June 2009 - Haskell-Cafe

Re: [Haskell] ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

[redirecting to haskell-cafe] Am Sonntag, 15. Februar 2009 00:25 schrieben Sie: > Hi Wolfgang, > > I was wondering if I can use FLTK as GUI backend for Grapefruit? This should be possible in principal. It just could be that my assumptions about how widgets are created and composed were too tight so that Grapefruit’s general interface doesn’t fit FLTK. In this case, please just tell me and I will try to make the interface more general. > I believe for this to make it happen, I would have to output FLTK's C++ > into C then create bindings for Haskell (via FFI). Is that doable or an > quite tall order? Recently, a student of mine has written a program which generates a Haskell Qt binding fully automatically from Qt header files. The generated binding consists of three layers. The first layer is C++ code which reexports Qt’s functionality as a pure C interface. The C interface is ugly for humans and not type safe (because C doesn’t know classes). The second layer consists of a couple of FFI declarations. The third layer is Haskell code which provides a nice interface similar to the original C++ interface. I still have to get the source code of the binding generator from that student but I hope this will happen soon. I want to publish it then on the web. It hope that it is possible to reuse this binding generator for other C++ libraries. > Jamie Clark Best wishes, Wolfgang

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lazy data structure for best-first search
by Martin Hofmann 07 Jul '09

07 Jul '09

I am looking for a good (preferably lazy) way to implement some kind of best-first search. The problem is, the expansion of the 'best' node in the search space forces other 'inferior' nodes to expand too. So my function expand :: Node -> ([Node],(Node -> [Node])) does not only return some successor nodes, but also a function to expand other Nodes with certain characteristics. So in fact, after one expansion, I need to fold over my complete search space, maybe updating other nodes and recompute the value of the cost function for the affected nodes. At the moment I am using StateT monad transformers, and retrieving the affected Nodes from a map (to avoid going through whole space), but this is not very elegant and I doubt whether it is efficient, too. I have already read through Spivey's paper "Algebras for combinatorial search". Unfortunately, he only proposes the use of heuristics and best-first search in combination with his framework for future work. Does anybody now some further papers on this topic? Thanks a lot in advance, Martin

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[Haskell] ANNOUNCE: ChristmasTree 0.1 (excuses for second attempt)
by S.Doaitse Swierstra 07 Jul '09

07 Jul '09

We are pleased to announce the availability of the package "ChristmasTree", which contains the code associated with our paper at the last Haskell symposium: @inproceedings{1411296, author = {Marcos Viera and S. Doaitse Swierstra and Eelco Lempsink}, title = {Haskell, do you read me?: constructing and composing efficient top-down parsers at runtime}, booktitle = {Haskell '08: Proceedings of the first ACM SIGPLAN symposium on Haskell}, year = {2008}, isbn = {978-1-60558-064-7}, pages = {63--74}, location = {Victoria, BC, Canada}, doi = {http://doi.acm.org/10.1145/1411286.1411296}, publisher = {ACM}, address = {New York, NY, USA}, } The name of the package stands for: "Changing Haskell's Read Implementation Such That by Manipulating Abstract Syntax Trees it Reads Expressions Efficiently" which, given the time of year, seems appropriate. Feel free to download and unpack this "present" at what for the Dutch is called "Sinterklaasavond" (http://en.wikipedia.org/wiki/Sinterklaas) Arthur Baars Marcos Viera Eelco Lempsink Doaitse Swierstra PS: the package uses our library supporting transformation of typed abstract syntax, which we placed in a separate package TTTAS

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How to read safely?
by Magicloud Magiclouds 07 Jul '09

07 Jul '09

Hi, Read often throws runtime errors, which breaks the robust of the problem. How to deal with it? Without lost too much proformance (so reads is a no). At least, if its error could be catched, that'd be better. -- 竹密岂妨流水过山高哪阻野云飞

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Could FFI support pass-by-value of structs?
by Maurício 07 Jul '09

