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copied!<p>In pure functional programming, there are no side-effects. For many types of software (for example, anything with user interaction) side-effects are necessary at some level.</p> <p>One way to get side-effect like behavior while still retaining a functional style is to use functional reactive programming. This is the combination of functional programming, and reactive programming. (The Wikipedia article you linked to is about the latter.)</p> <p>The basic idea behind reactive programming is that there are certain datatypes that represent a value "over time". Computations that involve these changing-over-time values will themselves have values that change over time.</p> <p>For example, you could represent the mouse coordinates as a pair of integer-over-time values. Let's say we had something like (this is pseudo-code):</p> <pre><code>x = <mouse-x>; y = <mouse-y>; </code></pre> <p>At any moment in time, x and y would have the coordinates of the mouse. Unlike non-reactive programming, we only need to make this assignment once, and the x and y variables will stay "up to date" automatically. This is why reactive programming and functional programming work so well together: reactive programming removes the need to mutate variables while still letting you do a lot of what you could accomplish with variable mutations.</p> <p>If we then do some computations based on this the resulting values will also be values that change over time. For example:</p> <pre><code>minX = x - 16; minY = y - 16; maxX = x + 16; maxY = y + 16; </code></pre> <p>In this example, <code>minX</code> will always be 16 less than the x coordinate of the mouse pointer. With reactive-aware libraries you could then say something like:</p> <pre><code>rectangle(minX, minY, maxX, maxY) </code></pre> <p>And a 32x32 box will be drawn around the mouse pointer and will track it wherever it moves.</p> <p>Here is a pretty good <a href="http://www.cs.brown.edu/research/pubs/techreports/reports/CS-03-20.html" rel="noreferrer">paper on functional reactive programming</a>.</p>

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