- Haskell-Cafe - Haskell.org

How to redirect a handle...
by Dimitry Golubovsky 19 Jan '06

19 Jan '06

Thanks everybody who answered. Indeed, forkProcess is something I completely overlooked... -- Dimitry Golubovsky Anywhere on the Web

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Packed String parameters
by Scherrer, Chad 18 Jan '06

18 Jan '06

To read a file using a packed string, I need to use this function, right? hGetPS :: Handle -> Int -> IO PackedString What's the Int? Do you have to specify the length in advance? I don't know how packed strings are implemented, and I'm having trouble finding much documentation. I'm trying to speed up this horribly slow thing: dot :: [Double] -> [Double] -> Double dot x y = sum $ zipWith (*) x y main = getContents >>= sequence_ . map (print . dot weights . map read . words) . lines The idea is to take a dot product of each line of a file with a fixed vector of weights, and send the result to stdout. I thought PackedString's might help, but I'm not sure what to do with that parameter. The input file is over 1 million lines long. Any ideas? Thanks, Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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How to redirect a handle within a thread (GHC 6.4.1 runtime)?
by Dimitry Golubovsky 18 Jan '06

18 Jan '06

Is it possible to redirect a Handle (say stdout) somewhere only within a running thread (started with forkIO) not touching the same handle for the main and other threads? I have a lot of code written with putStr(Ln) which was used in a program acting as a filter, i. e. stdout was redirected by the invoking shell. Now I want to run this code within a thread in other program, but output must go to a file (or a pipe, or anywhere else a file descriptor may be opened for). So fdToHandle is not good because I need to modify the `stdout' only for that thread, not to create a new Handle. Rewriting the code is not a convenient way (but will be done if nothing else helps) because then I will need to pass that handle around. Running that code in a separate executable is just a bit more convenient because I need to pass many parameters via command line (a thread inherits values of them), and therefore is also desired to be avioded. Functions in System.Process deal with external executables. Does there exist anything like runInteractiveProcess, but just forking a new process (the whole memory is copied, so everything will be inherited), but executes a particular function within the parent program? I'm afraid that if I just make a foreign call to `fork', the RTS may not understand this correctly. -- Dimitry Golubovsky Anywhere on the Web

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Avoiding name collisions by using value spaces instead of modules
by Brian Hulley 18 Jan '06

18 Jan '06

Hi - A main problem I've found with Haskell is that within a module, too many things are put into the same scope. For example data Tree a b = Leaf a | Node {elem::b, lhs::Tree a b, rhs::Tree a b} puts Leaf, Node, elem, lhs, and rhs, into the module's namespace, as well as Tree. This means that if you want another kind of tree in the same module you've got to try and think of another word for Leaf, or else prefix every value constructor with some prefix of the tycon to try and prevent name collisions. I propose that the above declaration should introduce a *new module* Tree, as a sub module of the containing module, and Leaf, Node, elem etc will be put into this module, and not the module containing the data declaration itself. Identifiers in this child module could then either be used qualified or unqualified as long as they don't conflict with anything in the parent (containing) module, thus in the parent module, we could have: a = Tree.Leaf 3 b = Node{elem = 6, lhs = Leaf 2, rhs = Leaf 3} c = Tree.elem b c = b^elem where ^ is just a helpful sugar which binds tighter than function application and can be used anywhere to allow an "object oriented" way of thinking where the first arg is put on the left of the function (more on this later). Also, we could generalise the GHC GADT style data declarations by allowing any declarations to appear in the where part, and allow modules to consist of a single data declaration (ie begin with the keyword data instead of module) so that the contents of the Data.Set module would instead be something like: data Ord a => Set a = .... where insert :: Set a -> a -> Set a ... This would allow Set to be imported qualified into other modules where we could refer to the Set type by the single word Set instead of Set.Set (which I always think looks very strange indeed) ie module M where import qualified Data.Set as Set f :: a -> Set a -- no longer need to write a -> Set.Set a In fact, the whole complexity asociated with hiding components of a module and allowing unqualified imports could be discarded, since the correct resolution for each identifier would be determined by its typed context, just as in OOP the resolution is determined by the type of the object. So to summarise so far, for any value f :: T0 -> T1 -> ... -> Tn, we construct the tuple (Q0, Q1, ... Qn) where for each i, Qi is obtained from Ti by ignoring all predicates and quantification and replacing each tyvar by the symbol '?'. For example, for module M where foo :: forall b. Eq a => Set (Tree a) -> Tree ([a],b) -> [Tree (a,b)] we ignore the forall and Eq, and replace tyvars and tuple the results to get: M.(Set (Tree ?), Tree ( (,) ( [] ?) ?), [] (Tree ( (,) ? ?))).foo as the fully qualified reference to the entity we've just declared. If we call the tuple (Set, ...) the "ty-tuple", which specifies a space of values (ie something that each value is considered to "belong" to - the analogue of the object in OOP), then we can define a projection from a space into a containing space as follows: 1) Let Qi == Ui Si0 Si1 ... Sik be any element of a ty-tuple P. Then we can form a new ty-tuple P' by replacing Qi by Ui. 2) Let Qi be any element of a ty-tuple P of arity n+1. Then we can form P' == (Q0, ..., Qi-1, Qi+1, ... Qn) of arity n In both cases, we can use P' to qualify the identifier to get a value reference as long as this still has enough information for disambiguating other uses of the identifier in M. Applying these transformations of the ty-tuple to the example above, any of the following could be used as a partial qualification of foo in M: (Set Tree, Tree ([],?), [Tree]).foo (Set, Tree, []).foo Tree.foo foo Rule 2 allows us to propagate identifiers back up to their ancestor modules as long as there are no conflicts along the way. The type inference algorithm could then be modified (I hope) to use the top-level annotation for a value declaration to determine the entities that identifiers refer to, by searching in the appropriate value spaces determined by the types of the args and return value. label :: Tree a -> Int -> (Tree a, Int) label (Leaf x) i = (Leaf i, i+1) The identifier Leaf is resolved in the current namespace augmented by the namespace for module Tree, thus we don't have to explicitly write Tree.Leaf even though the declaration of label occurs outside the Tree module itself. (I suggest that top-level type annotations should be mandatory since without them one just drowns in a sea of TI confusion when there is a type error. By making them mandatory the TI algorithm could make real use of them to resolve identifier bindings as in OOP) Similarly: foo :: a -> Set a foo x = singleton x There is no need to say Set.singleton x, because we know that the result of foo has type Set a therefore we search in the current module augmented by the contents of the Set module (every type is also a module) for a binding for "singleton" which maps from a to Set a. Finally (apologies for this long post), returning to the use of ^ to allow an object oriented way of thinking consider: insert :: a -> Set a -> Set a ps = singleton 3 qs = insert 4 ps rs = ps^insert 4 When resolving "insert" used in the binding for rs, the compiler should see that we are looking for some function Set Int -> Int -> Set Int, and hence will be looking in the current module augmented by the Set module. However the Set module only has a binding for insert with type a -> Set a -> Set a. So the compiler should generate a new function insert' from insert by moving the first Set a arg to the front. Summary: 1) Every value binding belongs to a value space represented by a ty-tuple which abstracts away from the details of the actual type 2) The space of ty-tuples forms a lattice, and in particular this means qualification is optional when there are no conflicts (we can use Leaf 3 instead of Tree.Leaf 3), and we can use partial qualification to supply just the disambiguation info needed. 3) The TI algorithm should use the type of the expression (generated top-down from the top-level annotation) to search in the correct value spaces to resolve identifiers 4) Record field selection doesn't need any special scoping rules, and is just one example of a general object-oriented way of thinking about function application. 5) We can get all the advantages of automatic namespace management the OOP programmers take for granted, in functional programming, by using value spaces as the analogue of objects, and can thereby get rid of complicated import/export directives and improve code readability with less typing (but making more use of typing - excuse the pun :-)) I'm in the process of trying to implement a pre-processor for Haskell which would use the algorithms sketched above (as well as fixing a few other things I think are wrong with Haskell such as the way the current layout rule allows one to write code that would break if you replaced an identifier with one that had a different number of characters in it etc), so any feedback on these ideas would be welcome. Thanks for reading so far! Brian Hulley PS Everything above is purely intended for resolving the kind of ad-hoc overloading that is not amenable to treatment by type classes (which transforms some subset of the ad-hoc-overloaded functions which share the same shape into a single function with implicit dictionary arguments (in the usual implementation)).

