- Haskell-Cafe - Haskell.org

purity and the season of good will
by Chris Dornan 31 Dec '11

31 Dec '11

(Whether readers would consider the post [or indeed this post] an act of trollery was mooted and mentioned several times in the original post - my thoughts at the end. I am writing this because I would have expected somebody to have said this by now. If it has been said then my sincere apologies. Meta issues, flames, etc., to me and not the list of course.) Pure versus Impure I suspect introducing C into this discussion may not help clarify the fundamental issue. As anyone with experience of programming in Haskell appreciates, there is a big difference between (e.g.) these two functions for taking the sum of two integers: plus :: Int -> Int -> Int plusIO :: Int -> Int -> IO Int The choice is a profound one and will affect their construction as well as the contexts they can be used and so forth. Haskell supports both styles of programming and allows them to be safely and freely intermixed with the type system guaranteeing to keep them properly separated. The type system and static semantics are critical to the Haskell approach to IO and 'purity'. I can't see how it can be meaningfully pushed into the background in any discussion or comparative discussion of these issues. In Short If the programming context in question doesn't provide any equivalent formal separation in its type system of plus-like functions and plusIO-like computations, that programmers use in practice to guide the construction of their programs, then how can the said programming context be considered equivalent to Haskell's treatment of pure functions and effects-generating computations. You just need to consider your own Haskell programming activity and how the type system keeps calculations and I/O actions separate to see this point. I doubt if any extra theory (or theology!) will add much insight. That T word Again (or not) Let's keep this forward-looking. With the email piling in as I write this, please consider whether this continuation of this discussion is likely to be helpful to the general readers of this list or whether it will confuse or discourage them, before adding to it. A Happy New Year to you, Chris From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Steve Horne Sent: 28 December 2011 17:40 To: Haskell Cafe Mailing List Subject: [Haskell-cafe] On the purity of Haskell This is just my view on whether Haskell is pure, being offered up for criticism. I haven't seen this view explicitly articulated anywhere before, but it does seem to be implicit in a lot of explanations - in particular the description of Monads in SBCs "Tackling the Awkward Squad". I'm entirely focused on the IO monad here, but aware that it's just one concrete case of an abstraction. Warning - it may look like trolling at various points. Please keep going to the end before making a judgement. To make the context explicit, there are two apparently conflicting viewpoints on Haskell... 1. The whole point of the IO monad is to support programming with side-effecting actions - ie impurity. 2. The IO monad is just a monad - a generic type (IO actions), a couple of operators (primarily return and bind) and some rules - within a pure functional language. You can't create impurity by taking a subset of a pure language. My view is that both of these are correct, each from a particular point of view. Furthermore, by essentially the same arguments, C is also both an impure language and a pure one. See what I mean about the trolling thing? I'm actually quite serious about this, though - and by the end I think Haskell advocates will generally approve. First assertion... Haskell is a pure functional language, but only from the compile-time point of view. The compiler manipulates and composes IO actions (among other things). The final resulting IO actions are finally swallowed by unsafePerformIO or returned from main. However, Haskell is an impure side-effecting language from the run-time point of view - when the composed actions are executed. Impurity doesn't magically spring from the ether - it results from the translation by the compiler of IO actions to executable code and the execution of that code. In this sense, IO actions are directly equivalent to the AST nodes in a C compiler. A C compiler can be written in a purely functional way - in principle it's just a pure function that accepts a string (source code) and returns another string (executable code). I'm fudging issues like separate compilation and #include, but all of these can be resolved in principle in a pure functional way. Everything a C compiler does at compile time is therefore, in principle, purely functional. In fact, in the implementation of Haskell compilers, IO actions almost certainly *are* ASTs. Obviously there's some interesting aspects to that such as all the partially evaluated and unevaluated functions. But even a partially evaluated function has a representation within a compiler that can be considered an AST node, and even AST nodes within a C compiler may represent partially evaluated functions. Even the return and bind operators are there within the C compiler in a sense, similar to the do notation in Haskell. Values are converted into actions. Actions are sequenced. Though the more primitive form isn't directly available to the programmer, it could easily be explicitly present within the compiler. What about variables? What about referential transparency? Well, to a compiler writer (and equally for this argument) an identifier is not the same thing as the variable it references. One way to model the situation is that for every function in a C program, all explicit parameters are implicitly within the IO monad. There is one implicit parameter too - a kind of IORef to the whole system memory. Identifiers have values which identify where the variable is within the big implicit IORef. So all the manipulation of identifiers and their reference-like values is purely functional. Actual handling of variables stored within the big implicit IORef is deferred until run-time. So once you accept that there's an implicit big IORef parameter to every function, by the usual definition of referential transparency, C is as transparent as Haskell. The compile-time result of each function is completely determined by its (implicit and explicit) parameters - it's just that that result is typically a way to look up the run-time result within the big IORef later. What's different about Haskell relative to C therefore... 1. The style of the "AST" is different. It still amounts to the same thing in this argument, but the fact that most AST nodes are simply partially-evaluated functions has significant practical consequences, especially with laziness mixed in too. There's a deep connection between the compile-time and run-time models (contrast C++ templates). 2. The IO monad is explicit in Haskell - side-effects are only permitted (even at run-time) where the programmer has explicitly opted to allow them. 3. IORefs are explicit in Haskell - instead of always having one you can have none, one or many. This is relevant to an alternative definition of referential transparency. Politicians aren't considered transparent when they bury the relevant in a mass of the irrelevant, and even pure functions can be considered to lack transparency in that sense. Haskell allows (and encourages) you to focus in on the relevant - to reference an IORef Bool or an IORef Int rather than dealing with an IORef Everything. That last sentence of the third point is my most recent eureka - not so long ago I posted a "Haskell is just using misleading definitions - it's no more transparent than C" rant, possibly on Stack Overflow. Wrong again :-( So - what do you think?

