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Yes, a lot of these are characteristic of functional programming, and many are being adopted. But I think that the idea of using mathematically pure functions--

by jekor 14y ago

Yes, a lot of these are characteristic of functional programming, and many are being adopted. But I think that the idea of using mathematically pure functions---programming without side effects or mutation---is a/the key idea behind functional programming. And it's this purity that divides the communities. You can take high-order functions and folds and put them in just about any language, and you could put OOP concepts like subtype polymorphism into functional languages. But there's a line that neither class of languages can cross over, and that's mutable state. I know that some developers are beginning to lean in the direction of functional programming by relying on const annotations and adopting a functional style. And I'm very excited and hopeful about the overall trend.

Chris_Newton 14y ago
But I think that the idea of using mathematically pure functions---programming without side effects or mutation---is a/the key idea behind functional programming. Indeed, and it is either a blessing or a curse depending on what kind of software you’re trying to write and the sort of situations you’re trying to model. For most of the programming work I’ve ever done myself, what I’d really like to use is languages that do have concepts like time and side effects, because they’re very useful, but which only use them in controlled ways and when the programmer actually wants them. I’ve often wondered whether such ideas might be elevated to first class concepts within future programming languages, complete with their own rules and specified interactions, just as today we have variables and types and functions, and a language will prevent errors like calling a function with only two values when it expects three or trying to use the value 20 where a string is expected. For now, I’m hoping that functional programming, in the pure sense, will raise awareness of the potential benefits of controlling side effects rather than allowing them more-or-less arbitrarily as most imperative languages today do. With a bit of luck, the generation of industrial languages in a few years will then borrow ideas that stand the test of time from today’s research into type/effect systems, software transactional memory, and so on, and perhaps we can have something of the best of both worlds.
- tikhonj 14y ago
  What you describe is exactly what Haskell does. You have pure functions by default, but you can have mutable state wherever you want it--it is just constrained to a particular scope by the type system. That is, you can use mutable references, mutable arrays and so on all you want inside a computation, and the compiler will guarantee that none of these references leak outside their scope. Haskell accomplishes this with "state threads" (ST[1]). [1]: http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Control-Monad-ST.html http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc... Similarly, Haskell has the IO type for containing any side effects at all. This type, as the name implies, can have not only mutable state but also arbitrary I/O--really anything you like. This has some very nice advantages: for example, you can create your own wrapper around the IO type which only exposes some IO operations. You could use this for ensuring that plugins can't do things like delete files, for example. (This is one of the example use cases for Safe Haskell[2], which is worth reading about.) [2]: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/safe-haskell.html#uses-of-safe-haskell http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/safe-... You can even reify time explicitly, rather than modelling it implicitly with mutable state. This leads to functional reactive programming (FRP[3]), a new and more declarative way to write things like UIs. FRP replaces event and callback oriented programming, fixing much of the spaghetti code common to most UI frameworks. [3]: http://stackoverflow.com/questions/1028250/what-is-functional-reactive-programming http://stackoverflow.com/questions/1028250/what-is-functiona... Really, I think "purely functional" is a very unfortunate term for Haskell from a marketing standpoint. "Purely functional" does not mean that we cannot have side-effects, or mutation, or what have you. It just means that side-effects aren't everywhere by default, and don't implicitly permute your otherwise pretty code.
  - deleted 14y ago
    [deleted]
  - Chris_Newton 14y ago
    I appreciate the comment, but what I have in mind looks very different to the way monads are used in Haskell today. For example, I don’t necessarily want pure functions by default in my idealised programming model. To me, a pure function is just a special case of a function whose effects are controlled and where the resources the function might interact with are readily identifiable, for which the sets of possible effects and interacting resources happen to be empty. Put another way, I don’t see anything wrong, or even inferior or undesirable, with performing a quicksort of an array or some fast matrix computations destructively. However, I’d like my language to make sure I couldn’t accidentally try to use the old data again afterwards (including, for example, in other threads running in parallel). I’d like to be sure that just because my algorithm updates one set of data in-place, it doesn’t also draw a window, block to wait for user input, or kick off a background thread that will reformat my hard drive in half an hour. And in the calling code, I’d like to find out very quickly that just because I created and populated an array over here, and then ran a quicksort that changed it over there, and then printed it out down there, nothing else had any chance to modify it in between. And I’d like to do all of that using syntax that is at least as clean and readable as today’s best languages, please, not the horrors that often seem to result when functional programming languages try to implement equivalent behaviour with realistic heavily structured/monadic values instead of the neat one-integer counter or append-only log that always seems to be used in the tutorial code samples. I suspect that to do that will require a language that provides dedicated, specialised tools to support the new programming model. Sure, you could probably do this sort of thing using monads in Haskell, just as you could do OOP in C, text processing in C++, or write in a functional style in Python, but the results are clumsy compared to using the right tool for the job. I’m not sure that tool exists yet for the kind of programming models I’d like to use, but that’s where I’m hoping the lessons of today’s functional languages and academic research will bear fruit over time.
jerf 14y ago
"But there's a line that neither class of languages can cross over, and that's mutable state." I'd add another qualifier for clarity; default or reliable mutable/immutable state. And this really comes up in libraries and such; either the language affords the use of immutability and you can count on libraries being themselves immutable, or the language affords the use of mutation and you can count on the libraries being based on mutation. Immutable languages can "borrow" mutability and mutable languages can "borrow" immutability (const, etc.), but the default matters, and there has to be some sort of default. (Though while I cast this as a binary, there are some small in-between choices that may be relevant; see Rust, for instance, whose "unique" pointers may be a successful split. If the most useful aspect of immutability is not strictly speaking the inability to mutate values, but instead to guarantee that values can not be mutated by other threads or functions you didn't expect, then what happens if you directly express only that constraint instead of the stronger and more annoying constraint of full immutability? I'm watching that language with interest.)

