OO programming literature is full of design patterns. Most books on object oriented programming dedicate a chapter or two to design patterns like factories and decorators. So what are the equivalent patterns in functional languages and why hasn't anyone written a book about them yet? Is there something special about functional languages that obviates the need for design patterns?
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OO and functional programming are two very different programming paradigms, and design patterns (DP) is a significant part of OO design and programing. DP do not have such role in functional programming. One could even say, that DP are not needed in functional programming -- there is no itch which DP is cure for.
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The simple fact is that many OO Patterns would be considered Idioms in functional languages (especially the original GoF patterns). For instance the Iterator pattern (built-in to languages like C# now) just isn't necessary in a Lisp or ML which has sequence operators. A lot of the patterns we use in O-O systems are there to help us get the "non essentials" out of the way so we can focus on coding objects. In other words, the patterns are solutions to the non-interesting parts of the application. We should leverage patterns to address common needs that have been solved before (like the patterns in Fowlers Patterns of Enterprise Application Architecture for dealing with things like database transmission, or xUnit Patterns for boosting your unit testing) so that we can focus on adding business value for the application. I'm sure that beyond the specifics of the GoF patterns, there are design patterns that will be applicable to functional programming as well. The thing is that O-O is the dominant paradigm. Writing a pattern book that targets functional developers...well frankly won't get a greenlight from a publisher. That's what it boils down to. There isn't enough of a market for Functional Patterns to have a significant number of books dedicated to the topic. | |||
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Insofar as the design methodology for FP is to design your types to accurately reflect the problem space and the implementation should follow automatically, the FP equivalent of a book on design patterns is something like Chris Okasaki's Purely Functional Data Structures. | |||||||||
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