June 2007 - Haskell-Cafe

Reinvention
by Andrew Coppin 27 Jun '07

27 Jun '07

I seem to be forever writing code that looks like this: decode :: String -> (SKI,String) decode (c:cs) = case c of 'S' -> (S,cs) 'K' -> (K,cs) 'I' -> (I,cs) '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs1 in (e0 :@: e1, cs1) In other words, lots and lots of functions with signatures like foo :: [Foo] -> (Bar,[Foo]) that likes to call itself recursively, or that gets called from somewhere else. Is there a Better Way(tm) to do this? It looks like such a common idiom that you'd expect to see "something" in the libraries somewhere. (I remember being puzzled that there was no library function for creating a Cartesian product of two lists. Puzzled until I realised that the monadic nature of lists make it utterly trivial to do this by hand anyway! So it's not always obvious to know what you're looking for...)

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A Query Language for Haskell Terms
by Pasqualino 'Titto' Assini 27 Jun '07

27 Jun '07

Hi, I am writing a Web application using HAppS. As all HAppS apps, it represents its internal state as a Haskell term (HAppS automagically provides persistence and transactions). It is a neat and efficient solution, you can write your data model entirely in Haskell and, at least for read-only transactions (queries) it will be operate as fast as possible as all data is in memory (if your transactions modify the application state, the transactions has to be recorded on disk to make it persistent, but this is pretty fast too). One major component, however seem to be missing, if we are effectively using Haskell as an in-memory database where is the "SQL for Haskell": a generic query language for Haskell terms? There are three basic functions that every web app has to provide, and all of them could be provided by a generic "Haskell SQL": -- query the application state -- transform (possibly monadically) the application state : the result of the query is the new state -- access control: what an user can see is what is returned by an internal access control query The availabilty of such a language would be a major boost for Haskell-based web applications as every application could be accessed via the same API, the only difference being the underlying application-specific data model. So my question is: what ready-made solutions are there in this space, if any? And if there are none, how would you proceed to design/implement such a language? The basic requirements, in decreasing order of importance, are: -- Safe, it must be possible to guarantee that a query: --- cannot cause a system crash --- completes by a fixed time of time --- uses a 'reasonable' amount of space --- cannot perform any unsafe operation (IO, or any unallowed read/write of the application state) -- Expressive (simple queries should be simple, complex queries should be possible) -- Simple to implement -- Efficient: --- Repeated queries should be executed efficiently time-wise (it is acceptable for queries to be executed inefficiently the first time) and all should be space-efficient, so it should not do unnecessary copying. -- User friendly: --- Simple to use for non-haskeller --- Short queries Ah, I almost forgot, it should also be able to make a good espresso. The problem can be broken in two parts: 1) How to implement generic queries on nested terms in Haskell? 2) How to map the queries, written as a string, to the internal Haskell query Regarding the first point, I am aware of with the following options: - SYB (Data.Generics..) - Oleg's Zipper - (Nested) list comprehensions (that are being extended with SQL-like order by and group by operators) Being rather new to Haskell all these options are rather unfamiliar so I would appreciate any advice on what should be preferred and why. Regarding the second point: The simplest solution would be to avoid the problem entirely by using Haskell directly as the query language. This is the LambdaBot way: queries are Haskell expression, compiled in a limited environment (a module with a fixed set of imports, no TH). Lambdabot avoids problems by executing the expression on a separate process in a OS-enforced sandbox that can be as restrictive as required (especially using something like SELinux). However, to get the query to execute efficiently it would probably have to be executed in a GHC thread and I am not sure how safe that would be. Looking at the discussion at http://haskell.org/haskellwiki/Safely_running_untrusted_Haskell_code it seems clear that there are many open issues. For example, how would I enforce limits on the space used by the query? So, it would probably be better to define a separate query language that is less expressive but more controllable than full Haskell, but what form should that take? Any suggestion/tip/reference is very welcome, titto

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HDBC and Sqlite3: SqlValue is restricted to SqlString
by Malte Milatz 27 Jun '07

27 Jun '07

It's always good to know there's a general Haskell mailing list to ask questions. I'm experimenting with the HDBC package in combination with Sqlite3. Here's my example table: sqlite> .schema foos CREATE TABLE foos (id integer primary key, flag bool, comment text); sqlite> select * from foos; id flag comment ---------- ---------- ---------- 1 0 a 2 1 b Here's my query in ghci: Database.HDBC> quickQuery conn "select * from foos;" [] [[SqlString "1",SqlString "0",SqlString "a"], [SqlString "2",SqlString "1",SqlString "b"]] What strikes me is that all values are represented as SqlStrings, although the type SqlValue would allow something more succint, e.g. SqlBool for the flag column. Is this a general limitation of the Sqlite backend, or is this by design (Sqlite seems to do typing differently than I had expected), or is it just that nobody has implemented a better behavior yet? Curiously, Malte

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Preferred way to get data from a socket
by Chad Scherrer 27 Jun '07

