- Haskell-Cafe - Haskell.org

how to create new repository on darcs.haskell.org?
by Bulat Ziganshin 05 Oct '06

05 Oct '06

Hello haskell-cafe, i want to make my library available via darcs repository on darcs.haskell.org. how i can arrange it? if it is required, i already have user account on haskell.org server -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

5 5

Mission: To take args from a list... generally
by tpledger＠ihug.co.nz 05 Oct '06

05 Oct '06

Joel Koerwer wrote: > Let's say I want to evaluate a function of type > (a->a->...->a->a), taking the arguments from a > list. If know the function ahead of time, I can > simply wrap it: > > foo a b c d = ... > wrapFoo (a:b:c:d:_) = foo a b c d > > But, as an exercise, I challenged myself to write > a function, multApply :: (a->a->...->a->a) -> [a] > -> a, that automatically does the wrapping for any > such function. This came up a while ago (but with a list of functions of different arities, all being fed one argument). I found Scott Turner's pure Haskell 98 solution very illuminating: http://www.haskell.org/pipermail/haskell-cafe/2000-November/001332.html Regards, Tom

2 1

seeking paper: Computing with lattices: An application of type classes
by Mathew Mills 04 Oct '06

04 Oct '06

I am not sure how to go about getting a copy of this paper. http://citeseer.ist.psu.edu/context/320255/0 I was hoping someone here could give me a pointer.

1 0

throwDynIO ?
by Carlos Pita 04 Oct '06

04 Oct '06

Hi all! I'm a newcomer to haskell programming with a lisp/scheme background. Last week I've been reading a lot about the monadic approach to IO and I should said I'm pretty fascinated about the flexibility it allows. Currently I'm trying to grasp haskell exception handling and although I find it mostly clear I'm a bit confused regarding the lack of a throwDynIO action "paralelling" throwIO, as there are dynamic variants for the other throw functions: throwDyn - throw throwDynTo - throwTo I understand that the same effect could be conjured up by throwIO (DynException (...))) magic. But the lack of symmetry still disturbes me. Am I missing something? Thank you in advance. Regards, Carlos

1 0

[Haskell] open assistant professorship at Utrecht University, Software Technology
by Doaitse Swierstra 04 Oct '06

04 Oct '06

We have an open 5 year position for an assistant professor. Current areas of interest are: - domain specific embedded languages (especially in the form of combinator languages) - programming language design and implementation - generic programming techniques - tools and methods for building complex systems out of a large number of artefacts in a controlled way - program verification - systems for generating informative feedback to users in the case of inconsistencies in specifications - advanced type systems and their implementation - software generation We strongly believe that the functional programming paradigm is an excellent starting point for many new developments in the above mentioned areas. In our research we try to identify real problems, to find solutions for them, to formalise these, and to build tools in order to convey the solutions to the problem holders. Thus we try to combine sound principles for solving real-life problems. Given the increased pressure on our finances cooperation with industry is of growing importance, which will also hold for this position. Further details can be found at: http://www.cs.uu.nl/vacatures/en/62612.html Doaitse Swierstra _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

1 0

question - which monad to use?
by Tamas K Papp 04 Oct '06

04 Oct '06

Hi, I have a computation where a function is always applied to the previous result. However, this function may not return a value (it involves finding a root numerically, and there may be no zero on the interval). The whole problem has a parameter c0, and the function is also parametrized by the number of steps that have been taken previously. To make things concrete, type Failmessage = Int -- this might be something more complex data Result a = Root a | Failure Failmessage -- guess I could use Either too f :: Double -> Int -> Double 0 -> Result Double f c0 0 _ = c0 f c0 j x = {- computation using x, parameters calculated from c0 and j -} Then c1 = f c0 0 c0 c2 = f c0 1 c1 c3 = f c0 2 c2 ... up to cn. I would like to 1) stop the computation when a Failure occurs, and store that failure 2) keep track of intermediate results up to the point of failure, ie have a list [c1,c2,c3,...] at the end, which would go to cn in the ideal case of no failure. I think that a monad would be the cleanest way to do this. I think I could try writing one (it would be a good exercise, I haven't written a monad before). I would like to know if there is a predefined one which would work. Thank you, Tamas

4 6

throwDynIO ?
by Carlos Pita 04 Oct '06

04 Oct '06

Hi all! I'm a newcomer to haskell programming with a lisp/scheme background. Last week I've been reading a lot about the monadic approach to IO and I should said I'm pretty fascinated about the flexibility it allows. Currently I'm trying to grasp haskell exception handling and although I find it mostly clear I'm a bit confused regarding the lack of a throwDynIO action "paralelling" throwIO, as there are dynamic variants for the other throw functions: throwDyn - throw throwDynTo - throwTo I understand that the same effect could be conjured up by throwIO (DynException (...))) magic. But the lack of symmetry still disturbes me. Am I missing something? Thank you in advance. Regards, Carlos

