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ANN: dtd-types 0.2.0.0
by Yitzchak Gale 01 Jun '11

01 Jun '11

The dtd-types package provides types for processing XML DTDs in Haskell. These types are intended to be compatible with and extend the set of types provided by John Millikin's xml-types package. The consensus seems to be to leave this as a separate package and not to merge it with xml-types. This version fixes a number of issues with the preliminary version. Among those issues were: omission of processing instructions, pointed out by John Millikin, and Haddock problems pointed out by Michael Snoyman. Thanks! Hackage: http://hackage.haskell.org/package/dtd-types/ Darcs: http://code.haskell.org/dtd/dtd-types Enjoy, Yitz

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Re: [Haskell-cafe] ANNOUNCE: CamHac: Haskell Hackathon in Cambridge, UK, 12-14 August 2011
by Simon Marlow 01 Jun '11

01 Jun '11

On 26/05/2011 15:35, Simon Marlow wrote: > CamHac is happening - come and spend a long weekend in Cambridge hacking > Haskell code in great surroundings with fantastic company! > > Full details on the wiki page: > > http://www.haskell.org/haskellwiki/CamHac > > When: Friday-Sunday 12-14 August 2011 > Where: the Horobin Room of Homerton Conference Centre [1] > > Registration is free! See the wiki page above for details. We're limited > to 50 people, and registration is on a first-come first-served basis, so > sign up soon. We've reached 50 registrations already! Luckily the venue has a larger room available, so we've changed our booking and we can now fit 72 haskellers in. We're now in the Leah-Manning room: http://www.homertonconference.com/Leah-Manning.html Cheers, Simon

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Enabling GADTs breaks Rank2Types code compilation - Why?
by dm-list-haskell-cafe＠scs.stanford.edu 01 Jun '11

01 Jun '11

I'm using GHC 7.0.2 and running into a compiler error that I cannot understand. Can anyone shed light on the issue for me? The code does not make use of GADTs and compiles just fine without them. But when I add a {-# LANGUAGE GADTs #-} pragma, it fails to compile. Here is the code: ==== {-# LANGUAGE Rank2Types #-} {-# LANGUAGE GADTs #-} mkUnit :: (forall t. t -> m t) -> m () mkUnit f = f () data Const b a = Const { unConst :: b } sillyId :: r -> r sillyId r = unConst (mkUnit mkConst_r) -- This line can't compile with GADTs where mkConst_r t = mkConst r t mkConst :: b -> t -> Const b t mkConst b _ = Const b ==== And the error I get is: Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: t -> Const r t The following variables have types that mention t0 mkConst_r :: t0 -> Const r t0 (bound at /u/dm/hack/hs/gadt.hs:11:11) In the first argument of `mkUnit', namely `mkConst_r' In the first argument of `unConst', namely `(mkUnit mkConst_r)' In the expression: unConst (mkUnit mkConst_r) I've found several workarounds for the issue, but I'd like to understand what the error message means and why it is caused by GADTs. Thanks in advance for any help. David

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Lazy Evaluation in Monads
by Scott Lawrence 01 Jun '11

01 Jun '11

I was under the impression that operations performed in monads (in this case, the IO monad) were lazy. (Certainly, every time I make the opposite assumption, my code fails :P .) Which doesn't explain why the following code fails to terminate: iRecurse :: (Num a) => IO a iRecurse = do recurse <- iRecurse return 1 main = (putStrLn . show) =<< iRecurse Any pointers to a good explanation of when the IO monad is lazy? === The long story === I wrote a function unfold with type signature (([a] -> a) -> [a]), for generating a list in which each element can be calculated from all of the previous elements. unfold :: ([a] -> a) -> [a] unfold f = unfold1 f [] unfold1 :: ([a] -> a) -> [a] -> [a] unfold1 f l = f l : unfold1 f (f l : l) Now I'm attempting to do the same thing, except where f returns a monad. (My use case is when f randomly selects the next element, i.e. text generation from markov chains.) So I want unfoldM1 :: (Monad m) => ([a] -> m a) -> [a] -> m [a] My instinct, then, would be to do something like: unfoldM1 f l = do next <- f l rest <- unfoldM1 f (next : l) return (next : rest) But that, like iRecurse above, doesn't work.

