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    <p>There's no community consensus on what are the essential concepts in functional programming. In <a href="http://www.cse.chalmers.se/~rjmh/Papers/whyfp.pdf" rel="nofollow noreferrer">Why Functional Programming Matters (PDF)</a>, John Hughes argues that they are higher-order functions and lazy evaluation. In <a href="http://research.microsoft.com/en-us/um/people/simonpj/papers/haskell-retrospective/" rel="nofollow noreferrer">Wearing the Hair Shirt: A Retrospective on Haskell</a>, Simon Peyton Jones says the real essential is not laziness but purity. Richard Bird would agree. But there's a whole crowd of Scheme and ML programmers who are perfectly happy to write programs with side effects.</p> <p>As someone who has practiced and taught functional programming for twenty years, I can give you a few ideas that are widely believed to be at the core of functional programming:</p> <ul> <li><p><strong>Nested, first-class functions</strong> with proper lexical scoping are at the core. This means you can create an anonymous function at run time, whose free variables may be parameters or local variables of an <em>enclosing</em> function, and you get a value you can return, put into data structures, and so on. (This is the most important form of higher-order functions, but some higher-order functions (like <code>qsort</code>!) can be written in&nbsp;C, which is not a functional language.)</p></li> <li><p>Means of composing functions with other functions to solve problems. Nobody does this better than John Hughes.</p></li> <li><p>Many functional programmers believe purity (freedom from effects, including mutation, I/O, and exceptions) is at the core of functional programming. Many functional programmers do not.</p></li> <li><p><strong>Polymorphism</strong>, whether it is enforced by the compiler or not, is a core value of functional programmers. Confusingly, C++ programmers call this concept "generic programming." When polymorphism is enforced by the compiler it is generally a variant of <a href="https://stackoverflow.com/questions/399312/what-is-hindley-milner">Hindley-Milner</a>, but the more powerful <a href="http://en.wikipedia.org/wiki/System_F" rel="nofollow noreferrer">System&nbsp;F</a> is also a powerful basis for functional languages. And with languages like Scheme, Erlang, and Lua, you can do functional programming without a static type system.</p></li> <li><p>Finally, a large majority of functional programmers believe in the value of <strong>inductively defined data types</strong>, sometimes called "recursive types". In languages with static type systems these are generally known as "algebraic data types", but you will find inductively defined data types even in <a href="http://www.htdp.org" rel="nofollow noreferrer">material written for beginning Scheme programmers</a>. Inductively defined types usually ship with a language feature called <strong>pattern matching</strong>, which supports a very general form of case analysis. Often the compiler can tell you if you have forgotten a case. I wouldn't want to program without this language feature (a luxury once sampled becomes a necessity).</p></li> </ul>
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    1. POWhat are the core concepts in functional programming?
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    1. USNorman Ramsey
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    1. COI think the polymorphism one is misleading. Generic programming in C++ covers a lot more than this (it typically also uses metaprogramming to enable several different implementations, depending on type - What you're talking about is more similar to .NET generics than C++ templates/generic programming). And this form of parametric type polymorphism has little to do with what OOP programmers call polymorphism. If you'd called it polymorphic types, it'd have been clearer, I think.
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    2. CO@Norman Ramsey - I like that you called out purity and I must admit I hadn't before heard that functional programming embodies polymorphism. I feel my answer gets right to the meat of functional programming but I found your write up informative. Thank you.
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