by jekor 14y ago

Chris_Newton 14y ago

But I think that the idea of using mathematically pure functions---programming without side effects or mutation---is a/the key idea behind functional programming. Indeed, and it is either a blessing or a curse depending on what kind of software you’re trying to write and the sort of situations you’re trying to model. For most of the programming work I’ve ever done myself, what I’d really like to use is languages that do have concepts like time and side effects, because they’re very useful, but which only use them in controlled ways and when the programmer actually wants them. I’ve often wondered whether such ideas might be elevated to first class concepts within future programming languages, complete with their own rules and specified interactions, just as today we have variables and types and functions, and a language will prevent errors like calling a function with only two values when it expects three or trying to use the value 20 where a string is expected. For now, I’m hoping that functional programming, in the pure sense, will raise awareness of the potential benefits of controlling side effects rather than allowing them more-or-less arbitrarily as most imperative languages today do. With a bit of luck, the generation of industrial languages in a few years will then borrow ideas that stand the test of time from today’s research into type/effect systems, software transactional memory, and so on, and perhaps we can have something of the best of both worlds.

tikhonj 14y ago
What you describe is exactly what Haskell does. You have pure functions by default, but you can have mutable state wherever you want it--it is just constrained to a particular scope by the type system. That is, you can use mutable references, mutable arrays and so on all you want inside a computation, and the compiler will guarantee that none of these references leak outside their scope. Haskell accomplishes this with "state threads" (ST[1]). [1]: http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Control-Monad-ST.html http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc... Similarly, Haskell has the IO type for containing any side effects at all. This type, as the name implies, can have not only mutable state but also arbitrary I/O--really anything you like. This has some very nice advantages: for example, you can create your own wrapper around the IO type which only exposes some IO operations. You could use this for ensuring that plugins can't do things like delete files, for example. (This is one of the example use cases for Safe Haskell[2], which is worth reading about.) [2]: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/safe-haskell.html#uses-of-safe-haskell http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/safe-... You can even reify time explicitly, rather than modelling it implicitly with mutable state. This leads to functional reactive programming (FRP[3]), a new and more declarative way to write things like UIs. FRP replaces event and callback oriented programming, fixing much of the spaghetti code common to most UI frameworks. [3]: http://stackoverflow.com/questions/1028250/what-is-functional-reactive-programming http://stackoverflow.com/questions/1028250/what-is-functiona... Really, I think "purely functional" is a very unfortunate term for Haskell from a marketing standpoint. "Purely functional" does not mean that we cannot have side-effects, or mutation, or what have you. It just means that side-effects aren't everywhere by default, and don't implicitly permute your otherwise pretty code.
- deleted 14y ago
  [deleted]
- Chris_Newton 14y ago
  I appreciate the comment, but what I have in mind looks very different to the way monads are used in Haskell today. For example, I don’t necessarily want pure functions by default in my idealised programming model. To me, a pure function is just a special case of a function whose effects are controlled and where the resources the function might interact with are readily identifiable, for which the sets of possible effects and interacting resources happen to be empty. Put another way, I don’t see anything wrong, or even inferior or undesirable, with performing a quicksort of an array or some fast matrix computations destructively. However, I’d like my language to make sure I couldn’t accidentally try to use the old data again afterwards (including, for example, in other threads running in parallel). I’d like to be sure that just because my algorithm updates one set of data in-place, it doesn’t also draw a window, block to wait for user input, or kick off a background thread that will reformat my hard drive in half an hour. And in the calling code, I’d like to find out very quickly that just because I created and populated an array over here, and then ran a quicksort that changed it over there, and then printed it out down there, nothing else had any chance to modify it in between. And I’d like to do all of that using syntax that is at least as clean and readable as today’s best languages, please, not the horrors that often seem to result when functional programming languages try to implement equivalent behaviour with realistic heavily structured/monadic values instead of the neat one-integer counter or append-only log that always seems to be used in the tutorial code samples. I suspect that to do that will require a language that provides dedicated, specialised tools to support the new programming model. Sure, you could probably do this sort of thing using monads in Haskell, just as you could do OOP in C, text processing in C++, or write in a functional style in Python, but the results are clumsy compared to using the right tool for the job. I’m not sure that tool exists yet for the kind of programming models I’d like to use, but that’s where I’m hoping the lessons of today’s functional languages and academic research will bear fruit over time.

jerf 14y ago

"But there's a line that neither class of languages can cross over, and that's mutable state." I'd add another qualifier for clarity; default or reliable mutable/immutable state. And this really comes up in libraries and such; either the language affords the use of immutability and you can count on libraries being themselves immutable, or the language affords the use of mutation and you can count on the libraries being based on mutation. Immutable languages can "borrow" mutability and mutable languages can "borrow" immutability (const, etc.), but the default matters, and there has to be some sort of default. (Though while I cast this as a binary, there are some small in-between choices that may be relevant; see Rust, for instance, whose "unique" pointers may be a successful split. If the most useful aspect of immutability is not strictly speaking the inability to mutate values, but instead to guarantee that values can not be mutated by other threads or functions you didn't expect, then what happens if you directly express only that constraint instead of the stronger and more annoying constraint of full immutability? I'm watching that language with interest.)