Why Haskell is suited to IO
github.com
github.com
1.) Haskell is "a bad language to write IO intensive programs in"
2.) "maintenance is more expensive as Haskell programmers are thin on the ground"
I was convinced of neither.
First, just because your own (potentially biased) benchmark shows that GHC performs "very favorably" doesn't mean this is true in general. For anyone to buy that argument, you need to provide code or at least a precise description of the experiment. These guys did a good job of that: http://www.yesodweb.com/blog/2011/02/warp-speed-ahead. Also, just because iteratees are fast, doesn't mean they're "good" for writing I/O intensive programs in. They're really difficult to understand, so most people end up using lazy I/O. Part of not being bad for I/O programs is making the task approachable, and I'm not convinced of that by this article.
The argument to dispel #2 is vague too.
> If you agree that programs that are more correct are also of higher quality then the you should also agree that the cost of maintenance for Haskell programs is lower than for imperative programs.
Okay... You can still write errors in Haskell, they're just less frequent (at least that's what this statement suggests). But how can you maintain them if no one is willing to actually do it? If it's N times easier to maintain a Haskell programmer, there need to be at least 1/N as many Haskell programmers as there are imperative programmers, and the article doesn't address this at all. As someone who likes Haskell, I'm actually curious to know if this is true.
> import qualified Data.ByteString.Lazy.Char8 as L
> main = L.getContents >>= print . L.count '\n'
edit: actually it beats "wc", and is roughly same speed as "wc -l".
I think that Haskell's forced purity is roughly as good at catching logic errors as the strict typing of C++ or the clean syntax of Python: It catches some stuff, some of the time, but never everything, and never reliably. At some point, the onus is on the programmer to write good code.
The pattern boils down to creating an expert system that is pure and encodes a state machine and creating a system within the IO monad that operates by querying the expert system. Both parts would be written in a synchronous style, but would use coroutines to mask the passing of control flow.
Um, awesome. You do that, and I'll continue to "print 'Hello world'".
I mean, precise joinery may be superior to hammers and nails when building a house, but it's an expensive proposition, and there are 5 people in the world who can do it properly.
Most of us monkeys, when coding, are aiming to accomplish some task in the notoriously incorrect real world. It is often much better to spend 1/5th the engery get to a 80-90% correct solution very fast, and then patch as necessary, for the very important reason that you don't even know if what you're attempting to do will work. In that mode, like carpenters, sometimes it's helpful to just bang it with a hammer.
If what you do is write compilers, well, OK. The description you're quoting sounds pretty good. But the world doesn't need tons of compiler writers, and those it does need typically have pretty stable specs in front of them, so maybe joinery is the right approach.
I think this guy gets it about right: http://alarmingdevelopment.org/?p=440
I actually like Haskell. But I do think it's the wrong tool for a lot of the work that people here on HN tend to do.
In Haskell, you pay upfront.
For example, expressing any control flow as a monad may be harder than just muddling through, but your components are guaranteed to compose afterwards.
(Disclaimer: I've only spent about a week learning me a Haskell for great good, and that was a couple weeks ago, so I'm not an authority by any measure.)
The Haskell community uses the word "pure" a lot, and I don't get it whatsoever. Even this musing says it's "easier to write correct pure code than correct imperative code." What does that mean? Is "pure" code the opposite of "imperative" code?
For example, f(x) = 2*(x+1) is pure. f(3) == 8, and it is completely safe (in terms of yielding the proper return value) to replace f(3) with 8 anywhere in your program.
Similarly, print world "foo" is referentially transparent - print world_in_which_foo_wasnt_printed "foo" == world_in_which_foo_was_printed. It's completely safe to make the substitution (restOfProgram (print world_in_which_foo_wasnt_printed "foo")) -> (restOfProgram world_in_which_foo_was_printed).
Referential transparency makes it easy to reason about programs. A function of type Int -> Int will never touch a variable of type OutsideWorld, and can always be replaced by it's output.
For those wondering what this has to do with monads, it's fairly simple. A pure program without monads requires a lot of plumbing concerning the world variable:
f: World, Int
world, result <- action1 world 3
world2, result <- action2 world
...
return (worldN, result)
[edit: fixed formatting.]
Pure means that functions are not allowed to have side effects. More formally, a pure function can only use its inputs (parameters) to generate its output. Therefore, a pure function can't modify a global variable, print to the screen etc. because those are all side effects (more on this later). The benefit of this is threefold:
* Testing is much safer and generally easier. You just have to test the various inputs and edge cases and you can pretty much guarantee the function is 100% correct. You can't get this guarantee in impure languages.
* Inherently concurrent. Since functions don't modify any global state, you're free to make this as concurrent as you want without worrying about "thread safety" and all that.
* Increased modularity. Pure functions are highly portable, since they don't modify state they can be used for any library if needed, really. In an impure language, you often have to modify functions for use in a more general sense as a library.
Of course, the real world has side effects. Almost every program we run is a side effect, since we're giving it input from the world and it uses that to print to a screen. So how can a program be pure at all???
Without going into too much detail, there are ways in Haskell to isolate impurity to a small part of the code which is clearly marked as impure. This impure code is responsible for say, reading a string from the user input. This string can then be used to make function calls to pure functions.
So the way you do things is to put all your business logic and such in pure functions, then the impure functions just deal with shuttling values to and from the real world. You can test pure functions with theorem checkers and you test impure functions with unit tests and integration tests.
Haskell has a steep learning curve since it has so many concepts which still simply haven't made it into more industry-standard languages. This is getting better with language such as Scala becoming more mainstream.
I really love Haskell and even if I don't use it on my day-to-day, it is great to hack out a Haskell program every once in awhile.
Hope this helps!
- Mitchell
> "lazy" is the opposite of "imperative."
should be: "lazy" is the opposite of "strict" or "eager". "Imperative", in this context, is the opposite of "functional".
More reading here: http://www.haskell.org/haskellwiki/Functional_programming#Pu...
edit: BTW, Haskell DOES have a steep learning curve, and the inherent recursion breaks my brain. Even simple things like:
`combos = []` `combos = combos + 'foo'`
hurt, because counterintuitively (to me, anyway) this winds up in infinite recursion. I'm very used to OOP so this sort of thing is hard on me. I'm old though.
edit: I'm a little bummed out by the downvotes on my original question. It was an honest academic question I had and was answered splendidly. I was not attacking Haskell whatsoever. I apologize if anyone took it as anything other than an honest question.
Great explanation, actually. But one question. I thought functional languages were "pure" throughout. I mean, I thought the whole point of a functional language is that there are no side effects and every expression returns some value. How is "pure" distinct from "functional"?
# Sum a list in Python using FP
items = [1,2,3,4,5]
function = lambda x, y: x + y
reduce(function, items) # => 15
# The equivalent in haskell:
foldr1 (+) [1,2,3,4,5]
edit: BTW, Haskell DOES have a steep learning curve, and the inherent recursion breaks my brain. Even simple things like:
`combos = []` `combos = combos + 'foo'`
hurt, because counterintuitively (to me, anyway) this winds up in infinite recursion. I'm very used to OOP so this sort of thing is hard on me. I'm old though.
Alas, you still have to be able to understand recursion to get anywhere.
It also admits many optimizations in the compiler, garbage collector and runtime system that are not possible for code without such safety guarantees.
It is this IO and thread throughput and scaling results that are being hinted at in this post, I think. And projects like Snap and Yesod are taking advantage. http://www.yesodweb.com/blog/2011/03/preliminary-warp-cross-...