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Yet another top-level IO proposal
by Robert Dockins 18 Jan '06

18 Jan '06

Fellow haskellers, I have a proposal I would like to enter into the eternal top-level IO debate. The proposal involves a minor language extension and some runtime support for thread local reference cells. I believe it has the potential to meet many of the needs of those requesting top level IO. My apologies for this rather lengthy message, but given the volatility of discussion on this matter in the past, it seemed best to lay out my thoughts as precisely as possible at the beginning. So without further ado, the proposal. The language extension: -- Add a new keyword 'threadlocal' and a top level declaration with the syntax threadlocal <identifier> <type expression> <initializer expression> -- The declaration consists of three parts 1) the name of the reference cell 2) the type of data stored in the cell and 3) an initializer action. -- The name of the cell declared with 'threadlocal' shares the function namespace and introduces a CAF with the type 'TLRef a', where 'a' stands for the type given in the declaration. -- The initializer action must be an expression with type 'TL a', where 'a' stands for the the type given in the 'threadlocal' declaration, and 'TL' is the thread-local initialization monad (a little like the ACIO monad, more on this below). The semantics: -- Each thread in a program (the main thread and threads sparked from forkIO and forkOS) has a "bank" of thread local variables. Writes to and reads from a thread- local cell are only written to/read from the bank of the thread performing the write/read. -- For any given bank, a thread-local cell may be "empty" (which means it holds no value) or "full" with a value of its declared type. -- There is a phantom bank of thread-local values belonging to no thread in which the value of all thread-local cells is "empty". This represents the state of thread local variables before program start. -- Whenever a thread is sparked (including the main thread) and before it begins executing, its thread-local variables are initialized. For each declared thread-local variable (in the transitive closure of imported modules), the declared initilzation action is run and the generated value initializes the thread-local cell for that thread. The initializer actions are run in an unspecified order. -- The primitives of the TL are strictly limited and include only actions which have no observable side effects (a proposed list of primitives is listed below). A TL action may read from (but NOT write to) thread-local cells in the bank of the sparking thread (the bank of the thread calling forkIO, or the special phantom bank for the main thread). -- Any exceptions generated during a thread-local initilization action are propigated to the thread which called forkIO/forkOS or, in the case of the main thread, directly to the runtime system just as though an uncaught exception bubbled off the main thread. -- New IO primitives are added to read from, write to and clear (set to empty) thread-local variables. Advantages: This proposal seems to hit most of the use cases that I recall having seen (including the very important allocate-a-top-level-concurrency-variable use case) and seems to provide a nice way to reinterpret some of the "magic" currently in the standard libraries. In addition, this proposal does not suffer from the module loading order problem that some previous proposals have; because thread local initializer actions depend only on the "previous" bank of values, the order in which they are run makes very little difference (only for the primitives that read clock time or some such). The value of a thread-local cell is always well-defined, even before the main thread starts. Values in a thread-local have a well defined lifetime that is tied to the owning thread. I think that efficient implementation is possible (maybe we can play some copy-on-write games?). I especially like that variables are only as "global" as desired for any given task; if a library writer uses thread-locals for some manner of shared "global" state, later users are always able to write programs that use more than one instance of the "global" state without needing to alter the library. Disadvantages: Requires a language extension (but I don't know of a serious alternate proposal that doesn't). Requires non-trivial runtime system support. Not sure what effect this has on garbage collection. Adds overhead to thread creation (this could perhaps be mitigated by introducing new primitives that distinguish heavyweight threads with their own thread-local banks from lightweight threads, which do not have separate thread-local banks). Its a bit complicated. You can shoot yourself in the foot (true of most of the other proposals). Some representative use cases: -- Implicit parameter style use case: You want to provide a default value that you expect will be rarely changed. Threading the parameter deeply through the code obsfucates meaning and the code is in the IO monad. Solution: * Define a thread-local variable for the value and set the initializer to set the variable to some default value if it was empty, or to copy the parent thread's value otherwise. * If desired, change the variable's value early in main (before any other threads are sparked); the new value will be propagated to all new threads and be available in main. * If desired, different threads can set different values of the parameter which will then be propagated to their sub-threads. -- Top level synchronization variable use case: You need an MVar to manage some "global" resource. Solution: * Define a thread-local variable to hold the MVar. Define the initializer to create a new MVar if the TLRef was empty, and to copy the parent thread's value otherwise. * Read the MVar from the thread-local var, and use it as usual. * Also, allows you to partition the program into sandboxes which use distinct MVars to manage distinct pools of the "global" resource without needing to change or complicate any underlying libraries. -- Running time statistics use case: You want to easily keep track of how long each thread in a program has been running in wall-clock time. Solution: * Define a thread-local variable with an initializer that reads the current wall-clock time. * Calculate the thread running time by taking the difference between the current wall-clock time and the time in the thread-local var. -- Give better semantics to standard handles use case: You want to make the handling of stdin, stdout and stderr in System.IO less "magic" and baked-in. Solution: * Define a thread-local variable for each of stdin, stdout, and stderr. The initializer action creates appropriate handles for each one in the case that the thread-local was empty, and copies the parent's value otherwise. * Nice feature: allows you to override the values of stdin, stdout and stderr for sub-threads, shell style. -- Give better semantics to getArgs use case: You want getArgs/withArgs and getProgName/withProgName to have better semantics. Solution: * Define a thread-local variable for the list of arguments and for the program name. Define an initializer which reads these values from C land when the thread-local value is empty and copies the parent thread's value otherwise. * Allows you to spark multiple threads in a single program with different "command line" arguments. The proposed list of new primitives: -- thread-locals maintained by the standard libs currentWorkingDirectory :: TLRef FilePath stdin :: TLRef Handle stdout :: TLRef Handle stderr :: TLRef Handle -- in the TL monad readTL :: TLRef a -> TL a -- ^ Reads a thread-local variable in the bank of the parent thread. -- Returns bottom if the cell is empty. tryReadTL :: TLRef a -> TL (Maybe a) -- ^ Reads a thread-local variable in the bank of the parent thread. -- Returns Nothing if the cell is empty. getClocktimeTL :: TL ClockTime getCPUTimeTL :: TL Integer getDefaultStdinHandle :: TL Handle getDefaultStdoutHandle :: TL Handle getDefaultStderrHandle :: TL Handle getDefaultArgs :: TL [String] getDefaultProgramName :: TL String newIORefTL :: a -> TL (IORef a) newMVarTL :: a -> TL (MVar a) newEmptyMVarTL :: TL (MVar a) newSTRefTL :: a -> TL (STRef a) -- in the IO monad readThreadLocal :: TLRef a -> IO a -- ^ bottom on empty tryReadThreadLocal :: TLRef a -> IO (Maybe a) -- ^ Nothing on empty writeThreadLocal :: a -> TLRef a -> IO () clearThreadLocal :: TLRef a -> IO () -- ^ Reset the thread-local to empty clearBank :: IO () -- ^ Clear all thread-local cells in the current thread ------------------------------------------------------------------------ ---------------- In order to get discussion flowing and experiment with the semantics I have put together a demonstration module which implements thread-local variables using the standard "unsafePerformIO" hacks. The module and an example usage are attached. If there is any interest in these ideas, I will post this proposal to the wiki. Please respond with thoughts and comments, Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

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Space usage problems
by Ian Lynagh 17 Jan '06

