- Haskell-Cafe - Haskell.org

($) not as transparent as it seems
by Steffen Schuldenzucker 03 Feb '11

03 Feb '11

Dear cafe, does anyone have an explanation for this?: >>> error (error "foo") *** Exception: foo >>> error $ error "foo" *** Exception: *** Exception: foo -- Steffen

9 10

hsmagick on OSX 10.6
by Max Cantor 03 Feb '11

03 Feb '11

Just out of curiosity, has anyone gotten this to work? I'm getting bus errors whenever I call readImage. If anyone out there has gotten it working and would tell me which versions they're using of the relevant bits, that would be very much appreciated. Also, I'll post all my debug info if there's someone who's gotten this to work. thanks, mc

2 1

Reader monad wrapping State monad
by michael rice 03 Feb '11

03 Feb '11

Given the first program, it seems that the unchanging first element of the tuple could be handled by a Reader monad, leading to the second program, where b becomes the state, but how do I get the constant a from the Reader monad? Michael ================== import Control.Monad.State type GeneratorState = State (Double,Double) sqrtST :: GeneratorState Double sqrtST = do (a,b0) <- get let b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put (a,b1) sqrtST) mySqrt a = let b = a/2.0 in fst ( runState sqrtST (a,b) ) {- *Main> mySqrt 2.0 1.4142135623746899 -} ================== import Control.Monad.Reader import Control.Monad.State type GeneratorState = State Double sqrtST :: GeneratorState Double sqrtST = do b0 <- get let a = ? b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put b1 sqrtST) mySqrt a = let b = a/2.0 in runReaderT (runState sqrtST b) a

4 6

Re: [Haskell-cafe] Haskell Functions
by Ozgur Akgun 03 Feb '11

03 Feb '11

(CCing haskell-cafe again) Is this an homework question? On 3 February 2011 20:30, Manolache Andrei-Ionut <andressocrates90(a)yahoo.com > wrote: > http://pastebin.com/GxQBh3hx <http://pastebin.com/GxQBh3hx> > > --- On *Thu, 2/3/11, Ozgur Akgun <ozgurakgun(a)gmail.com>* wrote: > > > From: Ozgur Akgun <ozgurakgun(a)gmail.com> > Subject: Re: [Haskell-cafe] Haskell Functions > To: "Manolache Andrei-Ionut" <andressocrates90(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Thursday, February 3, 2011, 10:59 AM > > > On 3 February 2011 18:33, Manolache Andrei-Ionut < > andressocrates90(a)yahoo.com<http://mc/compose?to=andressocrates90@yahoo.com> > > wrote: > > first this is the curent code :http://pastebin.com/UPATJ0r > > > There is no code on that page. (It has expired, probably?) > > -- > Ozgur Akgun > > > -- Ozgur Akgun

3 2

Another Question
by Navin Rustagi 03 Feb '11

03 Feb '11

hi , I am stuck in the following problem. I am maintaining a list of tuples of the form ([Char],Int,Int, Int,Int) . The purpose of maintaining the tuples is that the program reads from a file line by line , Matches the contents with the first element of the tuple and updates the tuple respectively. The precise function I am using is as follows tupup::Bool->[Char]->Int->Int->Int->Int->Char->([Char],Int,Int,Int,Int) tupup val elf els elr ell elx ys= if val then case ys of 'A' -> (elf, els+1,elr,ell,elx) 'G' -> (elf,els, elr +1,ell,elx) 'C' -> (elf,els,elr, ell +1,elx) 'T' -> (elf,els,elr,ell, elx +1) else (elf,els,elr,ell,elx) uptable::[[Char]]->[([Char],Int,Int,Int,Int)]->[([Char],Int,Int,Int,Int)] uptable (xf:xs) main_array = map (\(x,y,z,r,t)-> tupup (x==xf) x y z r t (secvalue xs) ) main_array It gives the error ERROR - Control stack overflow. I assume it is because of the lazy evaluation . Is there a way to enforce strict evaluation only for the function tupup. Thanks, Navin

4 3

Haskell Functions
by Manolache Andrei-Ionut 03 Feb '11

03 Feb '11

I need help with two functions....first this is the curent code :http://pastebin.com/UPATJ0r ->Function 1)removeTautologies :: Formula->Formula If in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable. Consider the following example: (A v B v -A) ^ (B v C v A) The first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) . I was tinking of using something like removeTautologies (f:fs)=filter rTf:removeTautologies fs where rT-is supposed to take the firs Literal from the clasue and search for a similar one,if one si found we compare the values if not the we go to the second literal. ->Function 2)pureLiteralDeletion :: Formula->Formula This is a little bit complicate but from What I get this function is suppose to implement a simplification step that assumes as true any atom in a formula that appears exclusively in a positive or negative form (not both). Consider the formula: (P v Q v R) ^ (P v Q v -R) ^ (-Q v R) Note that in this formula P is present but -P is not. Using Pure Literal Deletion it can be assumed that the value of P will be True thus simplifying the formula to (-Q v R). If the literal were false then the literal would simply be deleted from the clauses it appears in. In that case any satisfying model for the resulting formula would also be a satisfying model for the formula when we assume that the literal is true.