07 Jul '09

It's not usual, but it is allowed to have values of structs passed between functions directly instead of using pointers: /*****/ struct ex { int x; int y; int z; }; ex example_functions (ex p) { (...) } /*****/ Would it be possible to allow that in Haskell FFI by, say, allowing any instance of Storable to be used in a 'foreign' declaration? Like: -- data Ex = (...) instance Storable Ex where sizeOf _ = ... alignment = sizeOf (...) foreign import ccall "example_functions" exampleFunction :: Ex -> IO Ex -- Thanks, Maurício

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Catering for similar operations with and without state
by Phil 06 Jul '09

06 Jul '09

Hi, I'm trying to think around a problem which is causing me some difficulty in Haskell. I'm representing a stateful computation using a State Transform - which works fine. Problem is in order to add flexibility to my program I want to performs the same operation using different techniques - most of which require no state. My program at the moment is a stack of state monads/transforms. I have a random number generator as a state monad (seed=state), feeding a Box Muller (normal) generator implemented as a state transform (state is 'maybe' normal, so it only spits out 1 normal at a time), which in turn feeds another state machine. This all works fine, but I want to add flexibility so that I can chop and change the Box Muller algorithm with any number of other normal generators. Problem is most of them do not need to carry around state at all. This leaves me with a messy solution of implementing lots of state monads that don't actually have a state, if I want to maintain the current paradigm. This strikes me as really messy - so I'm hoping someone can point me in the direction of a more sensible approach? Currently I have my Box Muller implemented as below - this works: class NormalClass myType where generateNormal :: myType Double type BoxMullerStateT = StateT (Maybe Double) type BoxMullerRandomStateStack = BoxMullerStateT MyRngState instance NormalClass BoxMullerRandomStateStack where generateNormal = StateT $ \s -> case s of Just d -> return (d,Nothing) Nothing -> do qrnBaseList <- nextRand let (norm1,norm2) = boxMuller (head qrnBaseList) (head $ tail qrnBaseList) return (norm1,Just norm2) But say I have another instance of my NormalClass that doesn't need to be stateful, that is generateNormal() is a pure function. How can I represent this without breaking my whole stack? I've pretty much backed myself into a corner here as my main() code expects to evalStateT on my NormalClass: main = do let sumOfPayOffs = evalState normalState (1,[3,5]) -- (ranq1Init 981110) where mcState = execStateT (do replicateM_ iterations mc) 0 normalState = evalStateT mcState Nothing If it wasn't for this I was thinking about implementing the IdentityT transformer to provide a more elegant pass-through. I've never tried designing my own Monad from scratch but this crossed my mind as another possibillity - i.e. a Monad that either has a state of maybe double, or has no state at all? I may be talking rubbish here of course :-) I'm pretty daunted by where to even start - but I want to improve this as the quick and dirty solution of implementing a load of state monads with no state just to cater for the above method strikes me as very ugly, and as I can easily see ways of doing this in C or C++, I figure there must be a better approach in Haskell - I'm just thinking in the right way! Any advice or hints would be great, Cheers, Phil.

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Monoid wants a (++) equivalent
by Bryan O'Sullivan 06 Jul '09

06 Jul '09

I've thought for a while that it would be very nice indeed if the Monoid class had a more concise operator for infix appending than "a `mappend` b". I wonder if other people are of a similar opinion, and if so, whether this is worth submitting a libraries@ proposal over.

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Half-integer
by Andrew Coppin 04 Jul '09

04 Jul '09

I just wrote a small module for dealing with half-integers. (That is, any number I/2 where I is an integer. Note that the set of integers is a subset of this; Wikipedia seems to reserve "half-integer" for such numbers that are *not* integers.) module HalfInteger where data HalfInteger i instance (Eq i) => Eq (HalfInteger i) instance (Ord i) => Ord (HalfInteger i) instance (Integral i) => Show (HalfInteger i) instance (Integral i) => Num (HalfInteger i) half :: (Num i) => HalfInteger i fromNum :: (Integral i, RealFrac x) => x -> HalfInteger i toNum :: (Integral i, Fractional x) => HalfInteger i -> x isInteger :: (Integral i) => HalfInteger i -> Bool Note carefully that the set of half-integers is *not* closed under multiplication! This means that for certain arguments, there are two reasonable products that could be returned. (E.g., 1/2 * 1/2 = 1/4, so 0 or 1/2 would be a reasonable rounding.) I haven't put a lot of effort into the rounding details of (*) or fromNum; which answer you get is kind of arbitrary. (However, addition and subtraction are exact, and for multiplications where an exact result is possible, you will get that result.) The Show instance outputs strings such as fromInteger 5 fromInteger 5 + half fromInteger (-5) - half depending on the isInteger predicate. Now, the question is... Is this useful enough to be worth putting on Hackage?

