the FP paradigm for modelling a system of components. Let me elaborate: Let's say I have a component of type A, characterised by a set of data (a parameter like temperature or pressure,a PDE and some boundary conditions,etc.) and a component of type B, characterised by a different set of data(different or same parameter, different PDE and boundary conditions). Let's also assume that the functions/methods that are going to be applied on each component are the same (a Galerkin method for example). If I were to use an OOP approach, I would create two objects that would encapsulate each type's data, the methods for solving the PDE(inheritance would be used here for code reuse) and the solution to the PDE. On the other hand, if I were to use an FP approach, each component would be broken down to data parts and the functions that would act upon the data in order to get the solution for the PDE. This approach seems simpler to me assuming that linear operations on data would be trivial and that the parameters are constant. What if the parameters are not constant(for example, temperature increases suddenly and therefore cannot be immutable)? In OOP, the object's (mutable) state can be used. I know that Haskell has Monads for that. To conclude, would implementing the FP approach be actually simpler,less time consuming and easier to manage (add a different type of component or new method to solve the pde) compared to the OOP one? I come from a C++/Fortran background, plus I'm not a professional programmer, so correct me on anything that I've got wrong.

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  • Learning how to design knowledge and data flow [closed]

    - by max
    In designing software, I spend a lot of time deciding how the knowledge (algorithms / business logic) and data should be allocated between different entities; that is, which object should know what. I am asking for advice about books, articles, presentations, classes, or other resources that would help me learn how to do it better. I code primarily in Python, but my question is not really language-specific; even if some of the insights I learn don't work in Python, that's fine. I'll give a couple examples to clarify what I mean. Example 1 I want to perform some computation. As a user, I will need to provide parameters to do the computation. I can have all those parameters sent to the "main" object, which then uses them to create other objects as needed. Or I can create one "main" object, as well as several additional objects; the additional objects would then be sent to the "main" object as parameters. What factors should I consider to make this choice? Example 2 Let's say I have a few objects of type A that can perform a certain computation. The main computation often involves using an object of type B that performs some interim computation. I can either "teach" A instances what exact parameters to pass to B instances (i.e., make B "dumb"); or I can "teach" B instances to figure out what needs to be done when looking at an A instance (i.e., make B "smart"). What should I think about when I'm making this choice?

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  • Guidelines for creating referentially transparent callables

    - by max
    In some cases, I want to use referentially transparent callables while coding in Python. My goals are to help with handling concurrency, memoization, unit testing, and verification of code correctness. I want to write down clear rules for myself and other developers to follow that would ensure referential transparency. I don't mind that Python won't enforce any rules - we trust ourselves to follow them. Note that we never modify functions or methods in place (i.e., by hacking into the bytecode). Would the following make sense? A callable object c of class C will be referentially transparent if: Whenever the returned value of c(...) depends on any instance attributes, global variables, or disk files, such attributes, variables, and files must not change for the duration of the program execution; the only exception is that instance attributes may be changed during instance initialization. When c(...) is executed, no modifications to the program state occur that may affect the behavior of any object accessed through its "public interface" (as defined by us). If we don't put any restrictions on what "public interface" includes, then rule #2 becomes: When c(...) is executed, no objects are modified that are visible outside the scope of c.__call__. Note: I unsuccessfully tried to ask this question on SO, but I'm hoping it's more appropriate to this site.

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  • Why does Scala require functions to have explicit return type?

    - by garbage collection
    I recently began learning to program in Scala, and it's been fun so far. I really like the ability to declare functions within another function which just seems to intuitive thing to do. One pet peeve I have about Scala is the fact that Scala requires explicit return type in its functions. And I feel like this hinders on expressiveness of the language. Also it's just difficult to program with that requirement. Maybe it's because I come from Javascript and Ruby comfort zone. But for a language like Scala which will have tons of connected functions in an application, I cannot conceive how I brainstorm in my head exactly what type the particular function I am writing should return with recursions after recursions. This requirement of explicit return type declaration on functions, do not bother me for languages like Java and C++. Recursions in Java and C++, when they did happen, often were dealt with 2 to 3 functions max. Never several functions chained up together like Scala. So I guess I'm wondering if there is a good reason why Scala should have the requirement of functions having explicit return type?