1 0

irrefutable patterns for existential types / GADTs
by oleg＠pobox.com 04 Oct '06

04 Oct '06

It seems that irrefutable pattern match with existentials is safe. The fact that irrefutable pattern match with GADT is unsafe has been demonstrated back in September 2004. Let us consider the following regular existential data type > data TFoo where > Foo :: Show a => a -> TFoo > Bar :: Int -> TFoo Despite the 'where' syntax, it is NOT GADT; it is just a regular data type. We can write > test_foo vf = case vf of ~(Foo x) -> body Now, if 'x' does not occur in `body', we could have just as well written test_foo vf = body If `x' does occur in body and the scrutinee is not of the form `Foo', then 'x' is undefined, and so 'body' bottoms out when it demands the value of 'x'. No surprise, and no concern here. Let is now consider a GADT > data GTFoo a where > GFoo :: GTFoo Int > GBar :: GTFoo Bool > > test_gfoo :: GTFoo a -> a > test_gfoo vf = case vf of GFoo -> (1::Int) It can be faithfully emulated as follows > -- the data constructors Em_GFoo and Em_GBar must be hidden! > data EmulateGTFoo a = Em_GFoo | Em_GBar > > em_gfoo :: EmulateGTFoo Int > em_gfoo = Em_GFoo > em_gbar :: EmulateGTFoo Bool > em_gbar = Em_GBar The constructors Em_GFoo and Em_GBar should be hidden. The user should use `smart' constructors em_gfoo and em_gbar, which not only set the correct type (Int vs Bool) but also produce the witness, viz. Em_GFoo or Em_GBar. Now, the test_gfoo function should be written as > tesd_emulate_gfoo :: EmulateGTFoo a -> a > tesd_emulate_gfoo vf = case vf of Em_GFoo -> unsafeCoerce (1::Int) So, we test for evidence, Em_GFoo, and if it is presented, we proceed with unsafeCoerce, which generalizes Int to the desired type 'a'. We know this generalization is safe because the evidence Em_GFoo told us that 'a' was really Int. The similarity with Dynamics is uncanny. Now, had we used an irrefutable match instead, we would have proceeded with unsafeCoerce without checking for the evidence. The following code, written back in Sep 13, 2004, shows that the above is not an empty concern. The code did indeed typecheck, with the version of GHC (tidt-branch CVS branch) that existed at that time. Running the code produced the result one'd expect when reading an Int as a Bool. I think the code below was the reason GHC prohibited irrefutable GADT pattern matching since. > {-# OPTIONS -fglasgow-exts #-} > > module Main where > > import Data.IORef > > data T a where > Li:: Int -> T Int > Lb:: Bool -> T Bool > La:: a -> T a > > writeInt:: T a -> IORef a -> IO () > writeInt v ref = case v of > ~(Li x) -> writeIORef ref (1::Int) > > readBool:: T a -> IORef a -> IO () > readBool v ref = case v of > ~(Lb x) -> > readIORef ref >>= (print . not) > > tt::T a -> IO () > tt v = case v of > ~(Li x) -> print "OK" > > main = do > tt (La undefined) > ref <- newIORef undefined > writeInt (La undefined) ref > readBool (La undefined) ref

4 6

Mission: To take args from a list... generally
by Joel Koerwer 04 Oct '06

04 Oct '06

Thanks! Don pointed me to Oleg's page too. I *was* (90%) convinced that this is fundamentally different, but it sure looks similar. Kind of an inside-out polyvariadic function. Any given f only takes n args, but myApply doesn't know the value of n. In fact, a polyvariadic f would cause an infinite loop. At least a stack overflow. Always nice to know you're in good company though. Cheers, Joel On 10/4/06, David House <dmhouse(a)gmail.com> wrote: > Nice work! I haven't tried it out, but you seem to have on the right > trick here: typeclass hackery. An interesting related exercise to > develop a similar function using Template Haskell [1]. Also of note > might be Oleg's writings on true polyvariadic functions [2]. > > [1]: http://haskell.org/th > [2]: http://okmij.org/ftp/Haskell/types.html#polyvar-fn

1 0

Mission: To take args from a list... generally
by Joel Koerwer 04 Oct '06

04 Oct '06

Haskellers, Let's say I want to evaluate a function of type (a->a->...->a->a), taking the arguments from a list. If know the function ahead of time, I can simply wrap it: foo a b c d = ... wrapFoo (a:b:c:d:_) = foo a b c d But, as an exercise, I challenged myself to write a function, multApply :: (a->a->...->a->a) -> [a] -> a, that automatically does the wrapping for any such function. On #haskell Don Stewart suggested I look at printf, but I've yet to put much thought into whether that method will work here. I'm posting my solution in hopes to learn from your comments. This solution uses fundeps, multi-parameter classes, and overlapping instances. Note that I don't actually understand these things! :) ------------------------ MultApply.hs ------------------------------ {-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-} module MultApply where class MultApply func arg | func -> arg where multApply :: func -> [arg] -> arg instance MultApply (a->a) a where multApply f (x:xs) = f x multApply f _ = error "MultApply: one too few args" instance MultApply cascade a => MultApply (a -> cascade) a where multApply f (x:xs) = multApply (f x) xs multApply f _ = error "MultApply: n too few args" -- some random examples oneArg = multApply sqrt [25..] twoArg = multApply (+) [1..] fiveArg = multApply (\a b c d e -> sqrt ((a+b)^2+(d-e)^2)-5*c) [13..] -------------------End File ------------------------ Results in ghci: *MultApply> oneArg 5.0 *MultApply> fiveArg -47.981487827787404 To compose your own examples in ghci, you'll need -fallow-overlapping-instances on the command line. Cheers, Joel

2 1