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RE: Database interface - would like advice on oracle library bind ing
by Bayley, Alistair 29 Sep '03

29 Sep '03

Yes, I know of it. Krasimir's code uses similar functions and types in the three bindings (MySQL, ODBC, PostgreSql), but the bindings do not share a common interface (AFAICT). Because they are so similar, making them support a common interface shouldn't be a lot of work. I was hoping the earlier discussions on database interfaces would converge to a solution most interested parties agreed on. I'm still sold on the left-fold interface discussed previously. Oleg claims it is sufficient, as it can be automatically converted to a stream-like interface. I bit of me agrees with you that I should be consistent with previous work, and another bit says make it simple and elegant, and this leads me towards Oleg's proposals (left-fold + auto-generated value extraction functions + auto-generated stream functions). Do others on this list (and elsewhere) have any further opinions as to how a database interface ought to look? > -----Original Message----- > From: Tim Docker [mailto:timd@macquarie.com.au] > Sent: 29 September 2003 13:43 > To: Bayley, Alistair; haskell-cafe(a)haskell.org > Subject: RE: Database interface - would like advice on oracle library > bind ing > > > > Alistair Bayley wrote: > > > Still making slow progress on an Oracle database binding... > > now I'm trying to fit the API I have into some sort of > > abstract interface (like the one(s) discussed previously: > > > Since the previous dicussion I've discovered Krasimir > Angelov's HSQL interface as part of his HTOOLKIT: > > https://sourceforge.net/project/showfiles.php?group_id=65248 > > This has a uniform abstract interface similar to the one > discussed, apparently implemented for MYSQL, ODBC, and > PostgreSQL. I've only played with the MYSQL variant. > > If possible, it would seem sensible to provide a consistent > interface for your Oracle binding. If not, it may be worth > persuading him to generalise the interface. > > Then again, perhaps you knew about this already. > > Tim ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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RE: Database interface - would like advice on oracle library bind ing
by Tim Docker 29 Sep '03

29 Sep '03

Alistair Bayley wrote: > Still making slow progress on an Oracle database binding... > now I'm trying to fit the API I have into some sort of > abstract interface (like the one(s) discussed previously: Since the previous dicussion I've discovered Krasimir Angelov's HSQL interface as part of his HTOOLKIT: https://sourceforge.net/project/showfiles.php?group_id=65248 This has a uniform abstract interface similar to the one discussed, apparently implemented for MYSQL, ODBC, and PostgreSQL. I've only played with the MYSQL variant. If possible, it would seem sensible to provide a consistent interface for your Oracle binding. If not, it may be worth persuading him to generalise the interface. Then again, perhaps you knew about this already. Tim

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RFC: "Haskell for Perl Programmers" Presentation
by Shlomi Fish 29 Sep '03

29 Sep '03

At our local Perl Mongers club we have decided to hold a series of introductions to foreign language. Being a big fan of Haskell (except for any practical use), I wrote a "Haskell for Perl Programmers" intro: http://vipe.technion.ac.il/~shlomif/lecture/Perl/Haskell/slides/ It's more or less complete, but there may still be things to add or remove. Note that until now I did not write too much about the typing facilities of Haskell. I could, though. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ An apple a day will keep a doctor away. Two apples a day will keep two doctors away. Falk Fish

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RE: haddock depencies to std-modules
by Simon Marlow 29 Sep '03

29 Sep '03

> I use haddock to create documentation. Everything is fine > except of the > references to standard modules. If I am using standardtypes > like Int or > String haddock wants to create crossrefs to these modules and > fails because > of missing "sourcecode". > > Is there a simple way to fix this? I've heard of interface > files to create > right refs to existing docs, but that doesnt work. Whats the > right way? One way is to use the Haddock interface files that are distributed with GHC. I haven't put any effort into making the process easy yet - ideally Haddock should consult GHC's package database to find the interface files, but it doesn't currently do this. You'll find Haddock interface files in the same location as the documentation when you install GHC (eg. /usr/share/ghc-<version>/html/base/base.haddock on a Linux system). Cheers, Simon

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haddock depencies to std-modules
by Patrick Scheibe 29 Sep '03

29 Sep '03

hello I use haddock to create documentation. Everything is fine except of the references to standard modules. If I am using standardtypes like Int or String haddock wants to create crossrefs to these modules and fails because of missing "sourcecode". Is there a simple way to fix this? I've heard of interface files to create right refs to existing docs, but that doesnt work. Whats the right way? Cheers Patrick

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(Off-topic) Question about categories
by Peter J. Veger 28 Sep '03

28 Sep '03

> I'm not sure if anyone mentioned the examples of a poset and a monoid as categories. > There is no "internal" structure in these. In the former, the objects are the > elements and there is a morphism between a and b iff a <= b. A functor then becomes > an order preserving map. In the latter, there is one object and the morphisms are > the elements. The identity is the identity map and if x and y are two elements / > morphisms then composition is xy. A functor is then a homomorphism. > Dominic. It is probably better to use the example of a preordered set. A Preordered set is a set with a relation <=, that is reflexive (x<=x) and transtive (x<=y & y<=z --> x<=z) (but not necessarily antisymmetric, x<=y & y<=x --> x=y as for a poset). If, in a preordered set, a is the unique morphism: x -> y and b the unique morphism: y -> x, then (writing x for the identity morphism id(x)) a.b=y and b.a=x Categorically, x and y are isomorphic. Concluding that x=y is not in the categorical language. Of course, you should distinguish the category of preorders (posets, monoids) from preorder (poset, monoid) as a category. As for literature, you have the classic: Saunders Mac Lane: Categories for the Working mathematician but the working computer scientist may prefer: Andrea Asperti and Guiseppe Longo: Categories, Types, and Structures (MIT Press, 1991) Michael Barr and Charles Wells: Category Theory for Computing Science (3rd ed., Les Publication CRM, 1999) idem, Introduction to Category Theory (www.let.uu.nl/esslli/Courses/barr-wells.html) Peter J. Veger, Best, Netherlands

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(Off-topic) Question about categories
by Dominic Steinitz 27 Sep '03

27 Sep '03

Graham, I'm not sure if anyone mentioned the examples of a poset and a monoid as categories. There is no "internal" structure in these. In the former, the objects are the elements and there is a morphism between a and b iff a <= b. A functor then becomes an order preserving map. In the latter, there is one object and the morphisms are the elements. The identity is the identity map and if x and y are two elements / morphisms then composition is xy. A functor is then a homomorphism. Dominic.