17 Jan '06

Hi all, I am having space issues with some decompression code; I've attached a much simplified version as Test1.hs. At the bottom (foo/bar) is the equivalent of deflate. This should be a standalone module which doesn't know about the rest. In the middle (readChunks) is the equivalent of gunzip. It repeatedly calls foo until there is no more input left. At the top is a simple main function that calls them. If I do dd if=/dev/zero of=data bs=1000 count=3000" # making data around 3MB ghc --make Test1 -o Test1 -O -Wall ./Test1 then in top I see Test1 increasing memory usage to around 150MB. I think this is because the "let (ys, zs) = foo xs" means zs holds on to xs (it's hard to be sure as compiling for profiling is too happy to change the behaviour). I tried (Test2) changing foo to be a monad transformer over the calling monad, so the caller's remaining input was updated as we went along, but (as well as memory usage not obviously being fixed) this is giving me a stack overflow. Has anyone got any suggestions for making a constant space, constant stack version? Thanks Ian

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libtiff
by Green Bryan - bgreen 17 Jan '06

17 Jan '06

Does anyone know of libtiff having been wrapped for Haskell? If not, where could I find a good tutorial on wrapping a c library with functions that have variable argument parameters? Thanks, Bryan Green *************************************************************************** The information contained in this communication is confidential, is intended only for the use of the recipient named above, and may be legally privileged. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please resend this communication to the sender and delete the original message or any copy of it from your computer system. Thank You. ****************************************************************************

2 1

Shootout favouring C
by Daniel Fischer 17 Jan '06

17 Jan '06

Hi, Is it only my machime, or can you confirm that for the Ackermann benchmark, it's very good for C that they chose 9 and not a larger value? For 10, we are significantly faster and for 11,12,13, we can run rings around the C-programme: dafis@linux:~/Documents/haskell/shootout> time ./cacker3 10; time ./acker 10; time ./cacker3 11; time ./acker 11; time ./cacker3 12; time ./acker 12 Ack(3,10): 8189 real 0m0.664s user 0m0.660s sys 0m0.010s Ack(3,10): 8189 real 0m0.405s user 0m0.400s sys 0m0.000s Ack(3,11): 16381 real 0m6.255s user 0m6.220s sys 0m0.000s Ack(3,11): 16381 real 0m1.731s user 0m1.710s sys 0m0.000s Ack(3,12): 32765 real 0m35.270s user 0m35.050s sys 0m0.000s Ack(3,12): 32765 real 0m10.673s user 0m10.590s sys 0m0.000s dafis@linux:~/Documents/haskell/shootout> time ./acker 13; time ./cacker3 13 Ack(3,13): 65533 real 1m4.476s user 1m4.050s sys 0m0.010s Ack(3,13): 65533 real 2m50.645s user 2m47.020s sys 0m0.020s Cheers, Daniel

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Db and STRef
by Brandon Moore 16 Jan '06

16 Jan '06

John Goerzen wrote: ... > I didn't honestly follow the STRef discussion, or how something so > I/O-based could work there. > > -- John The only message I've found was Oleg mentioning the typing that keeps an STRef limited to the scope of the runST it came from. I don't think database access could live in ST either. I think the proposal was to build something like Oleg's Monadic Regions http://okmij.org/ftp/Haskell/types.html#monadic-regions on top of IO I'm assuming you understand how the type on runST and the STRef operations ensure that, even though you *can* smuggle out an STRef in the result from runST, you will never be able to use it again. The idea was to do the equivalent thing with databases: use fancy types to ensure that handle can only be used inside to origination withDB or withCursor or whatever, and the bracketing function can release the resource on the way out, without worrying about it being used again. Brandon > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Shootout rankings
by dons＠cse.unsw.edu.au 16 Jan '06

16 Jan '06

:D Haskell now ranked 2nd overall, only a point or so behind C: http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=all And still a bit more we can squeeze out... -- Don

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Re: What's the current bug report procedure?
by Simon Marlow 16 Jan '06

16 Jan '06

Seth Kurtzberg wrote: > All, > > I have a (minor) but to report in the networking (Network and Network.Socket) libraries. If the /etc/protocols file is missing, an error message is printed saying that a service could not be found. This occurs even when no service is involved (the port number is specified explicitly using the constructor PortNumber). > > I'm aware that the procedure changed; can you point me at the right place to report this and/or the right people/bug tracker/whatever? > > Thanks, > Seth Kurtzberg I believe that's been fixed, but the place to report bugs in the network library is currently the GHC bug tracker, at http://www.haskell.org/ghc/reportabug. Cheers, Simon

1 0

please send your cabal bug reports, wishlist, and patches!
by Isaac Jones 16 Jan '06

16 Jan '06

Greetings, I'm trying hard to get a better hold on the Cabal[1] project, and a more clear idea of all the outstanding work that needs to be done. I've gone through my mailbox to dig up stuff like this, but no doubt some has slipped between the cracks. I started a bug tracker / wiki a few weeks ago, and would like your help to make sure that it reflects _all_ of the work that needs to be done in Cabal. If you've sent me a feature request or bug report that hasn't been added to Cabal, can you please check if it is on the wiki, and if not, create a "ticket" (bug report) for it: Cabal Wiki & Bug Tracker: http://hackage.haskell.org/trac/hackage/ If you're not sure, or don't want to use the bug tracker, just email me. This bug tracker is a great place to make more complex feature requests, as it has full wiki markup. See this ticket for a nice example: http://hackage.haskell.org/trac/hackage/ticket/27 Also, if you've sent me any patches that are not yet applied, please re-send them to me. peace, isaac [1] http://www.haskell.org/cabal

1 0

formal methods & functional programming
by Abigail 16 Jan '06

16 Jan '06

Hi, I have been searching papers about tha raltionship between formal methods in software engineering and functinal programmming, but i haven't found enough information. can u hel me?. Thanks Abigail. __________________________________________________ Correo Yahoo! Espacio para todos tus mensajes, antivirus y antispam ¡gratis! Regístrate ya - http://correo.espanol.yahoo.com/

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HDBC Transactions with Sqlite3
by Tom Hawkins 15 Jan '06

15 Jan '06

I have HDBC running with Sqlite3, but I'm getting a SqlError due to a locked table. Please excuse my SQL ignorance, but what may be causing the problem? In SQL, are we not allowed to select, update, and delete from a table within a single transaction? If not, what are the rules for transactions? Thanks for any help! -Tom SELECT * FROM freeKey; SELECT * FROM nextKey; INSERT INTO nextKey VALUES (1); SELECT * FROM freeKey; SELECT * FROM nextKey; UPDATE nextKey SET key = 2 WHERE key = 1; SELECT * FROM freeKey; SELECT * FROM nextKey; UPDATE nextKey SET key = 3 WHERE key = 2; INSERT INTO freeKey VALUES (0); INSERT INTO freeKey VALUES (1); SELECT * FROM freeKey; DELETE FROM freeKey WHERE key = 0; SqlError {seState = "", seNativeError = 1, seErrorMsg = "step: database table is locked"}

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using cabal? add your package to the list!
by Isaac Jones 15 Jan '06

15 Jan '06

I started a list of packages using cabal (including those uplaoded to Hackage). Please either upload your packages to hackage, or add it here. I want this list so that I can try to figure out things like how much a given modification to Cabal will break stuff: http://hackage.haskell.org/trac/hackage/wiki/CabalPackages To add your package to the list, hit "Edit Page" at the bottom of the page. Feel free to make the page into a table, add links for the packages already there, reorganize, etc. peace, isaac