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ORM for haskell?
by Marc Weber 04 Jul '09

04 Jul '09

Some time ago I stumbled upon SQLAlchemy which is a great ORM wrapper library for python. It has a nice syntax I'd like to see in a haskell library as well. SQLAlchemy already provides some lazy features such as loading subitems on access etc. All haskell SQL libraries I know only let you run SQL statements but not much more. To start real business you no longer want to write many SQL commands. Example why it matters: schools - 1:n - teachers - 1:n - pupils If you want to list all schools which have a pupil with age > 3 you'd write an sql query like this: SELECT dictinct * FROM schools as s JOIN teachers t ON (t.school_id = s.id) JOIN pupils as p ON (p.teacher_id = t.id) WHERE p.age > 3 in SQLAlchemy it looks like this: session.query(School).join(School.teachers).join(Teacher.pupils).filter(Pupil.age > 3).all() difference? Because SQLAlchemy knows about the relations you don't have to remember alias names. So there is no chance to get that wrong. Another example: Updating the age of a pupil: row = SELECT * FROM pupils where age = 13; UPDATE pupils SET age = 14 WHERE id = <the id you got above> p = session.query(Pupil).filter(Pupil.age==13).one().age=14 session.commit() difference? You don't have to care about ids. you just assign a new value and tell the engine that it should commit. So again less chances to get something wrong. What about trees (eg web site navigation) id | title | parent_id 1 | top | null 2 | submenu | 1 3 | submenu2 | 1 should result in top - submenu - submenu2 using SQLAlchemy you can just do parent = session.query('nodes').filter(Node.id = 1) def print(node): print node.title print node.subnodes # this will run a subquery automatically for you returning submenu{,2} Again no sql. No chance to get something wrong? You can skim the manual to get a better idea how SQLAlchemy works http://www.sqlalchemy.org/docs/05/sqlalchemy_0_5_5.pdf I have to admit that I haven't used SQLAlchemy in a real project yet. However I can imagine doing so. Comparing this to what we have on hackage I'd say some work has to be done to get close to SQLAlchemy. The backend doesn't have to be a relational database. However I'd like to use this kind of abstraction in haskell. Is there anyone interested in helping building a library which a) let's you define kind of model of you data b) let's you store you model in any backend (maybe a relational database) c) does static checking of your queries at compilation time? Right now I'd say the best way to go is define the model in the application and not get the scheme from an existing database because there is not way to store all scheme details within a relational model. I think SQLAlchemy does it right by providing a way to define the model in python. Of course haskell doesn't have "objects" to store. But GADTs could be stored (data Foo = ...) So are there any volunteers who are interested in helping writing this kind of storage solution for haskell which could be used in real world business apps? Maybe this does already exist and I've missed it? Marc Weber

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Haskellers at University of Edinburgh? (hackathon)
by Eric Kow 02 Jul '09

02 Jul '09

Hi Haskellers, I'm looking for people at the University of Edinburgh who have a swipe card with access to the Informatics Forum and could hang around on 29 August (or perhaps 6 Sep). Thanks! PS. You may also be interested in some Hackathon news: we've reserved a room at the ICFP venue suitable for up to 50 people, but we can only have it for 30 August. I'm still holding out for a two-day space, but if I don't hear back by mid-week, we'll settle for a single Haskell Hack Day, preceded by a more informal Haskell Hacking Expedition the day before (for example, camping in in a wifi-enabled pub for the day). -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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