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Decimal type-level arithmetic.
by Serguey Zefirov 31 May '11

31 May '11

I would like to present my version of type arithmetic with decimal encoding: http://thesz.mskhug.ru/svn/hhdl/TyleA.hs It is not worth Cabal package in its current state, but I hope it would be useful for someone. It is easy to use, just say Plus (D1 :. D2 :. D0) D8 to get a type of 128. Or you can say $(tySize 128) if you're not afraid to use Template Haskell. I tested it on my current project, it works quite well with numbers around 128..512, about 10-20 times faster than Peano numbers. As operations over this representation are not lazy (Peano numbers oeprations are lazy for one argument of Plus and for part of one argument of Max), it is not well suited for big number of operations done at once. I encountered it when I autogenerated a big arithmetic expression for big polymorphic data type. I think it is quite rare situation.

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Thoughts about currying and optimisations
by Yves Parès 31 May '11

31 May '11

Hello Café, An idea came to me: unless the compiler notices that stuffA and stuffB are equivalent, would it be correct to suppose that A is better than B? stuffA x = if someComputationallyExpensiveTest x then doSomething else doSomethingElse stuffB x y = if someComputationallyExpensiveTest x then doSomething y else doSomethingElse y I explain: in stuffA, the function only depends on x, so when doing: a = stuffA xxx runs the expensive test once and for all, and a can directly be bound to doSomething or doSomethingElse so calling after: a foo a bar won't run the test Whereas: b = stuffB xxx is valid due to curryfication, but: b foo b bar will both run the expensive test

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Can't figure out cmap in hmatrix
by Mats Klingberg 31 May '11

31 May '11

Hello, I'm playing around a bit with the hmatrix package (http://hackage.haskell.org/package/hmatrix) but can't quite figure out how to make the cmap function in Numeric.Container work. An example: ghci> import Numeric.LinearAlgebra ghci> let v = fromList [1.0,2.0,3.0] ghci> v fromList [1.0,2.0,3.0] :: Data.Vector.Storable.Vector ghci> :t v v :: Vector Double ghci> cmap sqrt v <interactive>:1:1: No instance for (Container Vector e0) arising from a use of `cmap' Possible fix: add an instance declaration for (Container Vector e0) In the expression: cmap sqrt v In an equation for `it': it = cmap sqrt v ghci> :t cmap cmap :: (Container c e, Element b, Element a) => (a -> b) -> c a -> c b There is an instance for (Container Vector Double) but I assume that since the signature of cmap doesn't mention the type variable 'e' GHCi can't infer it. Googling hasn't helped me so far, except for digging up another post to this list with the same (?) problem, but no answer: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091390.html Is there a way to tell GHC what instance to use, or how should cmap be used? Thanks! Mats

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Sub class and model expansion
by Patrick Browne 31 May '11

31 May '11

Hi, I'm not sure if this is possible but I am trying to use Haskell’s type class to specify *model expansion*. Model expansion allows the specification of new symbols (known as enrichment) or the specification further properties that should hold on old symbols. I am trying to enrich simplified Monoids to Groups. The code below is not intended to do anything other than to specify simplified Groups as a subclass of Monoid. The main problem is that I cannot use ! on the LHS of equations in Group. Is it possible to expand the specification of Monoid to Group using Haskell type classes? Pat data M = M deriving Show -- Monoid with one operation (!) and identity e (no associativity) class Monoid a where (!) :: a -> a -> a e :: a a ! e = a -- A Group is a Monoid with an inverse, every Group is a Monoid -- (a necessary condition in logic: Group implies Monoid) -- The inverse property could be expressed as: -- 1) forAll a thereExists b such that a ! b = e -- 2) (inv a) ! a = e class Monoid a => Group a where inverse :: a -> a -- Haskell does not like ! on the LHS of equation for inverse in Group. -- (inverse a) ! a = e This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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ANN: quickcheck-properties
by Alexey Khudyakov 31 May '11

