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Top Answer
Answered Jan 26, 2021 · 130 votes
Your example of declarative programming above is not an actual program, so it's not a good example.
The main difference is between imperative and declarative. Functional is a particular kind of declarative.
C, C++, Java, Javascript, BASIC, Python, Ruby, and most other programming languages are imperative. As a rule, if it has explicit loops (for, while, repeat) that change variables with explicit assignment operations at each loop, then it's imperative.
SQL and XSLT are two well-known examples of declarative programming. Markup languages such as HTML and CSS are declarative too, although they are usually not powerful enough to describe arbitrary algorithms.
Here is an example computation (summing the income by gender, from a suitable data source) first written in an imperative language (Javascript) and then in a declarative language (SQL).
Imperative programming
var income_m = 0, income_f = 0;for (var i = 0; i < income_list.length; i++) { if (income_list[i].gender == 'M') income_m += income_list[i].income; else income_f += income_list[i].income;}-
Notice:
- explicit initialization of variables that will contain the running totals;
- explicit loop over the data, modifying the control variable ( i) and the running totals at each iteration;
- conditionals ( ifs) are only used to choose the code path at each iteration.
Declarative programming
select gender, sum(income)from income_listgroup by gender;-
Notice:
- memory cells to contain running totals are implied by the output you declare you want;
- any loop the CPU will need to perform (eg. over the income_list table) is implied by the output you declare you want and by the structure of the source data;
- conditionals (eg. case in SQL) are used in a functional way to specify the output value you want based on the input values, not to choose a code path.
Functional programming
As I mentioned above, SQL's case is a great example of the functional way of programming, which is a restricted subset of Declarative programming in which the desired computation is specified by composing functions.
Functions are things that accept inputs and return outputs (eg. case, sum()…)
Composition means chaining two or more together by specifying how the output of one is fed as the input to the next (typically by writing one inside the other.) Finally the whole composition, which is still itself a big function, is applied to the available inputs to get the desired output.
In this snippet I am declaring the output I want by composing the functions sum() and case. This is called functional programming:
select sum(case when some_flag = 'X' then some_column else some_other_column end)from ...-
If the composition of two or more functions and their application to the input data are the only constructs available in a given language, that language is said to be purely functional. In those languages you will notice the complete absence of loops, variable assignment and other typically imperative statements.
Edit: I recommend watching some of Anjana Vakil's talks on functional programming in Javascript, to get a better idea of what it's about.
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Top Answer
Answered Mar 30, 2022 · 2 votes
This question seems to originate from the misunderstanding that there's no state in Functional Programming (FP). While this notion is understandable, it's not true.
In short, FP is an approach to programming that makes an explicit distinction between pure functions and everything else (often called impure actions). Simon Peyton-Jones (SPJ, one of the core Haskell developers) once gave a lecture where he said something to the effect that if you couldn't have any side effects, the only thing you could do with a pure function would be to heat the CPU, whereafter one student remarked that that would also be a side effect. (It's difficult to find the exact source of this story. I recall having seen an interview with SPJ where he related the story, but searching the web for a quote in a video is still hard in 2022.)
- Changing a pixel on the screen is a side effect.
- Sending an email is a side effect.
- Deleting a file is a side effect.
- Creating a row in a database is a side effect.
- Changing an internal variable that, via cascading consequences, causes anything like the above to happen, is a side effect.
It is impossible to write (useful) software that has no side effects.
Furthermore, pure functions also don't allow non-deterministic behaviour. That excludes even more necessary actions:
- Getting a (truly) random number is non-deterministic.
- Getting the time or date is non-deterministic.
- Reading a file is non-deterministic.
- Querying a database is non-deterministic.
- Etc.
FP acknowledges that all such impure actions need to take place. The difference in philosophy is the emphasis on pure functions. A pure function has many desirable traits (predictability, referential transparency, possible memoization, testability) that makes it worthwhile to pursue a programming philosophy that favours such functions.
A functional architecture is one that minimises the impure actions to their essentials. One label for that is Functional Core, Imperative Shell, where you push all impure actions to the edge of your system. That would, for example, include an HTML counter. Actually changing an HTML element stays imperative, while the calculation required to produce the new value can be implemented as a pure function.