27 Jun '07

I've never used sockets before, but I need to now, and I need to be able to get a lot of data quickly. I was thinking about doing something like this (below), but I'm wondering if there's a way that would be faster. Is the obvious way of doing this "the right way"? I'm happy to install outside libraries if that would help, as long as they work on Linux and MS. Thanks! -Chad ----------------------8<------------------------------- import Network import qualified Data.ByteString.Lazy as B hostName = "myComputer" portID = PortNumber 54321 theData :: IO B.ByteString theData = connectTo hostName portID >>= B.hGetContents

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directory tree?
by Chad Scherrer 27 Jun '07

27 Jun '07

Haskell is great at manipulating tree structures, but I can't seem to find anything representing a directory tree. A simple representation would be something like this: data Dir = Dir {dirName :: String, subDirectories :: [Dir], files :: [File]} data File = File {fileName :: String, fileSize :: Int} Maybe these would need to be parametrized to allow a function splitting files by extension or that kind of thing. Anyway, the whole idea would be to abstract as much of the file stuff out of the IO monad as possible. I haven't used the "Scrap Your Boilerplate" stuff yet, but it seems like that could fit in here naturally to traverse a Dir and make changes at specified points. The only problem I can see (so far) with the approach is that it might make "big changes" to the directory tree too easy to make. I'm not sure immediately how to deal with that, or if the answer is just to post a "be careful" disclaimer. So, what do you think? Do you know of any work in this direction? Is there a way to make "dangerous 1-liners" safe? Is there a fundamental flaw with the approach I'm missing? Thanks much, Chad

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ANN: Scripting.Lua 0.1
by Gracjan Polak 27 Jun '07

27 Jun '07

Hi all, I'm pleased to announce the first public release of Scripting.Lua. The package hslua-0.1 contains Haskell FFI bindings for a Lua interpreter along with some Haskell utility functions simplifying Haskell to Lua and Lua to Haskell calls. Full Lua interpreter is included in the package. Example import qualified Scripting.Lua as Lua main = do l <- Lua.newstate Lua.openlibs l Lua.callproc l "print" "Hello from Lua" Lua.close l More information http://home.agh.edu.pl/~gpolak/hslua The Lua language http://www.lua.org -- Gracjan

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Re: [Haskell-cafe] Collections
by Andrew Coppin 27 Jun '07

27 Jun '07

Dan Piponi wrote: > Andrew said: > >> True enough - but that's a rather specific task. I'm still not seeing >> vast numbers of other uses for this... > > Graphs are one of the most ubiquitous structures in the whole of > computer science. Whether you're representing dataflows, or decoding > error-correcting codes, or decomposing an almost block matrix into > independent parts for multiprocessing, or figuring out which registers > to spill in a compiler, or programming neural networks, or finding the > shortest path between two cities, or trying to find dependencies in a > sequence of tasks, or constructing experimental designs, or using an > expert system to diagnose disease symptoms, or trying to find optimal > arrangements of marriage partners, or a million other tasks, graphs > appear everywhere! I see *trees* around the place a lot, but not general graphs. Maybe it's just the type of problems I attempt to solve?

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Parsec
by Michael T. Richter 27 Jun '07

27 Jun '07

I just wanted to say "thank you" to Daan Leijen for Parsec. It's nice to finally find a tool that does parsing that's: a) readable; b) understandable; c) tightly integrated with the target language. I've been dodging Parsec for a long time because my experiences with parser generators and parser libraries in the past have been unpleasant chores. Parsec is a ray of sunshine. Thanks, Daan. -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) The most exciting phrase to hear in science - the one that heralds new discoveries - is not "Eureka!" but "That's funny..." (Isaac Asimov)

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Re: [Haskell-cafe] Haskell version of ray tracer code is much slower than the original ML
by Philip Armstrong 26 Jun '07

26 Jun '07

On Thu, Jun 21, 2007 at 12:25:44PM +0100, Sebastian Sylvan wrote: >Try using floats for the vector, and strict fields (add a ! to the >fields in the data declaration). Because the optimisation page on the haskell wiki is very explicit about never using Float when you can use Double, that's why. An older revision used Float and it was slower than the current one. Making the datatypes strict also makes no difference. I have tried the obvious things :) >That's the simplest possible thing I can think of after about two >seconds of looking anyway. It appears that the ML version uses float >so I don't understand why you would use Double for the Haskell version >at all, and then think you could do any sort of valid comparisons >between them... OCaML floats are Doubles, at least on x86. cheers, Phil -- http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt

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Re: Propositional logic question
by Dave Tapley 26 Jun '07

26 Jun '07

Whoops, okay after two lines (thanks to oerjan) on #haskell I realise that yes, it is as easy as it should have been. For completeness: [A /\ B]1 ------------ (/\ E1) [A => (B => C)]2 A ------------------------------------------------- (=> E) B => C [A /\ B]1 ------------ (/\ E2) B B => C ------------------------------------------------- (=> E) C ------------------ (=> I)1 (A /\ B) => C ------------------------------------------------ (=> I)2 (A => (B => C)) => ((A /\ B) => C) Learning is fun :) Dave,

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