1 0

Inheritance without OOHaskell
by John Goerzen 14 Jan '06

14 Jan '06

Hello, I'd like to put forth two situations in which I miss the ability to use inheritance in Haskell, and then see if maybe somebody has some insight that I'm missing out on. Situation #1: In HDBC, there is a Connection type that is more or less equivolent to a class on a OOP language. In Haskell, we use a record with named fields that represent functions. Closures are used to permit those functions to access internal state. That works well enough. But there is no way at all to extend this. Say a database such as PostgreSQL has some extra features -- it would be nice for the PostgreSQL objects to support additional functions, whereas Connection objects from other databases might not support those functions. Any Haskell function could expect a Connection object (in which case it could access only the standard functions) or a PostgreSQL object (in which case it could access the standard plus the enhanced functions). The internal state of a Connection object is DB-specific, so there can be no general function to expose it. Situation #2: In Python, every exception is an object, and every object can be extended. Therefore, I could write an exception handler for, say, an IO error, and have it work for anything that's a subclass of the generic IO error -- even if these subclasses weren't known at the time the program was written. Other handlers could be as specific or as general as desired. In Haskell, it seems that all of this has to be anticipated in advance; it's not very easy to extend things. So my questions are: 1. I have sometimes used typeclasses instead of data records. This provides some of what I'm searching for, but has the unfortunate side-effect that one can't very easily build a list of objects that may not come from the same place but are nonetheless part of the class. For instance, had I used typeclasses for HDBC, I couldn't have a list of Connection objects where some are from MySQL, some from PostgreSQL, etc. 2. As a library designer, what is the most friendly way around these problems that adheres to the principle of least surprise for Haskell programmers? 3. How does one choose between a type class and a data record of functions when both would meet the general needs? 4. Is there a way to solve these inheritance problems without resorting to a library such as OOHaskell, which many Haskell programmers are not familiar with? Thanks, -- John -- John Goerzen <jgoerzen(a)complete.org> GPG: 0x8A1D9A1F www.complete.org "Value your freedom, or you will lose it, teaches history. `Don't bother us with politics,' respond those who don't want to learn."

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powerful type checking & false expectations that a program is correct
by Isaac Gouy 14 Jan '06

14 Jan '06

Programmers who use languages without static type checking sometimes claim that static type checking gives folk the false impression that once the program passes type checking the program is correct. That always seemed silly to me, but I'm starting to wonder ;-) Of course, the shootout programs are a silly entertainment; but after taking the time to optimise and write the programs, I'm a little surprised that there have been Haskell contributions which just don't give 'the correct answer' in a way that visual inspection with the expected answer would detect. For example ackermann Haskell GHC program regex-dna (new) Haskell GHC #2 program Oh well, they are just easy to fix formating bugs :-) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

1 0

Formated output with ghci
by Frank 14 Jan '06

14 Jan '06

A 'stupid' question (sort of an embarassing one): I want to do formated output by converting data types to strings and the bring them in a formated way together. I use ghci 6.4 and when I want to see this text then it is shown as a string (i.e. with "... " added around) and the formating characters in the text (e.g. \n) are quoted and not executed. (if I apply show to the text string, then the above is done twice!) This does occur if I use a defined function (something like 'toString' or 'text') and does not occur if I use show (but I have other reasons not to use show). Is there a cure to have output formated with ghci without using show to individually convert data types into strings? Here an example output: text s2 "last id used #2\n\n(#2,EntType {unEntType = \"Point\"})\n(#1,EntType {unEntType = \"Point \"})\n\n\n deleted\nRelX empty\n\nHCons (Region2start \"start of region\",RelX empty\n) ( HCons (Region2end \"end of region\",RelX empty\n) (HCons (Point2coord \"Coordinates of Poi nt\",\n(#2,\n[NV 0.0 100.0])\n(#1,\n[NV 0.0 0.0])\n\n) HNil))" Wrapping the text string into a data type and then apply show to this data type gives also the desired result. Is there an easier way? Help is highly appreciated module OutputTest where data X = X Int Int Float deriving (Show) x1 = X 3 4 4.5 text :: X -> String text (X a b c) = show a ++ show b ++ " \n" ++ show c -- example output -- >text x1 --"34 \n4.5" data T a = T a instance Show a => Show (T a) where show (T a) = "Tx " ++ show a tx1 = T x1 instance Show (T X) where show (T a) = text a -------- works: -- >tx1 --34 --4.5

2 1

What does the Haskell type system do with "show (1+2)"?
by Jeff.Harper＠handheld.com 14 Jan '06

14 Jan '06

What does the Haskell type system do with expressions such as these . . . ? show 1 show (1+2) The type of the subexpressions "1" and "1+2" are "ambiguous" since they have type "(Num a) => a". I'm under the assumption before "1+2" is evaluated, the "1" and "2" must be coerced into a "concrete" type such as Int, Integer, Double, etc, and before "show 1" is evaluated, the "1" must be coerced into a "concrete" type. Is my assumption correct? If so, how does Haskell know into which type to coerce the subexpressions? If I try to write a new function, "my_show", which converts an expression into a string representation that includes type information, I run into errors with expressions like "show 1" and "show (1+2)" because of the type ambiguity. class (Show a) => My_show a where my_show :: a -> String instance My_show Int where my_show a = show a ++ " :: Int" instance My_show Integer where my_show a = show a ++ " :: Integer" I can avoid the errors if I change it to "my_show (1::Int)" or "my_show ((1+2)::Int). I'm wondering what the difference is between, my_show and Haskell's built-in show that causes my_show to produce an error message when it is used with ambiguous types, but Haskell's show works okay with ambiguous types.

8 8

Splitting a string into chunks
by Adam Turoff 13 Jan '06

13 Jan '06

Hi, I'm trying to split a string into a list of substrings, where substrings are delimited by blank lines. This feels like it *should* be a primitive operation, but I can't seem to find one that works. It's neither a fold nor a partition, since each chunk is separated by a 2-character sequence. It's also not a grouping operation, since ghc's Data.List.groupBy examines the first element in a sequence with each candidate member of the same sequence, as demonstrated by: Prelude> :module + Data.List Prelude Data.List> let t = "asdfjkl;" Prelude Data.List> groupBy (\a _ -> a == 's') t ["a","sdfjkl;"] As a result, I've wound up with this: -- Convert a file into blocks separated by blank lines (two -- consecutive \n characters.) NB: Requires UNIX linefeeds blocks :: String -> [String] blocks s = f "" s where f "" [] = [] f s [] = [s] f s ('\n':'\n':rest) = (s:f "" rest) f s (a:rest) = f (s ++ [a]) rest Which somehow feels ugly. This feels like it should be a fold, a group or something, where the test is something like: (\a b -> (a /= '\n') && (b /= '\n')) Any thoughts? Thanks, -- Adam

5 7

Re: Haskell-Cafe Digest, Vol 29, Issue 36
by Jeff.Harper＠handheld.com 13 Jan '06

13 Jan '06

Christian Maeder wrote: > Jared Updike wrote: > > http://www.haskell.org/onlinereport/decls.html#default-decls > > http://www.haskell.org/tutorial/numbers.html#sect10.4 > > I still don't see, why it works for show but not for my_show. > > > On 1/12/06, Jeff.Harper(a)handheld.com <Jeff.Harper(a)handheld.com> wrote: > [...] > >> class (Show a) => My_show a where > >> my_show :: a -> String > > If I let this be > > class My_show a where > my_show :: a -> String > > >> instance My_show Int where > >> my_show a = show a ++ " :: Int" > >> > >> instance My_show Integer where > >> my_show a = show a ++ " :: Integer" > > What is the difference to the builtin Show class? I was wondering the same thing myself. Then, I reread the online Haskell report link. >From http://www.haskell.org/onlinereport/decls.html#default-decls: where n>=0, and each ti must be a type for which Num ti holds. In situations where an ambiguous type is discovered, an ambiguous type variable, v, is defaultable if: o v appears only in constraints of the form C v, where C is a class, and o at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and o all of these classes are defined in the Prelude or a standard library (Figures 6.2--6.3, pages -- show the numeric classes, and Figure 6.1, page , shows the classes defined in the Prelude.) Notice the last bullet item. class Show is part of the Prelude. However, class My_show is not part of the prelude. AS far as I can tell, that's the only reason "my_show 1" produces errors while "show 1" is okay.

