- Haskell-Cafe - Haskell.org

New Eq instance
by rodrigo.bonifacio 09 Aug '07

09 Aug '07

Hello, I had defined the follwing data type: data Step = Step Id Scenario Action State Response How can I define Step as an "Eq Instance", in such way that two steps are equals if they have the same Id (Id is defined as a synonimous for the String type). I tried the following code, but something is wrong.... instance Eq Step where Step id1 scenario1 action1 state1 response1 == Step id2 scenario2 action2 state2 response2 = id == id _ == _ = False ps.: sorry, this kind of basic question can be sent to this list? Thanks in advance. Rodrigo.

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Using haskell with emacs
by Paulo J. Matos 09 Aug '07

09 Aug '07

Hi all, I'm starting to learn haskell by my own, being currently mostly a Common Lisp, Scheme, C++ programmer... I've got the haskell emacs mode but can't find a manual. Moreover, I've found some keybindings on the net but nothing that allows me to start an interpreter in emacs and send definitions, one by one to the interpreter. Is this possible? Is there any good reference of the emacs keybindings for haskell mode? Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Using HList Records
by Rahul Kapoor 09 Aug '07

09 Aug '07

Using The Darcs version of HList (http://darcs.haskell.org/HList/) I can do simple things with (H)lists just fine, so --angus = Key 42 -- .*. Name "Angus" -- .*. Cow -- .*. Price 75.5 -- .*. HNil --main = putStrLn (show angus) works, On the other hand examples from the HList paper for extensible records like --key = firstLabel FootNMouth "key" --name = nextLabel key "name" --breed = nextLabel name "breed" --price = nextLabel breed "price" do not compile with errors --Main.hs:15:8-17: Not in scope: `firstLabel' --Main.hs:16:8-16: Not in scope: `nextLabel' A quick show "ghc-pkg describe HList" shows that --exposed-modules: HList --hidden-modules: Label4 CommonMain Variant GhcSyntax GhcRecord -- Record HZip TIC TIP HTypeIndexed HOccurs HArray GhcExperiments -- HListPrelude TypeEqBoolGeneric TypeEqGeneric1 TypeCastGeneric1 -- FakePrelude So the question is do I need to play around with the cabal file to expose the right stuff? Or has the syntax changed? I get errors running "./setup haddock" (configure, build, install, are fine). Are the HList api docs available anywhere online? Rahul

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Re: default for quotRem in terms of divMod?
by Christian Maeder 09 Aug '07

09 Aug '07

Isaac Dupree wrote: > In class Integral, divMod has a default in terms of quotRem. > (quot,rem,div,mod all have defaults as the both-function they're part > of.) I'm sure divMod is more natural than quotRem to implement for some > types... so why doesn't quotRem have a default in terms of divMod? it > has no default! Then the "minimal to implement" will change from > (toInteger and quotRem) to (toInteger and (quotRem or divMod)). > > Isaac while I don't care if quotRem or divMod should be implemented. I oppose to give both default implementations in terms of the other. Already for the class Eq either == or /= must be defined, with the unpleasant effect that an empty instance like: instance Eq T leads to a loop (when == or /= is called on elements of type T). The empty instance does not even raise a warning about unimplemented methods (since the default definition is used). I'd rather prefer to remove /= as method of Eq. Cheers Christian

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Defining new operators
by rodrigo.bonifacio 09 Aug '07

09 Aug '07

Hello, I have created the following function: dist :: String -> [[String]] -> [[String]] dist x y = [ x:e | e<-y ] eg.: dist "1M" [[], ["2M"], ["2M, 3M"]] = [["1M"],["1M","2M"],["1M","2M, 3M"]] How can I create an operator that perform the same function as dist? I want to write something like: "1M" ++ [[], ["2M"], ["2M", "3M"]] Thanks in advance. Rodrigo.

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Newbie question (again!) about phantom types
by peterv 09 Aug '07

09 Aug '07

I’m having difficulty to understand what phantom types are good for. Is this just for improving runtime performance? I read the wiki, and it says "this is useful if you want to increase the type-safety of your code", but the code below does not give a compiler error for the function test1, I get a runtime error, just like test2. Thanks, Peter -- CODE -- -- With phantom types data T1 a = TI1 Int | TS1 String deriving Show foo1 :: T1 String -> T1 String -> T1 String foo1 (TS1 x) (TS1 y) = TS1 (x++y) test1 = foo1 (TI1 1) (TI1 2) -- Shouldn't this give a compiler error instead of a runtime error? -- Without phantom types data T2 = TI2 Int | TS2 String deriving Show foo2 :: T2 -> T2 -> T2 foo2 (TS2 x) (TS2 y) = TS2 (x++y) test2 = foo2 (TI2 1) (TI2 2) No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.8/941 - Release Date: 07/08/2007 16:06

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mutually recursive types
by rodrigo.bonifacio 08 Aug '07

08 Aug '07

Hi, I am learning the haskell programming language and had tried to define the following types: type Scenario = (String, String, [Step]) type Step = (String, Scenario, String, String, String) Notice that Scenario depends on a list of steps and Step has a dependence with scenario. I know that this is a kind of "bad smell".... in Haskell, are there any pattern or language idiom to deal with cyclical dependences? Regards, Rodrigo.