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Overloaded Quotes for Template Haskell
by Michael D. Adams 31 Dec '11

31 Dec '11

What would it take to get an "-XOverloadedQuotes" flag of the same sort as the "-XOverloadedStrings" flag? I.e. "[| ... |]" would have type "Quasi m => m Exp" instead of "Q Exp" and any splices in that quotation expect the contents of that splice to have type "m Exp". (Obviously, top level splices would still have type "Q Exp" otherwise the compiler wouldn't know how to evaluate the monad.) I ran into this problem when writing a Template Haskell program in which part of it operates in a "StateT S Q a" monad instead of the usual "Q a" monad. (The "S" type stores the state of a memoization table of code fragments already generated. Without it, the code would loop infinitely when processing certain recursive structures.) It is fairly easy to declare an instance of "Quasi" for "StateT S Q", so in order to keep the code clean, I'd like to use quotations with splices in them (i.e. [| ... $( ... ) ... |] ) for expressing the generated code. However, quotations and splices are tied to the "Q" monad which means that as it is now I have to manually write "LamE ... VarP ... VarE ... etc." instead of using the much nicer quotation syntax. Michael D. Adams mdmkolbe(a)gmail.com

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Type classes in Typing Haskell in Haskell
by Alexander Bau 30 Dec '11

30 Dec '11

Hi, recently I tried the Typing Haskell in Haskell library. But I was wondering why this program type checks: > -- plusMfun is standard '+': Num a => a -> a -> a > test = let Just classEnv = ( addCoreClasses <:> addNumClasses ) > initialEnv > e = Ap ( Ap (Var "+") (Lit $ LitStr "3")) (Lit $ LitStr "5") > impl = ("foo",[([],e)]) > in > putStrLn $ pretty $ runTI $ tiImpls classEnv [plusMfun] [impl] I was expecting some kind of typechecking error, because [Char] is no instance of Num. But I get this: > ([isIn1 cNum (TAp tList tChar)], > ["foo" :>: > Forall [] > ([] :=> > (TAp tList tChar))]) The predicate says that if [Char] would be in Num then the type of the expression would be [Char]. But actually [Char] isn't in Num. So, how do I provoke a type check error in this case? Thanks in advance.

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Text.Regex.Base throws exceptions with makeRegexOptsM
by Omari Norman 30 Dec '11

30 Dec '11

Hi folks, I'm using Text.Regex.Base with the TDFA and PCRE backends. I want to compile regular expressions first and make sure the patterns were actually valid, so I used makeRegexOptsM, which indicates a bad regular expression by calling fail. That allows you to use makeRegexOptsM with Maybe or with (Either String) (assuming that Either String is an instance of Monad, which of course is defined in Control.Monad.Error.) Doing this with Maybe Regex works like it should--bad pattern gives you a Nothing. But if you want to see the error message by using Either String, an exception gets thrown with the bad pattern, rather than getting a Left String. Why is this? Seems like an odd bug somewhere. I am a Haskell novice, but I looked at the code for Text.Regex.Base and for the TDFA and PCRE backends and there's nothing in there to suggest this kind of behavior--it should work with Either String. The attached code snippet demonstrates the problem. I'm on GHC 7.0.3 (though I also got the problem with 6.12.3) and regex-base-0.93.2 and regex-tdfa-1.1.8 and regex-pcre-0.94.2. Thanks very much for any tips or ideas. --Omari

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IO typeclasses
by David Thomas 30 Dec '11

30 Dec '11

Is there any particular reason IO functions in the standard libraries aren't grouped into type-classes? This might allow for: 1) Testing IO code without actual input and output. (I have done this on a small scale, but it presently involves much ugliness). 2) Redirecting output of a function that neglects to take a handle without a bunch of calls to dup. 3) Forwarding IO over a connection to a remote system, allowing code written to work locally to be applied remotely, or vice-versa. 4) Wrapping dangerous IO actions in additional sanity checks. Thoughts?