1 0

Does anybody have a simple example of using continuation Monad?
by Marc Weber 13 Jan '06

13 Jan '06

I'm struggling with this example: http://www.nomaware.com/monads/html/contmonad.html#example After looking at it for the fourth time I got much more.. but still not enough.. because there are so much new things (when beeing translated into some kind of condition ?> thentodo :> elsetodo which is using ThenElse .... It wouldn't be any problem if the next example wasn't using continuation, too.. and that's about combining monads which is important, isn't it? At the tutorial there was mentioned that continuation monads are used for continuation passing style which I've looked up in wikipedia meaning something like splitting a task into different parts beeing executed delayed (for example because of user interaction filling a web form?) I think one simple example like ((+1).(\x->x**2)) in continuation style would make me understand a lot more.. Marc

3 2

Chameneos
by Chris Kuklewicz 12 Jan '06

12 Jan '06

Your "case" tweak was for an older version of Chameneos that used an older Ch channel implementation. But I was inspired by your improvement to use Int# instead of data Color, and I posted a version that seems faster than the winning one that was submitted. http://www.haskell.org/hawiki/ChameneosEntry It still has to box the Int# to put it into the MVar channels. But the fastest complement is now (3# -# a -# b) and "if (other ==# faded)" is faster than the previous "case other of Faded -> ; _ ->". At least on OS X / G4. Also, thanks for cleaning up the SumFile code.

9 27

multiline strings in haskell?
by Michael Vanier 12 Jan '06

12 Jan '06

Is there any support for multi-line string literals in Haskell? I've done a web search and come up empty. I'm thinking of using Haskell to generate web pages and having multi-line strings would be very useful. Mike

4 4

I/O and utf8
by Andreas Kägi 12 Jan '06

12 Jan '06

hello i want to read a file encoded in utf8 and at a later time output portions of it on the console. Is there an easy way to do this in haskell? using the standard i/o functions i can read the file but the output gives me \1071 ... instead of the unicode characters.

4 4

Haskell DB bindings
by oleg＠pobox.com 12 Jan '06

12 Jan '06

Krasimir Angelov wrote: > There are three active database libraries: HDBC, HSQL and Takusen. > It is quite disappointing from my point of view. Recently there was > the same situation with the GUI libraires. I think the dichotomy between lower-level Haskell libraries (whose API is closer/faithful to the underlying library) and higher-level, more idiomatic libraries will always exist, because there is need for both. Regarding GUI libraries, the bulk of wxHaskell is close to wxWindows. However, there are also more higher-level, more idiomatic interfaces built on the top of it. Takusen has its own lower-level layer -- which can be replaced with other suitable lower-level layer, for example, HSQL. By suitable I mean the layer should not improve the underlying database API too much, e.g., add finalizers to database and session handlers, because avoiding such finalizers was one of the main reason for the design of Takusen. It may be an interesting experiment to see if Takusen proper can be moved to use HSQL, for example, as the lower-level layer. Also, perhaps HSQL may find something useful to borrow from Takusen's lower-level layer.

1 0

Re: [Haskell-cafe] multiline strings in haskell?
by Michael Vanier 12 Jan '06

12 Jan '06

Yes, just like that ;-) Thanks! Now if somebody has a string interpolation library, I'd be a pretty happy camper ;-) Mike > mvanier: >> Is there any support for multi-line string literals in Haskell? I've >> done a web search and come up empty. I'm thinking of using Haskell to >> generate web pages and having multi-line strings would be very useful. > > Do you mean like this: > > string = " line one\n\ > \ line two is here\n\ > \ line three is this line\n" > > > $ echo 'putStr string' | ghci A.hs > line one > line two is here > line three is this line > > -- Don

2 2

Help compiling School of Expression graphics library?
by Rakesh Malik 11 Jan '06

11 Jan '06

I'm trying to build the GraphicsLib code in Linux, Fedora. I have the March '05 version of Hugs installed (and ghc, but that doesn't seem to be relevant to this). The first bit of puzzlement comes from the error messages I'm getting: ffihugs +G +L"X_stub_ffi.c" X.hs Warning: unknown toggle `G'; ignoring. Warning: unknown toggle `L'; ignoring. runhugs: Unable to initialise Hugs (Unrecognised option) Hugs is on my path ($HOME/hugs/bin), and I edited the Makefile to point the hugs_install home directory to $HOME/hugs. X_dir is correct, it has lib/libX11.a and include/X11/X.h. My gc version is: gcc (GCC) 4.0.0 20050519 (Red Hat 4.0.0-8) (I don't know whether that's relevant, but just in case.) Thanks for any help you can provide!

3 3

Shootout favoring imperative code
by Scherrer, Chad 11 Jan '06

11 Jan '06

Several people on this list have said that the shootout favors imperative code. Is this really the case? Why is it Clean seems to have no trouble (for the incomplete set of benchmarks that are written in it)? http://shootout.alioth.debian.org/clean.php How difficult would it be to translate the Clean algorithms into Haskell? -Chad

9 20

Intersection types for Haskell?
by Brian Hulley 11 Jan '06

11 Jan '06

Hi - I'm wondering if there is any possiblility of getting intersection types into Haskell. For example, at the moment there is no (proper) typing for: f g x y = (g x, g y) Ideally, I'd like to be able to write: f:: (a -> b & c -> d) -> a -> c -> (b,d) or f :: (a -> b a) -> c -> d -> (b c, b d) which is perhaps clearer and prevents bad types such as (Int -> String & Int -> Char) by construction. While it may be impossible (?) to infer such a type for f, would it be possible to make use of such an annotation (esp since Haskell with GHC extensions already has arbitary rank polymorphism)? Also, as a second point, could functional dependencies in type classes be written using a similar syntax eg instead of class Insert t c a | c a -> t where insert :: t -> c a -> c a we could write: class Insert (h (c a)) c a where insert :: h (c a) -> c a -> c a or class Insert t@(h (c a)) c a where -- re-using as-pattern syntax insert :: t -> c a -> c a to avoid having to have a special syntax just for functional dependencies and/or to be able to write more complicated fundeps more succinctly? Regards, Brian Hulley

4 7

FunctionalJ - a library for Functional Programming in Java
by Graham Klyne 11 Jan '06

11 Jan '06

A colleague alerted me to this, which I thought might be of interest here: http://www.theserverside.com/news/thread.tss?thread_id=38430 (I have already found that my Haskell experiences have influenced my Python programming; maybe there's also hope for my Java?) #g -- Graham Klyne For email: http://www.ninebynine.org/#Contact

2 1

rank-2 :(
by Bulat Ziganshin 10 Jan '06

10 Jan '06

Hello , those rank-2 types wil make me mad :) encodeHelper :: (MRef m r, Binary m a, BitStream m (StringBuffer m r)) => a -> m String encodeHelper x = do h <- newStringBuffer "" stringBufferDefaultCloseFunc put_ h x getStringBuffer h encode x = runST (encodeHelper x) i can't force `encode` to compile, either with or without any type signatures. actually, signature for `encodeHelper` was developed by Hugs, but now i don't know how to proceed further just now tried to force Hugs tell me its type. mission impossible :) DataAltBinaryClass> :t (runST (encodeHelper 1)) ERROR - Cannot justify constraints in application *** Expression : encodeHelper 1 *** Type : ST c [Char] *** Given context : () *** Constraints : (Num b, Binary (ST c) b) Binary class in my library includes monad in its header. May be it's the cause of problems? class Binary m a where -- | Convert value to sequence of bits and write it to the stream put_ :: (BitStream m h) => h -> a -> m () but it's needed because MArray types, for example, can be serialized only in the appropriate monad: instance Binary IO (IOArray a) where ... -- Best regards, Bulat mailto:bulatz@HotPOP.com

