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    <p>In the late 1970s, Barbara Liskov and others developed a boatload of large-scale "object-oriented design" techniques which are still widely used today and which apply unchanged to functional programming. They are easiest to apply with a language that has explicit interfaces and implementations, which means Standard ML (where interfaces are called "signatures" and implementations are called "structures" or "functors") or Objective Caml (where interfaces are called "module types" and implementations are called "modules"). If you prefer Scheme then the "unit" language developed by Matthew Flatt and Matthias Felleisen is built into PLT Scheme and is a very good way of expressing large-scale functions.</p> <p>In brief:</p> <ul> <li><p>Organize your applications around abstract types (classes in OO, "abstract types" in FP) and the operations on those types.</p></li> <li><p>Use encapsulation mechanisms (classes in OO, modules in FP) to hide the representations of your abstract types.</p></li> <li><p>Structure your application so that each implementation depends on other implementations indirectly, through their interfaces. This way you limit the amount of code you have to understand to build or modify any one piece of your application.</p></li> <li><p>Go to town!</p></li> </ul> <p>The main difference is that when you're writing functional programs, you don't use inheritance to reuse implementations. Instead you use higher-order functions, or you use <a href="http://www.cs.tufts.edu/~nr/pubs/maniaws-abstract.html" rel="noreferrer">modules which take other modules as parameters</a>.</p> <p><strong>Summary:</strong> at the architectural level, there's not a lot of difference, but when using functional languages you may need to hunt a little harder to find the encapsulation mechanisms that you need.</p>
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    1. POArchitectural Thinking in Functional Languages
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    1. USNorman Ramsey
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    1. COAwesome answer. In general, are there performance drawbacks to the module-taking-module-parameter approach, or does it just cost you a pointer? (Javascript is mental model that I have of this)
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      1. USDanielMason
    2. CO@DanielMason: For a naive compiler, you pay about the same overhead you would in an object-oriented language: the module is represented by a pointer to a vtable. But the better compilers do a good job inlining small functions across module boundaries, so they have less overhead. And MLton is a really good whole-program compiler, so there's no run-time overhead at all. http://mlton.org/
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    3. COThank you for the follow-up. That gives me a lot to delve into.
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