Different languages have varying approaches to how explicitly they model the distinction between pure functions and impure actions. Most languages (even 'functional' languages like F# and Clojure) don't explicitly make that distinction, so it's up to the programmer to keep that separation in mind.
Haskell is one language that famously does make that distinction. It uses the IO monad to explicitly model impure actions. I've attempted to explain IO for object-oriented programmers using C# as an example language in an article: The IO Container.
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Top Answer
Answered Aug 19, 2010 · 8 votes
Declarative is the way forward.
Imperative describes the actual UI steps you have to take as a user, rather than the results you're trying to achieve. If you write your scenarios this way they'll become really brittle and impossible to maintain. Imagine if someone put a confirmation box in that imperative scenario, and there were 80 similar scenarios which also required changing.
With declarative steps you only need to change it in the one place where the step is defined; that same declarative step is then reused for all the scenarios which need it.
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Top Answer
Answered Apr 16, 2018 · 1 votes
You need to read the original paper published by John Backs "Can Programming Be liberated from the Von Neumann Style? A functional style and its algebra of programs". It basically talks about two kinds of bottlenecks one which is physical hardware limitations and the other which is a kind of conceptual bottleneck formed due to the way programmers think of languages. On your second question. As earlier languages were more closer to the respective hardware implementations the programmer thinking used to be mimic the sequential flow of events. Functional languages give us a new way to look at programs wherein parallel executions or operations on a set of data work.
On the first question I would like to iterate a comment from wiki.c2.com
"What does the choice of programming language have on the hardware? A functional language which is compiled to run on a von Neumann machine will still suffer the bottleneck." The answer, is ReferentialTransparency--which makes parallel computation much more tractable (and capable of being automated). Effectively parallelizing imperative languages is still an active research topic.
http://wiki.c2.com/?CanProgrammingBeLiberatedFromTheVonNeumannStyle
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Top Answer
Answered Nov 28, 2018 · 11 votes
Do it in two steps:
- let some existing tool convert the MediaWiki mark-up into plain HTML;
- convert the plain HTML into text.
The following demo:
import net.java.textilej.parser.MarkupParser;import net.java.textilej.parser.builder.HtmlDocumentBuilder;import net.java.textilej.parser.markup.mediawiki.MediaWikiDialect;import javax.swing.text.html.HTMLEditorKit;import javax.swing.text.html.parser.ParserDelegator;import java.io.StringReader;import java.io.StringWriter;public class Test { public static void main(String[] args) throws Exception { String markup = "This is ''italic'' and '''that''' is bold. \n"+ "=Header 1=\n"+ "a list: \n* item A \n* item B \n* item C"; StringWriter writer = new StringWriter(); HtmlDocumentBuilder builder = new HtmlDocumentBuilder(writer); builder.setEmitAsDocument(false); MarkupParser parser = new MarkupParser(new MediaWikiDialect()); parser.setBuilder(builder); parser.parse(markup); final String html = writer.toString(); final StringBuilder cleaned = new StringBuilder(); HTMLEditorKit.ParserCallback callback = new HTMLEditorKit.ParserCallback() { public void handleText(char[] data, int pos) { cleaned.append(new String(data)).append(' '); } }; new ParserDelegator().parse(new StringReader(html), callback, false); System.out.println(markup); System.out.println("---------------------------"); System.out.println(html); System.out.println("---------------------------"); System.out.println(cleaned); }}
produces:
This is ''italic'' and '''that''' is bold. =Header 1=a list: * item A * item B * item C---------------------------<p>This is <i>italic</i> and <b>that</b> is bold. </p><h1 id="Header1">Header 1</h1><p>a list: </p><ul><li>item A </li><li>item B </li><li>item C</li></ul>---------------------------This is italic and that is bold. Header 1 a list: item A item B item C
Where do you download the java packages you are importing?
Here: Web Archive link of download.java.net/maven/2/net/java/textile-j/2.2
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softwareengineering.stackexchange.com › questionsBy researching around (books, Wikipedia, similar questions on SE, etc) I came to understand that Imperative programming is one of the major programming paradigms, where you describe a series of commands (or statements) for the computer to execute (so you pretty much order it to take specific actions, hence the name "imperative"). So far so good.
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