2 1

Fannkuch timings
by Daniel Fischer 10 Jan '06

10 Jan '06

On my 1.2 GHz Duron (SuSE linux), I get significantly different timings than those on the wiki. Sebastian Sylvan's, Kimberley Burchett's and Bertram Felgenhauer's all take roughly thrice as long as posted (that's rather consistent, but the factor is surprisingly large). Cale Gibbard's takes 18.44 secs for N = 9, which is rather bad, I think. Really surprising (to me) are the following (N = 10) Algo Wiki Here Clean imperative 2.1 s 4.54 s Fastest Pure 1.8 s 8.65 s Fastest Impure 1.4 s 1.77 s. So the ratio for the fast impure version is approximately 1.6GHz/1.2GHz, which is natural, but the ratio for clean impure is 2.2 and for the fast pure version, it's even 4.8! All are compiled with -O2 -optc-O3. Does that mean my system is really poor in producing binaries from pure Haskell? And why would that be so? Cheers, Daniel

4 5

hPutStrLn and hFlush
by Gracjan Polak 10 Jan '06

10 Jan '06

Hi all, A bit strange behaviour with hPutStrLn. Consider following program: main = do handle <- openFile "output.txt" WriteMode hPutStrLn handle (unlines contLines2) -- hFlush houtput where contLines2 = flip map [1..2000] $ \x -> show x ++ " been there done that" Outputs file which ends with following lines: 1989 been there done that 1990 been there done that 1991 been there done that 1992 been there done that 199 (END) So the output is truncated. When I uncomment hFlush, file is fully written. Is this expected/documented behaviour? Platform: WinXP, GHC version 6.4.1 -- Regards, Gracjan

6 8

In for a penny, in for a pound.
by David F. Place 10 Jan '06

10 Jan '06

Hi All, Regarding the Fannkuch Shootout Entry: If we are willing to specialize flop in the way shown on the wiki, another 8% can be gained by similarly specializing rotate: rotate 2 (x1:x2:xs) = x2:x1:xs rotate 3 (x1:x2:x3:xs) = x2:x3:x1:xs rotate 4 (x1:x2:x3:x4:xs) = x2:x3:x4:x1:xs rotate 5 (x1:x2:x3:x4:x5:xs) = x2:x3:x4:x5:x1:xs rotate 6 (x1:x2:x3:x4:x5:x:6:xs) = x2:x3:x4:x5:x:6:x1:xs rotate 7 (x1:x2:x3:x4:x5:x:6:x7:xs) = x2:x3:x4:x5:x:6:x7:x1:xs rotate 8 (x1:x2:x3:x4:x5:x:6:x7:x8:xs) = x2:x3:x4:x5:x:6:x7:x8:x1:xs rotate 9 (x1:x2:x3:x4:x5:x:6:x7:x8:x9:xs) = x2:x3:x4:x5:x: 6:x7:x8:x9:x1:xs rotate 10 (x1:x2:x3:x4:x5:x:6:x7:x8:x9:x10:xs) = x2:x3:x4:x5:x: 6:x7:x8:x9:x10:x1:xs rotate n (x:xs) = rot' n xs where rot' 1 xs = x:xs rot' n (x:xs) = x:rot' (n-1) xs Cheers, David -------------------------------- David F. Place mailto:d@vidplace.com

6 7

Re: [Haskell-cafe] Intersection types for Haskell?
by Brian Hulley 10 Jan '06

10 Jan '06

José Miguel Vilaça wrote: > Hi > > If I understand your problem than the following is a solution: > > -------------------------------------------------- > > {-# OPTIONS -fglasgow-exts #-} > > class Foo a b where > g :: a -> b > > type A = {- change the following -} Int > type B = {- change the following -} Char > > instance Foo A B where > g a = {- change the following -} ' ' > > type C = {- change the following -} Float > type D = {- change the following -} String > > instance Foo C D where > g c = {- change the following -} "" > > > f :: (Foo a b, Foo c d) => a -> c -> (b, d) > f x y = (g x, g y) Thanks for the workaround. However this does not seem to be quite so general as intersection types, because it would only allow me to define f for some specific g ie the g of Foo, rather than for any general function... Regards, Brian Hulley

1 0

RE: [Haskell] Haskell DB bindings (was Re: ANN: HDBC (HaskellDatabase Connectivity)
by Bayley, Alistair 10 Jan '06

10 Jan '06

[Moving to café, too] > There are three active database libraries: HDBC, HSQL and Takusen. It > is quite disappointing from my point of view. Recently there was the > same situation with the GUI libraires. The Haskell Community is quite > small to waste efforts, developing different libraries for the same > things. When I started with HSQL there were only two database > libraries: HaSQL for ODBC and libpq for PostgreSQL. They both are > dead, I think. I decided that it is useful to have one abstract API > that can cover all database bindings. I imagine something like JDBC, > ADO or DBI for Haskell. If you guys would like this to happen then > lets discuss what we want. I would be happy to work on single project > that can satisfy all needs. > > Cheers, > Krasimir > ... The Haskell Community is quite > small to waste efforts, developing different libraries for the same > things. Yes, that could be considered undesirable. However, there is the choice for users over which API to choose that best supports their needs. In the use of an enumerator, Takusen has chosen quite a different design space from HSQL and HDBC. Whether or not that proves to be a good idea, or attractive to users, remains to be seen :-) HaskellDB+HSQL seems to be to be the most mature, and actively maintained. Although I'm keen to progress Takusen, I find it hard at present to take the necessary time, so work seems to stall. I'd have no bone to pick with HaskellDB+HSQL being promoted as the default database API, but I still want to work on Takusen, because I think there are some ideas worth exploring. When I started Takusen, I was specifically interested in using HaskellDB to interface to Oracle. At the time, HaskellDB did not use HSQL, and HSQL lacked a unifying interface, so I started work on a low-level library to Oracle (I was also keen to learn to use the FFI). The design later evolved towards the enumerator approach (Oleg persuaded me that it was a good idea) and the HaskellDB goal was never reached. Now that HSQL supports Oracle, I can choose to use HaskellDB with Oracle, so I suppose my original goal has been satisfied. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

2 1

RE: Re[2]: [Haskell-cafe] I/O and utf8
by Bayley, Alistair 10 Jan '06

10 Jan '06

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Bulat Ziganshin > > i have the question about this issue - i also want to provide > autodetection mechanism, which relies on first bytes of text files to > set proper encoding. what is the standard rules to encode utf8/utf16 > encoding used for text in file in these first bytes? Are you asking about the byte-order-mark in UTF encodings? http://www.unicode.org/faq/utf_bom.html#BOM Note that UTF8 files typically lack the BOM, as UTF8 is meant to be backwards-compatible with US7ASCII, I think. Windows Notepad is one of the few programs that will insert it if a text file is saved as UTF8. Alistair. ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

Haskell DB bindings
by oleg＠pobox.com 10 Jan '06

10 Jan '06

[moved from the main Haskell-list: I'm afraid the HDB discussion on the Haskell mailing list had already been long] Tim Docker wrote on the Haskell mailing list > The differences between HDBC and HSQL have been recently discussed. > Where does Takusen fit into this picture? From the above, it sounds > like it has quite a different API. Are all 3 of these actively > maintained? Takusen is built around an enumerator -- left fold enumerator, to be precise. That is, a database query is considered to be `left-folding' over the result table. Early termination is supported -- the iteration can be finished at any time the iteratee decided it has had enough. Cursors -- derivatives (pun intended) of the enumerator -- are supported as well, and we have a withCursor function (although, due to laziness, we haven't implemented the type-based region management for cursors. But we have talked about it!) The best supported databases are Oracle and Sqlite. For them, we implement buffer pre-allocation, pre-fetching (useful for minimizing the communication overhead if rows are small), parameter binding, handling date-time datatypes. Parameter binding is not yet supported for PostgreSQL. All interfaces support DML/DDL statements, transactions and isolation modes (if the back-end supports that), transparent translation from the DB type to a Haskell type. The user doesn't need to do anything for the translation. Database NULLs are fully handled: if the Haskell datatype is a Maybe datatype, NULL is permitted and translated into Nothing. Otherwise, NULL raises an exception. Nulls are distinguished from empty strings, provided the database server does so. Takusen has two layers. The low-level API (which is not actually meant for the end user) reconciles the differences among various DB interfaces. Currently supported are Oracle, Sqlite, PostgreSQL, Stub (that is, no DB -- useful for testing), and the ntwdblib interface to MSSqlServer (Microsoft encourages a newer interface though). Krasimir Angelov wrote: > Can you explain how do you manage the lifetime of handles in Takusen? > It isn't clear to me where are the advantages of enumerators. The handles are not visible to the user. The user cannot close the handle (or forget to close the handle) because the user is not given the handle in the first place. Just as we left-fold over a list, we are not given the pointer to the ``current list''. We are merely given the current element of the list. Perhaps the following links might be of help: http://www.eros-os.org/pipermail/e-lang/2004-March/009643.html http://pobox.com/~oleg/ftp/Haskell/misc.html#fold-stream http://pobox.com/~oleg/ftp/papers/LL3-collections-enumerators.txt