2 1

ANNOUNCE: Database Supported Haskell (DSH)
by George Giorgidze 03 Feb '11

03 Feb '11

I am pleased to announce that Database Supported Haskell (DSH) has been released on Hackage [1]. DSH is a Haskell library for database-supported program execution. Using this library a relational database management system (RDBMS) can be used as a coprocessor for the Haskell programming language, especially for those program fragments that carry out data-intensive and data-parallel computation. Database executable program fragments can be written using the list comprehension notation (with modest syntax changes due to quasiquoting) and list processing combinators from the Haskell list prelude. Note that rather than embedding a relational language into Haskell, we turn idiomatic Haskell programs into SQL queries. DSH faithfully represents list order and nesting, and compiles the list processing combinators into relational queries. The implementation avoids unnecessary data transfer and context switching between the database coprocessor and the Haskell runtime by ensuring that the number of generated relational queries is only determined by the program fragment's type and not by the database size. DSH can be used to allow existing Haskell programs to operate on large scale data (e.g., larger than the available heap) or query existing database resident data with Haskell. Note that this package is flagged experimental and therefore not suited for production use. This is a proof of concept implementation only. To learn more about DSH, our paper entitled as "Haskell boards the Ferry: Database-supported program execution for Haskell" is a recommended reading [2]. The package includes a couple of examples that demonstrate how to use DSH. [1] http://hackage.haskell.org/package/DSH [2] http://www-db.informatik.uni-tuebingen.de/files/publications/ferryhaskell.p…

1 0

Yesod and concurrency
by Ertugrul Soeylemez 03 Feb '11

03 Feb '11

Hello everybody, how well do WAI, Yesod and the 'persistent' package play with concurrency? For example, I'd like to write a program, which concurrently provides two related sites as well as a few background workers, which do something with the database. My idea would look something like this: main :: IO () main = withMyAppPool $ \pool -> do forkIO $ worker1 ... -- background worker forkIO $ worker2 ... -- background worker forkIO $ worker3 ... -- background worker forkIO $ toWaiApp ... -- site 1 forkIO $ toWaiApp ... -- site 2 Will I run into problems with this? Thanks. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

2 5

PhD Studentships in Functional Programming, Nottingham
by Henrik Nilsson 03 Feb '11

03 Feb '11

Apologies for multiple copies. Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham nhn(a)cs.nott.ac.uk

1 0

problem with instance method
by Patrick Browne 03 Feb '11

03 Feb '11

Hi, I am studying type classes using examples from the literature [1]. The attached code is a formalization of basic object oriented ideas. The particular approach seems to be built on the concepts of: thing, object, and identifier. I have no intension to implement anything or significantly change the code below. Rather, I am trying to understand the code as it stands. I include a number of test runs which seem OK, but I cannot get the *obj* function to work. obj :: t -> i -> o t i obj t i = Obj t i Any hints would be appreciated. Thanks, Pat [1] ftp://ftp.geoinfo.tuwien.ac.at/frank/frank97executableAxiomaticSpecification.pdf -- A property of a thing data Color = Blue | Green | Red | White deriving Show -- A thing class Cars c where car :: Color -> c getColor :: c -> Color putColor :: Color -> c -> c paint :: Color -> c -> c paint color car = putColor color car data Car = Car Color deriving Show instance Cars Car where car c = Car c putColor color (Car c) = Car color getColor (Car c) = c -- Identifiers for objects class (Integral i) => IDs i where startId :: i newId :: i -> i newId i = succ i sameId, notSameId :: i -> i -> Bool sameId i j = i == j notSameId i j = not (sameId i j) instance IDs Integer where startId = 1 -- Objects consist of Things, with Identifiers class (IDs i, Show i) => Objects o t i where obj :: t -> i -> o t i getId :: o t i -> i getThing :: o t i -> t doThing :: (t -> t) -> o t i -> o t i doThing f o = obj (f (getThing o)) (getId o) same :: o t i -> o t i -> Bool same i j = sameId (getId i) (getId j) isId :: i -> o t i -> Bool isId i o = sameId i (getId o) -- A general type of Obj data Object t i = Obj t i deriving Show -- A particular Car Obj, which an instance of Objects class (in Haskell terms, not OO terms) instance Objects Object Car Integer where obj t i = Obj t i getId (Obj t i) = i getThing (Obj t i) = t -- Create some actual Objects x = (Obj (Car Blue) (startId::Integer)) y = (Obj (Car Green) (newId startId::Integer)) -- Some tests on car thing, seem OK -- getColor (Car Blue) -- putColor Green (Car Blue) -- getColor (putColor Green (Car Blue)) -- Some tests on objects, seem OK -- same x y -- Obj (Car Blue) (newId startId::Integer) -- Obj (Car Blue) (startId::Integer) -- getThing (Obj (Car Blue) (startId::Integer)) -- getId (Obj (Car Blue) (startId::Integer)) -- isId 2 (Obj (Car Blue) (startId::Integer)) 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

3 2