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Haskell as specification language
by Luc Taesch 25 Sep '03

25 Sep '03

out of curiosity, is haskell already been used as a "specification" language ? i was thinking in a business term, rather than mathematical one. (i.e. one than " normal mortal can read, even with a bit of training ;-) I.e. one would specifiy a model, an application ( possibly run it on samples), and hand it over to developper, typically in other languages/environment... I am aware of the contracts paper of SPJ, for instance... others you may think of ? does a spec. always leads to a DSL , for instance ? I imagine a specification for an application resulting in a DSL for the "data model"/ " process model " part, (which is the generic/ reusabel part, IMH) plus something more specific to the effective application targeted... (I cannot resist explaining my long term view : if IT outsourcing really takes off one day, one key factor will be proper specification, which is in a dear state in the current practice nowadays "in the field") ref: http://search.ft.com/search/article.html?id=030923007625

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RE: Modeling multiple inheritance
by Simon Peyton-Jones 25 Sep '03

25 Sep '03

When Mark Shields and I tackled this problem we came up with Object-Oriented Style Overloading for Haskell http://research.microsoft.com/~simonpj/Papers/oo-haskell/index.htm It describes an (unimplemented) extension to Haskell, rather than modelling it in Haskell itself, but you may find it interesting none the less. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Brandon Michael Moore | Sent: 24 September 2003 22:22 | To: haskell-cafe(a)haskell.org | Subject: Modeling multiple inheritance | | I'm trying to build a nicer interface over the one generated by | jvm-bridge. I'm using fancy type classes to remove the need to mangle | method names. I would like methods to be automatcially inherited, | following an inheritance hierarcy defined with another set of type | classes. | | My basic classes look like this | class HasFooMethod cls args result | cls args -> result where | foo :: cls -> args -> result | | If I have classes A and B with foo methods like | foo_JA_Jint :: ClassA -> Jint -> Bool | foo_JB_Jboolean :: ClassB -> Bool -> Jint | then I can make instances | instance HasFooMethod ClassA Jint Bool | instance HasFooMethod ClassB Bool Jint | | Now I can just use foo everywhere. I would like to avoid declaring an | instance for every class though. In java methods are inherited from a | superclass, and I would like to inherit methods automatically as well. In | the bindings jvm-bridge generates a method is invoked with a function | mangled after the highest ancestor that defined that particular | overloading, so the implementation of HasFooMethod at a particular | overloading is the same for any descendant. | | So I defined a class to model the inheritance relationships | | class SubType super sub | sub -> super where | upCast :: sub -> super | | Now I can define a default instance of HasFooMethod: | instance (HasFooMethod super args result, | SubClass super sub) => | HasFooMethod sub args result where | foo sub args = foo (upCast sub) args | | This will propagate foo methods down the inheritance hierarcy. If a new | class C is derived from A, I just need to say | | instance SubClass ClassA ClassC | | and ClassC gets a foo method. (In the actually code I piggy-back on a | transitive subclass relation jvm-bridge defines that already includes an | upcast method, so upCast has a default that should always be acceptable). | | The problem comes when interfaces are added to the mix. Interfaces are | treated just like classes by jvm-bridge, and even though no implementation | is inherited from instances in Java, the method accessors generated by | jvm-bridge should be inherited. | | One problem is that the subclass relationship needs the functional | dependency so that the default instance of HasFooMethod will respects the | functional dependencies of HasFooMethod, so I can't declare subclass | instances for multiple inheritance. On the other hand, if I don't use the | functional dependency on HasFooMethod I end up needing to annotate most of | the return values in a program. I run into similar problems trying to use | numeric literals as arguments, because they are also overloaded. | | Does anyone know of clever solutions that would model multiple inheritance | while preserving the functional dependencies (unsafe compiler flags are | fine too), or ways to reduce the pain of overloading resolution without | the functional dependency? | | One alternative is generating seperate HasFooMethod instances for every | class in the system. The problem is that this would require alterating the | bit of jvm-bridge that uses JNI to find information on classes, which | currently only reports newly defined methods. JNI is black magic to me. | | Thanks | Brandon | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: Database interface - would like advice on oracle library bind ing
by Bayley, Alistair 25 Sep '03