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  • Expected time for lazy evaluation with nested functions?

    - by Matt_JD
    A colleague and I are doing a free R course, although I believe this is a more general lazy evaluation issue, and have found a scenario that we have discussed briefly and I'd like to find out the answer from a wider community. The scenario is as follows (pseudo code): wrapper => function(thing) { print => function() { write(thing) } } v = createThing(1, 2, 3) w = wrapper(v) v = createThing(4, 5, 6) w.print() // Will print 4, 5, 6 thing. v = create(7, 8, 9) w.print() // Will print 4, 5, 6 because "thing" has now been evaluated. Another similar situation is as follows: // Using the same function as above v = createThing(1, 2, 3) v = wrapper(v) w.print() // The wrapper function incestuously includes itself. Now I understand why this happens but where my colleague and I differ is on what should happen. My colleague's view is that this is a bug and the evaluation of the passed in argument should be forced at the point it is passed in so that the returned "w" function is fixed. My view is that I would prefer his option myself, but that I realise that the situation we are encountering is down to lazy evaluation and this is just how it works and is more a quirk than a bug. I am not actually sure of what would be expected, hence the reason I am asking this question. I think that function comments could express what will happen, or leave it to be very lazy, and if the coder using the function wants the argument evaluated then they can force it before passing it in. So, when working with lazy evaulation, what is the practice for the time to evaluate an argument passed, and stored, inside a function?

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  • Why is it good not to rely on changing state?

    - by Slomojo
    This question arises out of the question Is Haskell worth learning? Generally a few often repeated statements are made, about how Haskell improves your coding skills in other languages, and furthermore, this is because Haskell is stateless, and that's a good thing. Why? I've seen someone compare this to only typing with the left hand, or perhaps closing your eyes for a day and just relying on touch. Surely there is more to it than that? Does it relate to hardware memory access, or something else which is a big performance gain?

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  • Map multiple functions over a single data item

    - by Linus Norton
    I'm in the process of learning Scala and I came across a scenario today where I need to map multiple functions over a single piece of data and wondered if there was a formal name for this. It sort of feels like the inverse of map. I'm not sure this is the correct way of expressing it, but this is what I did: dmap(x: Object, fns: List[Function]) = fns.map(_(x)) Is there a built in way to do something similar? Is there a formal name for this function?

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  • What is the most compatible, widely used production language to export knowledge and skills gained from Haskell?

    - by World Engineer
    I like Haskell, plain and simple. While Haskell is used in production software, it's not especially widely deployed from what I've seen. What is the most similar and still widely used language in regards to production projects so that I might have a snowball's chance of using something similarly awesome in industry? Also is the same language from the first part available on large numbers of platforms? If not, what is the best alternative that has wide platform deployment? I'd like a single language to put on my to-do list rather than a massive swarm or family. Hard evidence would be a plus.

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  • How can I resolve gstreamer dependencies in Ubuntu

    - by michael
    Hi, Can you please tell me how can I resolve these dependencies on ubuntu: checking for GSTREAMER... configure: error: Package requirements (gstreamer-0.10 >= 0.10 gstreamer-app-0.10 gstreamer-base-0.10 gstreamer-pbutils-0.10 gstreamer-plugins-base-0.10 >= 0.10.25 gstreamer-video-0.10) were not met: No package 'gstreamer-app-0.10' found No package 'gstreamer-pbutils-0.10' found No package 'gstreamer-plugins-base-0.10' found No package 'gstreamer-video-0.10' found I have tried: $ sudo apt-get install *gstreamer-video* Reading package lists... Done Building dependency tree Reading state information... Done E: Regex compilation error - Invalid preceding regular expression $ sudo apt-get install *gstreamer-app* Reading package lists... Done Building dependency tree Reading state information... Done E: Regex compilation error - Invalid preceding regular expression $ sudo apt-get install *gstreamer-base* Reading package lists... Done Building dependency tree Reading state information... Done E: Regex compilation error - Invalid preceding regular expression

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