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FW: RE [Haskell-cafe] Monad Description For Imperative Programmer
by peterv 08 Aug '07

08 Aug '07

Kaveh> "A monad is like a loop that can run a new function against its variable in each iteration." I’m an imperative programmer learning Haskell, so I’m a newbie, but I’ll give it a try ☺ Making mistakes is the best way to learn it ;) There are lots of different kinds of monads, but let’s stick to the IO monad first, which you seem to refer to. No *an IO monad is not a loop at all*. Instead, from an imperative programmer’s point of view, the following might be better: “an IO monad is a delayed action that will be executed as soon as that action is needed for further evaluation of the program.” The simple program main = getLine >>= putStrLn can be visually represented as (see attachment) The “world” (=a representation of your computer’s hardware) is passed to the main function, which passes it to all actions that it encounters during its lazy evaluation, causing the executing of the actions as an effect. The red wire through which the “world flows” is a “single thread”, it cannot be split (because the physical world cannot be copied!!!), so no unwanted side effects can ever occur, making IO safe in Haskell. When you write your IO program, this world object is never available (the IO type is a special internal type), so the red wire is erased from the diagram, and the getLine and putStrLn boxes become “delayed actions”. Imperative programmers like myself might initially be confused when they see Haskell’s do notation, because it looks like the actions are strict statements as in C/C++/Pascal/Java/C#/etc, but they are not. For example, try the following program: main = do last [ putStrLn "NOT executed although it is first in the list, as it is not used by the main function!", putStrLn "This action IS executed because it is evaluated by the main function." ] This is of course all due to Haskell’s laziness which only evaluates just those expressions that it needs to evaluate the main function. One thing to note in the diagram above is that the getLine box has TWO outputs, the String and the World. But functions can only have a single output, but this can be tuple. Hence the passing of the world from one box to the other is a bit more complicated. It is this pattern of extracting both values from the output and passing them to the next function and other related combinations that form the generic monad class, which can be used for many more things than IO. See http://haskell.org/haskellwiki/IO_inside for a much deeper and more correct explanation ☺ And for the pros here, did this newbie make any sense? Probably not ;-) Oh no, yet another monad explanation!!! Now the universe will most certainly collapse… No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.0/929 - Release Date: 31/07/2007 17:26

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Bug in GHCi?
by Dusan Kolar 08 Aug '07

08 Aug '07

Hello all, Maybe this is a wrong place to report, but I have repeatedly performed "funny" calculation in GHCi with strange time report. The version of GHCi is: ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Prelude> My machine: OS WinXP Pro SP2, Intel Core 2, 2GB RAM. How to reproduce error: :set +s :set -02 :cd <path to the file in the attachment> :l Primes take 10000 primes1 <provides correct result and shows correct timing, about 636 seconds> take 10000 primes2 <provides correct result, BUT INCORRECT TIMING - about -326 seconds> Yes, that is not a typo. The report of time elapsed is: minus three hundred and twenty six seconds. I'll try the newest version of the GHCi, but it is strange anyway and maybe it should be investigated... Thanks, Dusan module Primes ( primes1 ,primes2 ,primes3 ,primes2' ,primes3' ) where candidates = map (\x -> (True,x)) [2..] candidates' = [2..] skipmap f toSkip list = domap toSkip list where domap n (x:xs) | n > 1 = x : domap (n-1) xs | True = f x : domap toSkip xs domap _ [] = [] primes1 = sieve candidates where sieve (pp@(isp,p):xs) | isp = p : (sieve $ skipmap (\(_,v) -> (False,v)) p xs) | True = sieve xs primes1' = sieve candidates' where sieve (p:xs) | p > 0 = p : (sieve $ skipmap (\n -> if n>0 then (-n) else n) p xs) | True = sieve xs sieve (p:xs) = p : sieve [y | y <- xs, y `mod` p /= 0] sieve' (p:xs) = p : sieve [y | y <- xs, y `mod` p > 0] primes2 = sieve [2..] primes3 = 2 : sieve [3,5..] primes2' = sieve' [2..] primes3' = 2 : sieve' [3,5..]

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OS swapping and haskell data structures
by Alex Jacobson 08 Aug '07

08 Aug '07

If you create a Data.Map or Data.Set larger than fits in physical memory, will OS level swapping enable your app to behave reasonably or will things just die catastrophically as you hit a memory limit? -Alex-

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