25 Sep '03

> From: oleg(a)pobox.com [mailto:oleg@pobox.com] > > I'd like to remark first that many real databases let us avoid opening > many cursors at once. It seems to be more efficient to do as much as > possible within the engine ... I agree (and Tom Pledger makes the same comment). It's rare that I want to have more than one cursor open at once. When I've seen other developers do it, they're often doing something that would be better accomplished as a join. And that last time I remember doing it, it was a nested loops situation (which, on reflection, probably could have been done as a join... oh well). OTOH, I have seen interleaved fetches used to simulate a join where the specific DBMS product struggled to perform the join efficiently itself. In this case the developer is making up for a deficiency in the DBMS, and I don't want to exclude the option from the interface (if you agree with what you read on http://www.dbdebunk.com , then most DBMS products are deficient in a number of ways). > Still, opening several cursors may be unavoidable. The left fold > approach may still help -- we _can_ mechanically invert a left fold > combinator (to be precise, a half-baked version of it) into a lazy > list. Please see a separate message "how to invert the left fold" Found it. I'll have a look... > I believe the extract functions can be constructed automatically -- > similar to the way Quickcheck constructs test cases. I believe that > instead of I though this might be possible, but I had no idea how to do it. I'll have a look at the Quickcheck source to see how it's done, unless you can suggest a better example. > buffers. It's hard for me to write the corresponding code because I > don't have Oracle handy (besides, I like Informix better). Is it > possible to come up with a "stub" that uses flat files? We are only > interested in fetching rows. It doesn't matter if these rows come from > a file or from a database. That would make prototyping the interface > quite easier. I wasn't interested so much in prototyping the interface, as trying to write an implementation that supported the interface(s) discussed previously. I intended to provide the left-fold interface, and was wondering if that was all that was needed (for selects). Still, this would be a good exercise for me, at least so I can figure out how to generate extraction functions. > From: Tom Pledger [mailto:Tom.Pledger@peace.com] > Here's one approach: find the OCI equivalent of JDBC's > ResultSetMetaData, and use it to drive the allocation and freeing of > buffers. I've considered this, and I think it's the next route I'll take (the OCI supports it). At the least it'll give me the ability to perform arbitrary queries (at present I have to know the types of the result set columns and construct the extraction function manually). I've also considered stuffing more information into the Cursor type (which I've introduced), and using this in a modal fashion to decide what to do at each point. > Here's another: ... > Make getInt (and friends) behave differently depending on the mode > of the cursor they're passed: either allocate a buffer and return > _|_, decode and return the current column of the current row, or > free a buffer and return _|_. Not wanting to sound too dumb, but how do you return _|_ ? Thanks, Alistair. ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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Database interface - would like advice on oracle library binding
by Bayley, Alistair 24 Sep '03

24 Sep '03

(2nd attempt; mailman thinks I'm not a list member, but it still keeps sending me mail.) Still making slow progress on an Oracle database binding... now I'm trying to fit the API I have into some sort of abstract interface (like the one(s) discussed previously: http://haskell.org/pipermail/haskell-cafe/2003-August/004957.html ). 1. Is the left-fold the best/only interface to expose? I think yes, but that doesn't allow fine-grained control over cursors i.e. being able to open many cursors at once and interleave fetches from them. Or does it? 2. I'm finding it hard to write a doQuery function that takes an extraction function that isn't a pig to write. Some advice would be useful here... (and a long-ish explanation follows): The Oracle Call Interface (OCI) requires that I allocate buffers for the result of a single row fetch, before the first row is fetched. So a query involves: - prepare statement etc - allocate buffers (one for each column - call OCI C function "DefineByPos") - fetch row - extract/marshal data from buffer into Haskell types (which are then processed by fold function) - fetch, marshal (repeat until no more rows) - free buffers i.e. the same buffers are re-used for each row. The problem for me is how to specify the left-fold function in terms of the low-level API. If I want to supply extraction functions (to extract Haskell values from result buffer), how do I manage the buffer allocation in the doQuery function? The buffer allocate/free code also needs to know the column positions and types, in the same manner as the extract functions. I want to be able to write code like this: results <- doQuery dbconn sqltext [] \row results -> do name <- stringv row 1 address <- stringv row 2 return (name,address):results .. where the stringv function extracts/marshals a Haskell String from the result buffer. The intermediate approach I currently have means I have to pass an IO action into the doQuery function that, when evaluated, allocates the buffer and returns two more actions: - an action that extracts the row as a tuple - another action that frees the buffer The doQuery function evaluates the initial action (to allocate the buffer), uses the extract action to build the result (at present a list), and when there are no more rows, uses the free action to free the buffer. This approach is quite awkward (especially w.r.t. writing extract functions), and it's hard for me to see how to build a better interface. Hard, because of the memory management requirements. -------------------- Here's a chunk of the code. A lot of it is OCI plumbing, but I hope you can see how awkward it is to create an extract function (see ex3 at the bottom). Given pointers to the buffer, extract a string of variable length (you have to terminate it yourself). > fetchStringVal :: OCIColInfo -> IO String > fetchStringVal (_, bufptr, nullindptr, retsizeptr) = do > retsize <- liftM cShort2Int (peek retsizeptr) > nullind <- liftM cShort2Int (peek nullindptr) -- unused > pokeByteOff (castPtr bufptr) retsize nullByte > val <- peekCString (castPtr bufptr) > return val Free a single column's buffer. > freeColBuffer :: OCIColInfo -> IO () > freeColBuffer (_, bufptr, nullindptr, retsizeptr) = do > free bufptr > free retsizeptr > free nullindptr Create a buffer for a string column, and return the extract and free IO actions. > getExtractFnString :: Int -> ErrorHandle -> StmtHandle -> IO (IO String, IO ()) > getExtractFnString posn err stmt = do > c <- defineCol err stmt posn 2000 oci_SQLT_CHR > return ((fetchStringVal c), (freeColBuffer c)) doQuery uses the extractFns action to create the result buffer, and the two actions (extract and free) which are passed to doQuery2. > doQuery2 :: ErrorHandle -> StmtHandle -> IO a -> IO () -> [a] -> IO [a] > doQuery2 err stmt extractData freeMem results = do > rc <- fetch err stmt > if rc == oci_NO_DATA > then do > freeMem > return results > else do > v <- extractData > doQuery2 err stmt extractData freeMem (v:results) > doQuery :: Session -> String -> (ErrorHandle -> StmtHandle -> IO (IO a, IO ())) -> IO [a] > doQuery (Sess env err con) qry extractFns = do > stmt <- getStmt env err > prepare err stmt qry > execute err con stmt > (extractData, freeMem) <- extractFns err stmt > doQuery2 err stmt extractData freeMem [] The interface provided by doQuery means I have to write extract functions like this. Here's one for a select that returns three String columns. It's quite awkward... > ex3 :: ErrorHandle -> StmtHandle -> IO (IO (String, String, String), IO ()) > ex3 err stmt = do > (fetchcol1, freecol1) <- getExtractFnString 1 err stmt > (fetchcol2, freecol2) <- getExtractFnString 2 err stmt > (fetchcol3, freecol3) <- getExtractFnString 3 err stmt > return > ( do { s1 <- fetchcol1; s2 <- fetchcol2; s3 <- fetchcol3; return (s1, s2, s3) } > , do { freecol1; freecol2; freecol3 } > ) ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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Re: Generating "setMember" functions for record structures
by Peter Simons 23 Sep '03