1 0

Is there a notion for identity?
by Tim Walkenhorst 09 Jan '06

09 Jan '06

{- Disclaimer: I'm rather new to Haskell and completely new to this board. I may not use the correct terminology in all cases, but I hope my intention becomes clear when you have a look at the code-snippets. -} Hey ho, Is there any way two find out whether two variables refer to the same cell? That is I'm looking for a notion of identity (compareable to == in Java) as opposed to equality (== in Haskell). The motivation is that I need to find out whether two infinite structures are identical. Consider this Haskell implementation of the simple typed lambda calculus (STLC) with the primitive Type "TyUnit" and the primitive value "TmUnit": data Type = TyUnit | TyArrow Type Type deriving (Show,Eq) data Term = TmUnit | TmVar String | TmAbs String Type Term | TmApp Term Term deriving Show {- Context is some LIFO structure which stores Argument-Type bindings: -} data Context = {- ... -} ctxEmpty :: Context addBinding :: Context -> String -> Type -> Context getBinding :: Context -> String -> Type typeof :: Context -> Term -> Type typeof _ TmUnit = TyUnit typeof ctx (TmVar s) = getBinding ctx s typeof ctx (TmAbs s ty1 t) = let ctx' = addBinding ctx s ty1 ty2 = typeof ctx' t in TyArrow ty1 ty2 typeof ctx (TmApp t1 t2) = let (TyArrow ttp tte) = typeof ctx t1 tta = typeof ctx t2 {- (1) problem here: -} in if tta==ttp then tte else error "..." -- eval omitted... The STLC does not provide a notion for recursion as it stands. My idea was to simulate recursion by using infinte structures of type Type. Consider this definition of the omega-combinator: omegaType :: Type omegaType = TyArrow omegaType omegaType omega, omegaAbs :: Term omega = TmApp omegaAbs omegaAbs omegaAbs = TmAbs "x" omegaType (TmApp (TmVar "x") (TmVar "x")) Now it would be nice if "typeof ctxEmpty omega" would return omegaType. Unfortunately it doesn't. Instead it loops on line ((1)), since omegaType and omegaType will never be compared to be equal using (==). If we had a function `sameAddress` we could write "tta `sameAddress` ttp || tta==ttp" instead of "tta==ttp" in line ((1)). If I interpret the chapter about infinite lists in the bird book correctly, this should be sound. omegaType will be represented by a cyclic graph in the runtime system and will therefore always refer to the same cell. The issue becomes a little more complicated if we consider: omegaType, omegaType' :: Type omegaType = TyArrow omegaType omegaType omegaType' = TyArrow omegaType' omegaType' omega, omegaAbs, omegaAbs' :: Term omega = TmApp omegaAbs omegaAbs' omegaAbs = TmAbs "x" omegaType (TmApp (TmVar "x") (TmVar "x")) omegaAbs' = TmAbs "x" omegaType' (TmApp (TmVar "x") (TmVar "x")) In this case it wouldn't be clear whether " omegaType `sameAddress` omegaType' " should be True or False, as a clever compiler might detect that both are identical and make them refer to the same address (, correct me if I'm wrong). This might be a reason that there is no `sameAddress`. My questions are: Is there any way to implement "sameAddress" in Haskell? Or is it at least possible to implement it with GHC using some compiler-specific notation? Thanks, Tim

3 4

Parsec Question
by Gerd M 09 Jan '06

09 Jan '06

I'm trying to use parsec for parsing a custom input stream. As far as I understood the manual correctly I need to define the primitive parser: type MyParser a = GenParser (SourcePos,Tok) () a mytoken :: (Tok -> Maybe a) -> MyParser a mytoken test = token showToken posToken testToken where showToken (pos,tok) = show tok posToken (pos,tok) = pos testToken (pos,tok) = test tok The problem is, since SourcePos is an abstract datatype, how can I actually run this parser without explicitly using values of type SourcePos in the input stream? Many thanks in advance! _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

3 3

Re: [Haskell] Re: haskell.org Public Domain
by Brian Sniffen 09 Jan '06

09 Jan '06

> > It might be nice to at least include some disclaimers of warranty. I'm not a lawyer. But those US copyright lawyers I've spoken with have expressed doubts about anybody's ability to put things into the public domain. Certainly, if you put it in the public domain, you can't also disclaim a warranty. I suspect that some license that is a superset of the following would be best: * The documentation of GHC * The documentation of Hugs * The Haskell 98 report I think that works out to be "BSD, with an option for authors to copyleft pages or code as they wish." On the other hand, I also think MoinMoin has lots to offer over Mediawiki---not least ease of maintenance and improvements such as license markers. -- Brian T. Sniffen bts(a)alum.mit.edu or brian.sniffen(a)gmail.com http://www.evenmere.org/~bts

1 0

Expanding do notation
by David F. Place 09 Jan '06

09 Jan '06

Hi All, Is there a program for expanding 'do' notation? I am trying to understand why the following code (from the Fannkuch entry) doesn't hold onto the list 'p' causing a space leak. main = do n <- getArgs >>= return . read . head let p = permutations [1..n] mapM_ (putStrLn . concatMap show) $ take 30 p putStr $ "Pfannkuchen(" ++ show n ++ ") = " putStrLn . show $ foldl' (flip (max . steps 0)) 0 p If I add a line which refers to 'p' at the end, there is a space leak. print (head p) Thanks. Cheers, David ---------without extra line ./Latest 9 +RTS -K256m -c -sstderr 123456789 ... Pfannkuchen(9) = 30 244,215,012 bytes allocated in the heap 613,972 bytes copied during GC 3,920 bytes maximum residency (1 sample(s)) 931 collections in generation 0 ( 0.06s) 1 collections in generation 1 ( 0.00s) 1 Mb total memory in use INIT time 0.00s ( 0.01s elapsed) MUT time 2.20s ( 2.72s elapsed) GC time 0.06s ( 0.13s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.26s ( 2.86s elapsed) %GC time 2.7% (4.5% elapsed) Alloc rate 111,006,823 bytes per MUT second Productivity 97.3% of total user, 76.9% of total elapsed --------with extra line ./Latest 9 +RTS -K256m -c -sstderr ... Pfannkuchen(9) = 30 [1,2,3,4,5,6,7,8,9] 244,216,248 bytes allocated in the heap 19,804,816 bytes copied during GC 10,586,416 bytes maximum residency (6 sample(s)) 931 collections in generation 0 ( 0.96s) 6 collections in generation 1 ( 1.28s) 18 Mb total memory in use INIT time 0.00s ( 0.01s elapsed) MUT time 2.30s ( 2.72s elapsed) GC time 2.24s ( 3.15s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 4.54s ( 5.88s elapsed) %GC time 49.3% (53.6% elapsed) Alloc rate 106,180,977 bytes per MUT second Productivity 50.7% of total user, 39.1% of total elapsed -------------------------------- David F. Place mailto:d@vidplace.com

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Where does memory go?
by Joel Reymont 09 Jan '06