31 May '11

Hello It always puzzled me why there are no packages for for testing general type classes laws. (Monoid laws, monad laws etc). It looks like ideal case for quickcheck and smallcheck. I'm glad to present package quickcheck-properties[1]. It containt set of generic properties for semiroups, monoids, groups and unsatisfactory imlementatation of functor's properties. Despite its name package do not depend on quickcheck and could be used with smallcheck as well. ==================== Examples ==================== Here are few examples: >>> quickCheck $ eq $ prop_Monoid (T :: T [Int]) +++ OK, passed 100 tests. >>> quickCheck $ eq $ prop_Monoid (T :: T (Maybe [Int])) +++ OK, passed 100 tests. prop_Monoid tests all monoid properties: associativity, left and right identity. T is used to fix type of monoid to test. It could be done with type signature for prop_Monoid but writing signature for three parameter function is just too cumbersome. eq compares expression in properties using == operator. Many (most?) of properties have form > expression = another expression Easiest solution is to compare them with ==. But is's not satisfactory. Data type may not have Eq instance. Eq instance may provide structural equality but some form of equivalince is needed. etc. It's left to library user to decide how to compare values. In this contrived example lists are compared by length. > eqOn length $ prop_Monoid (T :: T [Int]) ==================== Problems ==================== There are unsolved problems. So it's announcement with question. Functor/Applicative/Monads laws involve arbitrary functions. It would be natural to generate them using quickcheck but I don't know how to generate random functions. Any suggestions? [1] http://hackage.haskell.org/package/quickcheck-properties -- Aleksey Khudyakov

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PhD studentship in Nottingham
by Graham Hutton 31 May '11

31 May '11

Dear all, I are currently advertising a PhD studentship in functional programming -- the advert is copied below, and attached. If you know of any good candidates who many be interested in this, or there is a local mailing list for advertising such things, I would be much obliged if you could pass on the advert. Best wishes, Graham +------------------------------------------------------------------+ PhD Studentship in Functional Programming School of Computer Science University of Nottingham, UK Applications are invited for a fully-funded PhD studentship in the Functional Programming Lab in the School of Computer Science at the University of Nottingham, starting on 1st October 2011, under the supervision of Prof Graham Hutton. The suggested topic for the studentship is to develop a practical implementation of the worker/wrapper transformation, a simple but powerful unifying paradigm for program optimisation, but other ideas for possible topics are welcome too. The studentship is for three and a half years, and includes a maintenance grant of 13,590 UK pounds per year, and tuition fees. Applicants for the position will require a first-class Honours degree (or equivalent) in Computer Science and/or Mathematics, some experience in functional programming, and an aptitude for mathematical subjects. Additional desirable attributes include a Masters degree, and/or some experience in programming language semantics, implementation and optimisation. Due to the nature of the funding, the position is only open to UK citizens, or EU citizens who have resided in the UK for the last three years. The successful applicant will work under the supervision of Prof Graham Hutton in the FP Lab in Nottingham, a leading centre for research on functional programming. The group currently comprises 5 academic staff, 1 research fellow, and 10 PhD students. In order to apply, please submit the following to Prof Graham Hutton (graham.hutton(a)nottingham.ac.uk) by 24th June 2011: an up-to-date copy of your CV (including the results of all your University examinations to date) along with a brief covering letter that describes your experience in functional programming, your reasons for wishing to pursue a PhD in this area, and any ideas you have regarding possible research directions. Closing date for applications: 24th June 2011. +------------------------------------------------------------------+ -- Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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