23 Sep '03

Peter Gammie writes: > Haskell Report, Sec 3.15.3 Great, that's exactly what I need. Thanks a lot to all who replied! Peter

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Generating "setMember" functions for record structures
by Peter Simons 23 Sep '03

23 Sep '03

Hi, in one of my programs, I have to modify individual fields of a pretty large record structure quite frequently. Assume the structure is: > data State = State { foo :: [Int] > , bar :: [String] > } Then I'd like to have functions setFoo and setBar, which copy a State but replace the field in question with a given value. This is implemented as: > setFoo :: State -> [Int] -> State > setFoo st x = State { foo = x > , bar = bar st > } > setBar :: State -> [String] -> State > setBar st x = State { foo = foo st > , bar = x > } Now, defining these helper functions for large records is not what I call an interesting job. Thus I wonder, is there any way to automate this task? I thought about using Template Haskell, but I haven't used it at all yet and am uncertain about how difficult accomplishing this would be. I also thought about using "Data.Generics", which I know better, but the generic approach feels like over-kill for this task. In order to use it, I'd have to re-design my data structure as well. So this isn't too attractive ... Any ideas? Comments? Source Code? :-) Peter

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garbage collection and Ptr/ForeignPtr/StablePtr
by Bayley, Alistair 23 Sep '03

23 Sep '03

Should I expect that Ptr memory allocated with malloc is garbage collected when no longer used by the Haskell runtime? The FFI spec doesn't say so explicitly (that I could see); AFAICT this is how the pointer types behave: Ptr : freed when no longer used/referenced ForeignPtr : like Ptr, but will also run a finaliser when freed StablePtr : not freed or collected (I haven't tested this; I've only read the FFI spec.) ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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Re: Thanks: (was Question about categories)
by Bill Halchin 23 Sep '03

23 Sep '03

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Thanks: (was Question about categories)
by Graham Klyne 22 Sep '03

22 Sep '03

Thanks to everyone who responded to my question about category theory. The responses turned up some interesting and useful material on the web, which I'm still digesting. Notably [1] [2]. There's something in the introduction of [2] that I think will help me get a handle on this: [[ ... objects are not collections of "elements," and morphisms do not need to be functions between sets (thus morphisms cannot be applied to "elements" but only composed with other morphisms). Any immediate access to the internal structure of objects is prevented: all properties of objects must be specified by properties of morphisms ... ]] My earlier supposition was that an "object" was roughly in correspondence with a member of a set, but I'm coming to perceive that "object" corresponds more with the notion of a set or collection of some (unspecified) things. #g -- [1] http://www.cs.toronto.edu/~sme/presentations/cat101.pdf [2] http://www.di.ens.fr/users/longo/download.html which has a link to content of the (apparently) out-of-print book: [[ CATEGORIES TYPES AND STRUCTURES An Introduction to Category Theory for the working computer scientist Andrea Asperti Giuseppe Longo FOUNDATIONS OF COMPUTING SERIES, M.I.T. PRESS, 1991 ]] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ------------ Graham Klyne GK(a)NineByNine.org

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RE: garbage collection and Ptr/ForeignPtr/StablePtr
by Simon Marlow 22 Sep '03

22 Sep '03

> Should I expect that Ptr memory allocated with malloc is > garbage collected > when no longer used by the Haskell runtime? The FFI spec > doesn't say so > explicitly (that I could see); AFAICT this is how the pointer > types behave: > Ptr : freed when no longer used/referenced > ForeignPtr : like Ptr, but will also run a finaliser when freed > StablePtr : not freed or collected No, all memory that you allocate with the FFI must be freed explicitly. You can arrange for memory to be deallocated when no longer referenced by using a ForeignPtr, and using the finalizer to free the memory. Bear in mind that if your application has large, or strict, memory requirements, then relying on the ForeignPtr method may not be the best solution, because finalizers can be unpredictable. Cheers, Simon

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Re: An IO Question from a Newbie
by Jon Cast 22 Sep '03

22 Sep '03

> On Fri, 12 Sep 2003 16:27:59 -0600 > Matt O'Connor <angagon(a)earthlink.net> wrote: > > > Hello all. I'm new to functional programming and Haskell, but have > > been programming in C and Java for a while. I've been going through > > the tutorials and whatnot on haskell.org. I've read from the Gentle > > Introduction to Haskell about IO and some of the other stuff and I > > have a question about it. > > > > main = do putStr "Type Something: " > > str <- getLine > > putStrLn ("You typed: " ++ str) > > > > When compile and run this code the "Type Something: " isn't displayed > > until the putStrLn. So there is no prompt. The output looks like > > this. > > > > s > > Type Something: You typed: s > > > > But if I change the putStr "Type Something: " to a putStrLn or put a > > \n at the end of the string it displays the text immediately (ie, when > > I want it to). Is there a good reason for this? Am I doing something > > wrong? Or do I need to call some kind of standard output flush? > > Yes, the problem is that the first line is not being flushed. Importing > hFlush and stdout from IO (or System.IO) and adding hFlush stdout after > you putStr will fix it. Alternatively, you could change the buffering > on stdout to NoBuffering. I don't know if there is a standard > putStrWithFlush function or just a better answer than this. A > putStrWithFlush would be trivial to write or more generally to a > HOF that flushed after performing the action passed to it. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Just thought I'd point out (in case it isn't clear) that this function is flushAfter h a = liftM2 const a (hFlush h). Not much of an improvement... Jon Cast

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(Off-topic) Question about categories
by Graham Klyne 19 Sep '03