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Re: [Haskell-cafe] On the purity of Haskell
by Chris Smith 30 Dec '11

30 Dec '11

Sorry to cut most of this out, but I'm trying to focus on the central point here. On Thu, 2011-12-29 at 22:01 +0000, Steve Horne wrote: > In pure functional terms, the result should be equivalent to a fully > evaluated value - but putStrLn isn't pure. It cannot be fully > evaluated until run-time. And here it is, I think. You're insisting on viewing the performing of some effect as part of the evaluation of an expression, even though the language is explicitly and intentionally designed not to conflate those two ideas. Effects do not happen as a side-effect of evaluating expressions. Instead they happen because you define the symbol 'main' to be the effect that you want to perform, and then set the runtime system to work on performing it by running your program. Evaluation and effects are just not the same thing, and it makes no sense to say something isn't "evaluated" just because the effect it describes haven't been performed. It's exactly that distinction -- the refusal to conflate evaluation with performing effects -- that is referred to when Haskell is called a pure language. -- Chris Smith

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ANN: ekg-0.2 - Remote monitoring of processes
by Johan Tibell 30 Dec '11

30 Dec '11

(I forgot to announce v0.1 so this is a combined announcement.) I'm proud to announce the ekg [1] library. The library lets you remotely monitor any running Haskell program, using your web browser or an automated monitoring program. The library lets you monitor garbage collector and memory usage statistics (similar to what's provided by +RTS -s, but live), but also lets you defined your own counters that you can update from within your program. For some more examples see: http://blog.johantibell.com/2011/12/remotely-monitor-any-haskell.html http://blog.johantibell.com/2011/12/more-monitoring-goodies.html 1. http://hackage.haskell.org/package/ekg

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Re: [Haskell-cafe] On the purity of Haskell
by Никитин Лев 30 Dec '11

30 Dec '11

Of course we take in matter what computation "getStr :: IO [Char]" means. Of course we take in matter what computation "putStr :: String -> IO () means (I know, putStr is not computation, puStr x is computation, but it's not important) And If I want to write "echo" program, I have to combine these computation in such manner: echo = do s <- getStr putStr s But for geometry I have similarly situation. I know arithmetics and I know Pythagoras theorem and I combine arithmetics operations to get length of hypotéinusa: h x y = sqrt $ x * x + y * y. What is difference of IO actions and PURE operation *? I have to know meaning of getStr action but I have to know meaning of * operation. Side effects appears in impure languages when we can different results of function (procedure etc) with same parameters. For example, when our function use global (modifible) variable. But we WANT that ours function (Int -> Int, Char -> [a], b -> IO a - it's not matter) will be PURE, becourse it makes life of programmers easy. But I don't understand importance of thinking about "side effects" in dilemma of language vs compiled pogramm. > 30.12.2011, 02:21, "Heinrich Apfelmus" <apfelmus(a)quantentunnel.de>: > >> Gregg Reynolds wrote: >>> Donn Cave wrote: >>>> Quoth Gregg Reynolds wrote: >>>> >>>> Look again at the sentence you trimmed off the end: >>>>>> Of course, the point is that this result is an *IO action* of >>>>>> type IO Int, it's not the Int you would get "when executing >>>>>> this action". >>>> I believe that more or less points to the key to this discussion. >>>> If it didn't make sense to you, or didn't seem relevant to the >>>> question of pure functions, then it would be worth while to think >>>> more about it. >>> Ok, let's parse it out. "…it's not the int you would get 'when >>> executing this action". Close, but no cooky: it's not any kind of >>> int at all (try doing arithmetic with it). "IO Int" is a piece of >>> rhetoric for the mental convenience of the user; Haskell does not and >>> cannot know what the result of an IO action is, because it's outside >>> the scope of the language (and computation). (The "Int" part of "IO >>> Int" refers to the input, not the output; it's just a sort of type >>> annotation.) It's not even a computation, unless you want to take a >>> broad view and include oracles, interaction, etc. in your definition >>> of computation. >> Why would IO Int be something special or mysterious? It's an ordinary >> value like everything else; it's on the same footing as [Char], Maybe >> Int, Int -> String, Bool, and so on. I see no difference between the >> list [1,2,3] :: [Int] and the action "pick a random number between 1 >> and 6" :: IO Int . >> >> Best regards, >> Heinrich Apfelmus >> >> -- >> http://apfelmus.nfshost.com >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Convert Double to Data.Fixed
by Eugene Kirpichov 30 Dec '11

30 Dec '11

Hi cafe, How do I most efficiently convert a Double to a Data.Fixed? I'm asking because I want to convert fractional seconds to the seconds field of Data.Time.TimeOfDay, which is Pico = Data.Fixed.Fixed E12. For now the fastest thing I came up with was fromIntegral (round (sec*1000000)) / fromIntegral 1000000, but this frankly sucks and is rather slow, there must be a better way. -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Windows: openFile gives permission denied when file in use
by Michael Snoyman 30 Dec '11

30 Dec '11

Hi all, I just received a bug report from a client that, when an input file is open in FrameMaker, my program gives a "permission denied error". This bug is reproducible with a simple Haskell program: import System.IO main = do putStrLn "here1" h <- openFile "filename.txt" ReadMode putStrLn "here2" I tried writing a simple C program using fopen, and it ran just fine. Does anyone have experience with this issue, and know of a workaround? Thanks, Michael

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