09 Jan '06

I compiled a simple one-liner: main = print "Blah". This is the GC report: 5,620 bytes allocated in the heap 0 bytes copied during GC 0 collections in generation 0 ( 0.00s) 0 collections in generation 1 ( 0.00s) 1 Mb total memory in use Where did the memory go? What is 5K and what is (more suprisingly) 1Mb? Thanks, Joel -- http://wagerlabs.com/

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Re: [Haskell-cafe] x86 code generation going wrong?
by Chris Kuklewicz 09 Jan '06

09 Jan '06

Brian Sniffen wrote: > The first couldn't even complete on my 2.26 GHz Celeron! It's only got > 512 MB of RAM, which may be part of the problem. I should not leak memory but it may be an optimization problem. Try explicitly using "ghc -O2 -funbox-strict-fields". > > Stack space overflow: current size 1048576 bytes. > Use `+RTS -Ksize' to increase it. > ./test1 < sum-file-test-input 25.33s user 2.89s system 18% cpu 2:32.02 total > > ./test2 < sum-file-test-input 4.79s user 0.20s system 94% cpu 5.276 total > > ./test3 < sum-file-test-input 4.46s user 0.14s system 99% cpu 4.623 total > > -- > Brian T. Sniffen > bts(a)alum.mit.edu or brian.sniffen(a)gmail.com > http://www.evenmere.org/~bts >

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fastest Binary library!
by Bulat Ziganshin 08 Jan '06

08 Jan '06

Hello yes, i did it! today i spend time to optimize my own Binary library and got the (de)serialization speed about 50 mb/s with my 1 ghz cpu. it is a peek speed for unboxed arrays, in real life GC times and other overhead expenses will need much more time than (de)serialization itself. but at least noone can now say that serialization is a bottleneck of Haskell program :) i uploaded current version to http://freearc.narod.ru/Binary.tar.gz Main.hs in package is a testing program which builds, puts to memory buffer and then gets back a 40mb array and prints runtime of each operation Joel, if you are interested in switchinh to my library - write me. i have ideas about supporting your 150 records using your own DDL. smth like this: data Command = Cmd1 field1 Word32 333 field2 Word16 0 | Cmd2 field3 String "" field4 [SubRecord] [] you will describe all your records just one time and then these definitions will be compiled for the language/library you are using -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Project postmortem II /Haskell vs. Erlang/
by Joel Reymont 08 Jan '06

08 Jan '06

Simon, Please see this post for an extended reply: http://wagerlabs.com/articles/2006/01/01/haskell-vs-erlang-reloaded Thanks, Joel On Dec 29, 2005, at 8:22 AM, Simon Peyton-Jones wrote: > | Using Haskell for this networking app forced me to focus on all the > | issues _but_ the business logic. Type constraints, binary IO and > | serialization, minimizing memory use and fighting laziness, timers, > | tweaking concurrency and setting up message channels, you name it. > > That's a disappointing result. Mostly I think Haskell lets you > precisely focus on the logic of your program, because lots else is > taken > care of behind the scenes. You found precisely the reverse. > > It'd be interesting to understand which of these issues are > - language issues > - library issues > - compiler/run-time issues > > My (ill-informed) hypothesis is that better libraries would have > solved > much of your problems. A good example is a fast, generic > serialisation > library. > > If you felt able (sometime) to distil your experience under headings > like the above, only more concretely and precisely, I think it might > help to motivate Haskellers to start solving them. -- http://wagerlabs.com/

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do {x<-[1,2,3]; True <- return (odd x); return x}.. why? (do notation, monads, guards)
by Marc Weber 08 Jan '06

08 Jan '06

Here is a simple program implementing the above function in 4 different ways.. See my comments to get to know where I have problems: ---------- begin test.hs ---------- module Main where import IO import Control.Monad.List {- list1,2 are both implementations of the same function f=[1,3] ;-) I've both rewritten with the translation rules for do notation to better understand what's going on and where the differences are -} list1=do { x <- [1,2,3]; True <- return (odd x); return x} list2=do { x <- [1,2,3]; guard (odd x); return x} -- <- provided by xerox list1rewritten :: [Int] list1rewritten=let ok x = let ok2 True = do return x --1r1 ok2 _ = fail "ok2" --1r2 in return (odd x) >>= ok2 --1r3 ok _ = fail "outer" --1r4 in [1,2,3] >>= ok {- The outer let .. in >>= is used to "call" the inner >>= for each element of [1,2,3] (the list Monad causes this) True <- return (odd x): really nice trick...! if x is odd then line --1r1 is matched the values is returned otherwise line --1r2 is matched calling fail which is implemented as = [] ignoring the message hence no element is added but I'm not sure which implementation of >>= is used in --lr3: It should satisfy (Monad m) => m Bool -> (Bool -> m Int), right ? Looking at the definition taken from GHC/Base.lhs: class Monad m where (>>=) :: forall a b. m a -> (a -> m b) -> m b and a sample implementation: instance Monad [] where m >>= k = foldr ((++) . k) [] m I wonder how a, b (from m a and m b) and m (from class Monad m) are renated? Can you tell me how the implementation declaration of m a -> (...) -> m b differs in these cases: eg: 1. a = Int, b=String 2. the other way round: a=String b=Int? -} list2rewritten :: [Int] list2rewritten = let ok x = guard (odd x) >> return x ok _ = fail "I think never used?" in [1,2,3] >>= ok {- Here ok is feeded with 1,2 and three due to the list Monad again? So fail will never be called, right? I also know that guard returns either the monad data type constructor mzero or return () But how is this used in combintation with >> return x::Int to return either [] or [x] ? -} main=do -- print result of all implementations to show that they are equal sequence [ print x| x <- [[1,3], -- [1,3] should be the result list1, list1rewritten, list2, list2rewritten ] ] -------------- end ------------------------- I hope there will be some time when I can say: Monads.. I don't bother anymore I'm practicing every night while dreaming.... ;-) Marc

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Re: Shootout favoring imperative code
by Isaac Gouy 07 Jan '06

07 Jan '06

I sent a private email and the response to it has appeared on this mailing-list, so let me just correct some of the interpretations that have been made. > > You can say that again! > Ah..sarcasm, I know that one. No, it was emphatic agreement (the ordinary usage of that phrase). > > Is a persecution complex required for Haskell > > programming :-) This one is sarcasm. Hi Sebastian :-) > > I'm waiting for the complaints that binary-trees was designed > > to favour functional programming languages ;-) > ;-) == sarcasm Actually no - binary-trees was written with functional programming languages very much in mind. best wishes, Isaac __________________________________________ Yahoo! DSL Something to write home about. Just $16.99/mo. or less. dsl.yahoo.com

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Re: Shootout favoring imperative code
by Chris Kuklewicz 06 Jan '06

06 Jan '06

igouy(a)yahoo.com wrote: >>There is no need to beat a dead horse, though. This benchmark sets out >>to test fgets / atoi, and that is all. There are better benchmarks to >>spend time on. > > > You can say that again! > Ah..sarcasm, I know that one. Actually, I submitted a slightly faster sum-file entry for Haskell tonight. So I did kick the horse around, and I learned how to use -funbox-strict-fields. > Is a persecution complex required for Haskell programming :-) The sum-file benchmark is not about my cleverness. It is designed to test the language's library routines. I quote Brent: > Yes -- it was designed as a test of the standard I/O > system. > > -Brent See...It is like the "startup" benchmark -- which just tests how long it takes to start the program (and print "Hello World."). > > ackermann, sum-file, random, startup (aka hello world) are all > left-over from the old Doug Bagley Great Computer Language Shootout - > they are just little snippets of nothing that provide an easy starting > point - takfp and harmonic are much the same. takfp and ackerman and harmonic can be good tests of how well the langauage handles recursion. > > I'm waiting for the complaints that binary-trees was designed to favour > functional programming languages ;-) ;-) == sarcasm > > best wishes, Isaac > Cheers, Chris

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