19 Sep '03

(I'm asking here because I believe there is some overlap between category theory and functional programming cognoscenti...) It has been suggested to me that categories may be a useful framework (more useful than set theory) for conceptualizing things for which there is no well-defined equality relationship. (The discussion is about resources in WWW.) I've done a little digging about category theory, and find some relatively approachable material at: http://www.wikipedia.org/wiki/Category_theory http://www.wikipedia.org/wiki/Class_(set_theory) and nearby. But I'm hitting a mental block with this (and other places I've look don't add anything I can grok). The definition of a category depends on the definition of a morphism, and in particular the existence of an identity morphism for every object in a category. The definition of an morphism is in terms of equality of compositions of morphisms: for f : A -> B we have Id[B]. f = f = f . Id[A] My problem is this: how does it make sense to define an equality of morphisms without some well-defined concept of equality on the underlying objects to which they apply? That is, given object X and an object Y, it is possible to examine them and determine whether or not they are the same object. And if the underlying objects have such a concept of equality, what prevents them from being sets or members of sets? But categories are presented as being more general than sets. Does anyone see the cause of my non-comprehension here? #g ------------ Graham Klyne GK(a)NineByNine.org

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How to detect finite/infinite lists?
by Juanma Barranquero 19 Sep '03

19 Sep '03

Extremely-newbie questions: Is there any way to know if a list is finite or infinite, other than doing: length l and waiting forever? :) I ask because I was learning Haskell by writing some pretty naive implementation of surreal numbers, where I used lists for left and right surreal sets, and I wanted to do some operations on the sets (like finding the maximum/minimum), but obviously just on finite sets. I vaguely suspect the answer is going to be: "No, because lists are lazy (at least when they are :) and there's no general way to know beforehand how many elements they're going to have". But still, if I write x = [1..5] the compiler knows pretty well x is not going to grow any new member... (Unrelated) Is there any standard function to do: interleave [] _ = [] interleave _ [] = [] interleave (x:xs) (y:ys) = x : y : interleave xs ys It's pretty easy to implement as shown or via zipWith, but given that Haskell 98 already includes some "basic" functions (like cycle, iterate, etc.) I wonder if I've missed this one somehow. Thanks, /L/e/k/t/u

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Re: How to detect finite/infinite lists?
by Juanma Barranquero 18 Sep '03

18 Sep '03

On Thu, 18 Sep 2003 14:20:46 -0400, "David Barton" <dlb(a)patriot.net> wrote: > that's halting problem equivalent. Yes, I know it's a halting problem, in the general case. But the compiler could try to say if it knows it *is* a finite list. So: x = [1..10] y = [1..] z = someFunctionOrOther knownToBeFinite x -> should return Just True knownToBeFinite y -> should return Just False knownToBeFinite z -> should return Nothing ("I don't know"). or something along these lines. > Nope. Ok. Thanks, /L/e/k/t/u

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RE: Fast I/O with sockets
by Simon Marlow 17 Sep '03

17 Sep '03

> I was wondering how to read and write from a Socket in the most > performant way. According to the library documentation from GHC, > 'hGetBuf' and 'hPutBuf' are the way to go. But my problem with these > functions is that they expect to read exactly the number of bytes I > gave them! > > The IO computation > > hGetBuf sock ptr 1024 > > wants to read 1024 bytes, no less. This doesn't work for me, because I > need to drive an interactive line-based protocol session. I need the > read operation to consume as much data as is available and then to > return immediately, so that I can process the command right away. > > As an alternative, I looked at 'hGetLine', but this doesn't work for > me either, at least not very well, because in my protocol, lines are > terminated by "\r\n" -- not just "\n", like hGetLine expects it. hGetLine should work fine, indeed that's what I used in the Haskell Web Server. Each line will have an extra '\r' at the end, but you just need to allow for that. We really should have non-blocking versions of hGetBuf and hPutBuf, and in fact I have the code sitting in my tree. I'll take a look and see if it can be committed. Cheers, Simon

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Re: Improvement on this function
by Jon Fairbairn 17 Sep '03

17 Sep '03

On 2003-09-17 at 10:31EDT Gordon James Miller wrote: Something I think is more café than language: > Hello all. > > I'd be interested in getting some feedback on how to do this linear > interpolation function a little more cleanly. The normal way that this > is taught is to find the set of indices in the x list that bracket the > input value, then use the slope between these points to calculate the y > value for the input value. > > I had a version working at one point using !! to access particular > elements but I wasn't sure which one was the better solution. !! can be expensive. > linterp :: [Double] -> [Double] -> Double -> Double > linterp (x1:x2:xs) (y1:y2:ys) x > | x <= x2 || xs == [] = linterpPair x1 x2 y1 y2 x > | otherwise = linterp (x2:xs) (y2:ys) x > where linterpPair x1 x2 y1 y2 x = x1 + (x - x1) * (y2 - y1) / (x2 - > x1) It seems to me that you have too much going on in one function. It would be better to break it into the part that finds the point where the interpolation is supposed to happen and the function that does the interpolation. You sort-of do this, but don't go far enough for my taste. You don't say whether the xs are supposed to be in increasing order. Also it's not obvious that your version does the right thing in the case where x < head xs, and linterpPair doesn't look right to me. Something like: linterp :: [Double] -> [Double] -> Double -> Double linterp xs ys x = linterpPair (neighbours (zip xs ys)) where linterpPair ((x1,y1),(x2,y2)) = y1 + (x - x1) * (y2 - y1) / (x2 - x1) neighbours all@(p@(x1,y1):rest) | x >= x1 = head (dropWhile ((<x). fst . snd) (all `zip` rest)) would be more to my taste, though it would be better to handle the errors rather than leave them to the runtime system. I think I would also define the function as linterp :: [(Double, Double)] -> Double -> Double since that seems more natural as the data are connected that way, and it would eliminate a zip. -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

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Querying DNS from Haskell
by Peter Simons 16 Sep '03

16 Sep '03

Hi, is anyone aware of a library for Haskell, which allows to query domain name servers for MX, A, and PTR records in a non-blocking way? My first guess was to create an Interface to ADNS [1], but I figured, maybe someone else has already solved this problem? Peter [1] http://www.gnu.org/software/adns/

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RE: simpler I/O buffering [was: RE: An IO Question from a Newbie]
by Simon Marlow 16 Sep '03

