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It's not clear what the "most apps" are. There are userfacing apps, server, games, trading. Usecases differ. In some cases FP can be applied well, in some it can not. Lets say, even if you couldn't apply pure FP well in most cases, maybe it can to a very important one... Also there's multiparadigm.
Some impressive results are being achieved with functional languages today. For example a recent success in this area is the Amazon competitor Jet.com. Functional languages are increasing in number and popularity which shows increasing applicability to real world challenges.
Just because Amazon or some other company was successfully using it doesn't mean it supports the counter-argument. One could also count every other company that uses an other paradigm... Nothing argumentative achieved by this. inter
Many concepts from functional programming are very useful for most applications, such as map, filter and reduce.
features and principles from functional programming (e.g. first-order functions, immutability, monads) find wide application in established and new programming languages and at a higher level in system design (Google MapReduce, Twitter Finagle etc.)
Its a question what most people think how the world works.Because apps should solve realworld problems.Even if some argue the world is inherently FP,most people like to think they effect things and move around in a dynamic but stateful world.Any representation of state in FP is counter FP/ unelegant
why just use a specific paradigm? Much like discussions of which design patterns and frameworks are better, crafting an exclusive one for your purpose might take time, but is always the better option.
Functional programming means: Program with functions that only use explicit input and return output. As opposed to procedural where you might have no output but side-effects inside the function. Or functions that implicitly use another input. This idea makes understanding and testing code easier.
Y-combinator founder has noted that his and associate's choice of Lisp as basis for one of the first online webstores, had allowed them to beat others by (among other) implementing any new features way faster than competition due to the language used.
"Functional programming" is ill-defined.
Human brain thinks about world in stateful manner. Switching to stateless thinking requires more training than learning stateful paradigm.
"The problem is that most common code inherently involves state -- business apps, games, UI, etc. There's no problem with some parts of an app being purely functional; in fact most apps could benefit in at least one area. But forcing the paradigm all over the place feels counter-intuitive."