16 Sep '03

> Writing a sequence of characters across the interface I'm > proposing is a > request by the writing program for those characters to appear > at their > destination "soon". Ideally, "soon" would be "immediately"; > however, the > characters' appearance may deliberately be delayed ("buffered"), for > efficiency, as long as such delay is "unobtrusive" to a human > user of the > program. Buffering timeouts would depend on the device; for > a terminal, > perhaps 50-100 ms would be appropriate. Such an interval > would tend not > to be noticeable to a human user but would be long enough to > effectively > collect, say, an entire line of output for output "in one > piece". The use > of a reasonable timeout would avoid the confusing behavior where a > newline-less prompt doesn't appear until the prompted data is entered. So, let me see if I've understood: the situation would be exactly as now, except that the buffer automatically gets flushed if it stays non-empty for more than a certain time interval. I don't think I like it: whatever time interval you choose will be too long. If you're doing communication with another program, for example, you really don't want to have the dialog stalled for 100ms doing nothing. Sure, you could insert a flush, but only a programmer knowledgeable about buffering will know to do that, or a programmer who has debugged the program in detail to find out why it is sitting idle most of the time. So, in the end the programmer still needs to know about buffering. I'd prefer it if the program didn't work at all when the flush is missing, because it tells you immediately that something is wrong. Cheers, Simon

1 0

Fast I/O with sockets
by Peter Simons 16 Sep '03

16 Sep '03

Hi, I was wondering how to read and write from a Socket in the most performant way. According to the library documentation from GHC, 'hGetBuf' and 'hPutBuf' are the way to go. But my problem with these functions is that they expect to read exactly the number of bytes I gave them! The IO computation hGetBuf sock ptr 1024 wants to read 1024 bytes, no less. This doesn't work for me, because I need to drive an interactive line-based protocol session. I need the read operation to consume as much data as is available and then to return immediately, so that I can process the command right away. As an alternative, I looked at 'hGetLine', but this doesn't work for me either, at least not very well, because in my protocol, lines are terminated by "\r\n" -- not just "\n", like hGetLine expects it. Finally, I considered 'hGetContents', which has the nice property of returning a lazy-evaluated list of the data read from the socket. But unfortunately, I don't see how I could implement something like a "read timeout" on the socket when using this function. Any ideas, anybody? Peter

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RE: An IO Question from a Newbie
by Simon Marlow 16 Sep '03

16 Sep '03

> > But there's one significant difference between C and > Haskell, which is > > applicable in the case of Matt's program. In C, any line-buffered > > output streams are automatically flushed when a read from an > > unbuffered or line-buffered stream can't be satisfied from > its buffer. > > Interesting. I didn't know this. Maybe we should match this > behaviour, or > provide a write-string-and-flush function. It seems like this issue > is causing an undue amound of trouble. I wrote GHC's IO library, and deliberately didn't include this feature. The previous version of the library did have such a feature, specifically for stdin/stdout. Note that the report doesn't say we must do this. The reason I didn't include the feature is because I can't see a way to do it right. Flushing *all* line-buffered handles (the ANSI C way) doesn't seem right. Flushing stdout just because we read from stdin is not right, because the two streams might refer to completely different I/O objects. Perhaps we should attempt to detect when there are two streams connected to the same I/O object (file, pipe, tty, whatever) and enable the magic flushing then. But now do I have to explain to people how this works? I suppose we could take the view that extra flushing is basically harmless, so it doesn't matter that we flush a bunch of Handles more often than we need to. The advantage of the current scheme is that it is easy to understand and explain; the library doesn't try to do clever stuff behind your back. The disadvantage is that it catches people out, and it sometimes requires you to import IO in an otherwise Prelude-only program. I'm more-or-less agnostic - if there's a way to avoid catching people out without introducing too much overhead or complicated rules that we have to explain, then I'll happily implement it. Cheers, Simon

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threaded red-black tree
by Lex Stein 16 Sep '03

16 Sep '03

Hi, No one responded to my question several weeks ago about a purely functional implementation of a threaded, red-black tree. My message was sent about the same time as that flurry of silly emails about "how to respond to homework questions". Was my message not responded to because it was classified as a homework question? I assure you this is officework, not homework. I ended up porting Okasaki's red-black tree implementation; hacking it apart with a bunch of mutation for the threading of the list through the tree. However, I'm still missing a deletion function and I haven't been able to find a prototype (Okasaki's red-black tree module lacks delete). My study of the RB-tree deletion routine in CLR hasn't yet yielded an adaptation for a functional environment. Any advice would be much appreciated. Thanks, Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein(a)eecs.harvard.edu cell: 617-233-0246

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Instance checking and phantom types
by Nick Name 15 Sep '03

15 Sep '03

Hi all, I have an example wich I don't understand: ---------------begin class C t data T = T instance C T data C t => T1 t = T1 f1 :: T1 () f1 = T1 data C t => T2 t = T2 t f2 :: T2 () f2 = T2 () ----------------end The first function, f1, is accepted both by hugs and ghc, unlike the second wich is rejected. Why does this happen? Shouldn't f1 be rejected with "no instance C ()" Thanks for help Vincenzo

2 3

Read file problems
by Frederic BELLOC 15 Sep '03

15 Sep '03

HI, I have a recurrent probleme with function readFile and Monad : it is that i try to get a line, transform it and stock it in a list and what i want is that function return me the list. But hugs say me that in readFile my_file >>= \s -> map cons_line (lines s) readFile is a IO String type but is used here as [[Char]] type... and i don't know what to do... Ghc say me that is the lambda abstraction that fails with "map cons_line (lines s)" , he couldn't match IO against [] ! Help me please, i understand well (i hope) how Monad work but here i don't see a solution. Sorry for my poor english but i'm already a student. Thanks for all. -- C, socks and sun ;-) and haskell bugs :-(

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zlib binding providing Handle to compressed file.
by David Roundy 15 Sep '03

15 Sep '03

I've just created a binding to the gzopen function of zlib which causes it to create a Handle. The code currently only supports ReadMode and WriteMode, and hFileSize won't work properly when reading a file. In fact, pretty much nothing but plain old reading and writing will work, but such is life. Anyhow in case anyone is interested, I'm attaching the code. It creates a pipe using pipe(2) and spawns a thread to pass the data between the pipe and gzread or gzwrite. It's not pretty, but it's better than any other solution I could think of. Suggestions or criticisms are welcome. -- David Roundy http://www.abridgegame.org/darcs

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An IO Question from a Newbie
by Matt O'Connor 15 Sep '03

15 Sep '03

Hello all. I'm new to functional programming and Haskell, but have been programming in C and Java for a while. I've been going through the tutorials and whatnot on haskell.org. I've read from the Gentle Introduction to Haskell about IO and some of the other stuff and I have a question about it. main = do putStr "Type Something: " str <- getLine putStrLn ("You typed: " ++ str) When compile and run this code the "Type Something: " isn't displayed until the putStrLn. So there is no prompt. The output looks like this. s Type Something: You typed: s But if I change the putStr "Type Something: " to a putStrLn or put a \n at the end of the string it displays the text immediately (ie, when I want it to). Is there a good reason for this? Am I doing something wrong? Or do I need to call some kind of standard output flush? Thanks. Oh, I'm using ghc. Matt -- "Not everone knows Josiah Royce's definition of a liar as a man who willfully misplaces his ontological predicates, but everyone who has ever told a lie will recognize its accuracy." Ch. 94: Truth - "The Great Ideas: A Synopticon of Great Books of the Western World"

5 6

YAHT Problem
by Mark Espinoza 14 Sep '03

14 Sep '03

1 0

No safety in numbers
by Konrad Hinsen 14 Sep '03

14 Sep '03

I am trying to write a larger piece of code using only type constraints for all the numbers, not specific types, in order to be able to choose the precision as late as possible. This works rather well (something I can't say of many other languages), but one problem I keep running into is constants. Something I need frequently is, for example, the Boltzman constant, 0.0083144708636327096 in my unit system. I can certainly type 0.0083144708636327096 everywhere in the code and make things work, literals have the nice property of being overloaded. But for the sake of readibility, I prefer to give this beast a name and have the explicit value only once in my code. So I create a module "Constants" with something like k_B = 0.0083144708636327096 The trouble is that k_B then becomes "Double" by default (or any other type I declare it to be). And this ruins all the code where I make a reference to it, with Hugs telling me "Inferred type is not general enough" because all my "generic" types become unified with Double. The only solution I can see is to write a function "fromDouble" in analogy to "fromInteger" and put this in front of all named constants. But how could I implement such a function? I might also declare all my constants to be "Rational" and use "fromRational", but I don't know much about the "Rational" type. Do I have to worry about insufficient or compiler-dependent precision? Konrad.

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Simple IO Monad problem
by kuq32tr02＠sneakemail.com 13 Sep '03

13 Sep '03

Hello, I'm starting to use Haskell for writing actual programs using monads and I'm already lost. I have the following script: #!/usr/bin/runhugs > module Main where > import System(getArgs) > main = do putStr "Hello, World\n" > strs <- getArgs > map putStrLn strs Which gives the following error: runhugs: Error occurred Reading file "./mailalias.lhs": Reading file "/usr/lib/hugs/lib/System.hs": Reading file "./mailalias.lhs": Type checking ERROR "./mailalias.lhs":5 - Type error in final generator *** Term : map putStrLn strs *** Type : [IO ()] *** Does not match : IO a Can someone please explain what I'm doing wrong? Thanks!

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Re: Standalone Parser for Haskell Grammar
by Peter Robinson 10 Sep '03

10 Sep '03

Thanks to both of you, you helped me a lot! :-) Peter On Wednesday 10 September 2003 14:56, you wrote: > You may find an error correcting combinator based parser in the > directory uust, to be found at cvs.cs.uu.nl > > Just check out the whole tree and follow the building and installing > instructions. The parser etc can be found in the UHC section. It makes > use of the parser combinators found elsewhere in the tree, > > Doaitse Swierstra > > On woensdag, september 10, 2003, at 12:33 PM, Peter Robinson wrote: > > Hello! > > Does anyone know a reasonable standalone Parser for the Haskell > > Grammar? The > > only one i found was hsparser but it's still an alpha release and i > > get a few > > errors during compiling. I know i could write one using Happy but i > > don't > > want to reinvent the wheel... > > regards > > Peter > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Standalone Parser for Haskell Grammar
by Peter Robinson 10 Sep '03

10 Sep '03

Hello! Does anyone know a reasonable standalone Parser for the Haskell Grammar? The only one i found was hsparser but it's still an alpha release and i get a few errors during compiling. I know i could write one using Happy but i don't want to reinvent the wheel... regards Peter

3 2

Prefix and postfix operators
by Eugene Nonko 09 Sep '03

09 Sep '03

Hello, Are there any plans to extend Haskell parser to support prefix and postfix operators? It will be just great for domain-specific languages. It always kind of bothered me that unary minus is special. Thanks, Eugene

4 3

Why not haskell?
by Nick Name 09 Sep '03

09 Sep '03

Inspired by the various replies to the "haskell for non-haskell sake" I wonder why so much people uses haskell "only for prototipying" or producing code in other languages. I am just curios to hear from people who do not use haskell for project releases, or just think it's not suitable for a mature project, what exactly they find bad in current haskell implementations, or perhaps in the standard. Vincenzo

4 3

Library documentation
by Konrad Hinsen 05 Sep '03

05 Sep '03

Is there any place where I can find an introductory documentation for the Haskell libraries? To be more concrete, say I am interested in using Data.Array.ST.STUArray. I would need some explanation of the ST monad, and then an explanation of how I can use an STUArray in an ST monad. Plus ideally examples. But the same applies to most of the libraries. I can't find any documentation other than the very terse reference or the parts that happen to be covered by textbooks. Konrad.

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RE: haskell reify, was sequencing data structures
by Simon Marlow 05 Sep '03

05 Sep '03

> I wonder whether keeping the names of data constructors in info tables > is expensive? > > I would have guessed not. (more debugging info such as > function names and src locations is likely to be more > expensive however). > > Anyway, would it be possible/feasible for GHC to keep names > in info tables > all the time, not just for profiling? At least for data-constructors? That's a distinct possibility. If you send patches we'll incorporate them. Cheers, Simon

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RE: haskell reify, was sequencing data structures
by Simon Marlow 05 Sep '03

05 Sep '03

> - There needs to be some support from the > compilers/interpreters. Hugs > already has this. Ghc has some of it, but I abuse the > profiling system > in order to get the names of constructors to be present > on the heap. > I'm not happy with this. Under normal compilation GHC > doesn't keep > constructor names around, as far as I know. However, the > new debugger > in GHC must do something to get names of things, so perhaps the > -prof hack is no longer needed? I believe the debugger that Robert Ennals is working on tries to infer constructor names from the symbol table of the binary. Personally, I'd like to see debugging info placed in a separate segment of the binary, so it won't be loaded with the code but can be used by a debugger. Cheers, Simon

2 1

haskell html-colorizer
by henning＠ikso.net 04 Sep '03

04 Sep '03

Are anyone aware of a haskell html-colorizer a' la the emacs-mode? Regards /Henning

8 7

Problem with Infinite Lists
by Wamberto Vasconcelos 03 Sep '03

03 Sep '03

Folks I am new in this forum, so apologies if this has been asked before. If this is the case, please point me to the direction of the answer and I won't bother you again! Also, could you make sure you CC your answers to me? I am not sure how long it takes for my subscription to be activated and I don't want to miss out on the action :-) The Problem I have defined this function to calculate the Fibonacci numbers: all_fib :: [Float] all_fib = 1:(1:(add_fib all_fib)) where add_fib (x:y:rs) = (x + y):(add_fib (y:rs)) Which seems to work: Main> take 20 all_fib [1.0,1.0,2.0,3.0,5.0,8.0,13.0,21.0,34.0,55.0,89.0,144.0,233.0,377.0, 610.0,987.0,1597.0,2584.0,4181.0,6765.0] However, when I tried Main> filter even (take 20 all_fib) ERROR - Illegal Haskell 98 class constraint in inferred type *** Expression : filter even (take 20 all_fib) *** Type : Integral Float => [Float] What is going on here? Thanks in advance for any help/hint. -- Wamberto Vasconcelos, PhD wvasconcelos(a)acm.org Department of Computing Science http://www.csd.abdn.ac.uk/~wvasconc University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK Phone +44 (0)1224 272283 Fax +44 (0)1224 273422

5 4

Histograms
by Konrad Hinsen 03 Sep '03

03 Sep '03

How would one best (read: most efficiently) create a histogram in Haskell? More precisely, given a list of integers in a known range, I want a count of how often any given number occurs. In imperative languages this would be done by iterating over the list and updating the count for the element under consideration. Without updatable data structures, the most straightforward solution I can think of would start by sorting the list, but for a long list that is very inefficient. Any suggestions? Konrad.

2 1

OT - Book on Programming
by Matthew Donadio 03 Sep '03

03 Sep '03

Hi all, This is a little off-topic, but I know there are a lot of people here with teaching experience. I am looking for a good book on programming. I recently picked up a gig doing ActionScript programming for a multimedia company. They have very good designers, but not very good programmers. So, they decided to hire a professional programmer to help them out with some of their projects. They have several people in the company who understand the mechanics of programming, but don't really understand the concepts. I am looking for a book to recommend to them. I would really like to find a book that just discusses programming, and avoids any one particular language. It would have to cover the common imperative controls, as well as basic data structures. Thanks. -- Matthew Donadio (m.p.donadio(a)ieee.org)

4 3

RE: Help on Homework help
by Simon Peyton-Jones 03 Sep '03

03 Sep '03

Good write-up. One suggestion: it'd be good to suggest *strongly* that people only send their homework-style questions to Haskell-café, not to the main Haskell list. Given the existence of the fomer, using the latter is likely to irritate, which isn't likely to elicit helpful responses. I know I could make the change myself, but you'll do it better :-) Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Andrew | J Bromage | Sent: 03 September 2003 06:52 | To: haskell-cafe(a)haskell.org | Subject: Help on Homework help | | G'day all. | | In response to the recent poll, and because I promised to, here is a | first attempt at a page to direct people to, should they ask for | homework help in the wrong way: | | http://haskell.org/hawiki/HomeworkHelp | | Comments, criticisms and contributions are most welcome. | | Cheers, | Andrew Bromage | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Help on Homework help
by Andrew J Bromage 03 Sep '03

03 Sep '03

G'day all. In response to the recent poll, and because I promised to, here is a first attempt at a page to direct people to, should they ask for homework help in the wrong way: http://haskell.org/hawiki/HomeworkHelp Comments, criticisms and contributions are most welcome. Cheers, Andrew Bromage

1 0

Poll: How to respond to homework questions
by Tom Pledger 03 Sep '03

03 Sep '03

Hi. I'm curious about what the people on this list consider appropriate, as responses to homework questions. Even if there isn't a consensus, it may be interesting to see how opinion is divided. Please consider the following. (A) Give a perfect answer. (B) Give a subtly flawed answer. (C) Give an obfuscated answer. (D) Give a critique of what the questioner has tried so far. (E) Give relevant general advice without answering the specific question. I mean them to include giving references, e.g. a Wiki URL to relevant general advice would count as (E). Which of those do you approve of? Which do you disapprove of? Please email me a concise vote, e.g. OK: B D E Not OK: A and I'll post a collated result next week. - Tom

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IO StateTransformer with an escape clause
by Thomas L. Bevan 01 Sep '03

01 Sep '03

Hi, I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point. I tried constructing a StateT s MaybeIO a monad but the state is lost when the rest of the monad collapses. Tom

3 2