September 2005 - Haskell-Cafe

Growing Trees
by Tom Hawkins 22 Sep '05

22 Sep '05

I'm porting an ML program to Haskell, but am having difficulty with particular data structure: a tree where each node has references to the children as well as the parent... data Tree a = TreeRoot { stuff :: a , children :: [Tree] } | TreeNode { stuff :: a , parent :: Tree , children :: [Tree] } But because of these bidirectional links, every time I add a node I must reconstructing the entire tree. There is also the add coding complexity of threading the tree through various functions to simulate state. What are my [monadic] options? -Tom

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RE: [Haskell-cafe] killing a running thread interactively
by Simon Marlow 22 Sep '05

22 Sep '05

There's no getAllThreadIds or similar right now. You have to save the ThreadIds yourself, perhaps in a global variable. Writing to a file doesn't work because there's no instance Read ThreadId. Cheers, Simon On 21 September 2005 21:14, S. Alexander Jacobson wrote: > More particularly, is there a getAllThreadIds function somewhere? > > -Ale- > > On Wed, 21 Sep 2005, S. Alexander Jacobson wrote: > >> Is the general pattern to write all threadIds to a file, and then >> have a separate function that takes the file and kills them? >> >> Or is there something more clever? >> >> -Alex- >> >> On Wed, 21 Sep 2005, Simon Marlow wrote: >> >>> On 16 September 2005 20:42, S. Alexander Jacobson wrote: >>> >>>> If I am running a server interactively. (using ghci). >>>> Is there any way to kill its running threads without terminating >>>> the interpreter? >>> >>> If you can get ThreadIds for the threads, yes. GHCi doesn't >>> (currently) create a new thread for each expression evaluation, so >>> attempting to kill that thread might kill your GHCi session (it >>> shouldn't but that's another story). >>> >>> Cheers, >>> Simon >>> >>> ** CRM114 Whitelisted by: simonmar(a)microsoft.com ** >>> >> >> ______________________________________________________________ >> S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com >> > > ______________________________________________________________ > S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Newbie question: how to run HaXML in Unix
by Huong Nguyen 22 Sep '05

22 Sep '05

Hello everybody, This seems so simple question, but I did not find the solution. HaXML is installed. Do you know how I can run HaXML in Linux/Unix (which command I should write). Thank you.

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mistake in Gentle Introduction
by Michael Vanier 22 Sep '05

22 Sep '05

On this page: http://www.haskell.org/tutorial/modules.html it refers to the process of hiding imported names from a module and gives the example: import Prelude hiding length whereas the correct syntax is import Prelude hiding (length) I spent nearly an hour beating my head against this. Can someone fix this in the documentation? Mike

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Really confused
by Mark Carter 22 Sep '05

22 Sep '05

I'm trying to define a type called Stream. For now, assume that that the type has 3 fields: uid, x, y. X and y represent a point in space that a stream occupies, and uid is a unique identifier for the stream. The uid should be "auto-generated". It's important that streams have an identity so that they can be referred to and manipulated. Streams will be put together into a list, and they'll eventually need to be able to point to one another. It would be nice to be able to print the uid, just to show that it works. Now it occured to me that what one might want to generate the uids is a monad, let's call it UID. It'll have a function get, which returns another identifier. I assumed that the best solution for the problem would be in terms of monads, because successive calls to get return different results; i.e. there's a bit of state going on inside. The values returned can start from 1, and increment by 1 each time. I had a look at some documentation at: http://www.haskell.org/tutorial/monads.html and I'm afraid my brain just froze. I get the idea that data SM a = SM (S -> (a,S)) maps a state to a result, and a new state. OTOH, looking at instance Monad SM where -- defines state propagation SM c1 >>= fc2 = SM (\s0 -> let (r,s1) = c1 s0 SM c2 = fc2 r in c2 s1) return k = SM (\s -> (k,s)) just confuses me no end. Any pointers, like am I taking completely the wrong approach anyway? I'm also puzzled as to how the initial state would be set. ___________________________________________________________ Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com

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Re: [Haskell] Re: [Haskell-cafe] Haskell versus Lisp
by David F. Place 21 Sep '05

21 Sep '05

On Sep 20, 2005, at 3:43 PM, Glynn Clements wrote: > That, in a nutshell, is Lisp's key strength. It uses the same > structure for code as for data, which makes it very easy to add new > language features. > I assume that you refer to `eval' and the fact it operates on conses and symbols. Beyond the extremely contrived example of a metacircular interpreter, what are some examples of the benefits of this feature of lisp? What are some examples of language features that are easy to add? -------------------------------- David F. Place mailto:d@vidplace.com

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RE: [Haskell-cafe] killing a running thread interactively
by Simon Marlow 21 Sep '05

21 Sep '05

On 16 September 2005 20:42, S. Alexander Jacobson wrote: > If I am running a server interactively. (using ghci). > Is there any way to kill its running threads without terminating the > interpreter? If you can get ThreadIds for the threads, yes. GHCi doesn't (currently) create a new thread for each expression evaluation, so attempting to kill that thread might kill your GHCi session (it shouldn't but that's another story). Cheers, Simon

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Haskell-style proof tools?
by Robin Green 21 Sep '05

21 Sep '05

Does anyone know of a prover / proof assistant / proof verifier which uses a vaguely Haskell-like syntax? That is to say, it allows you to express theorems in Haskell-style syntax, print proof steps in Haskell-style syntax, etc. -- Robin

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Basic type classing question.
by Karl Grapone 21 Sep '05

21 Sep '05

Hi, I've just started learning Haskell, and I must admit I'm finding it a bit hard to get my head around the typing system... If I wanted to represent Employees and Departments in a Haskell program I could use data declarations like so: data Employee = Emp ... data Department = Dept ... This seems limited in (at least) two ways: I can't dynamically add fields to an employee or department, and once I pull a field out of an instance I lose type information. What I want to be able to do is add and remove fields while the system is running, I suppose via hs-plugins, and I should be prevented from, for example, accidentally taking an employees first name and using it as a departments address. My first attempt was the following, which isn't even valid and doesn't appear to buy me much anyway. class DataContainer c class DataContainer c => DataField f c a where extract :: f -> a apply :: (a -> a -> a) -> f -> f -> f data DataContainer c => Field c a = Fld c a instance DataField Field c a where extract (Fld _ a) = a apply f (Fld c a1) (Fld _ a2) = Fld c (f a1 a2) data Employee = Emp instance DataContainer Employee data Department = Dept instance DataContainer Department type EmployeeName = Field Employee String type EmployeeAddress = Field Employee String type EmployeeIdentifier = Field Employee Integer type DepartmentAddress = Field Department String type DepartmentIdentifier = Field Department Integer ... The 'DataField instance Field' declaration gives kind errors regarding how many type arguments Field is applied to. I don't claim to understand kinds at this point. Even if it did work, apply doesn't appear to force the arguments to be of the same type, and the declared type synonyms aren't enough to prevent me getting employee names and addresses confused. Is there a correct way to express what I'm trying to do? Or is it a bad idea to start with? Thanks Karl

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Re: [Haskell-cafe] Trapped by the Monads
by Mark Carter 21 Sep '05

21 Sep '05

Greg Buchholz wrote: > Have you read... > > http://haskell.org/hawiki/ThatAnnoyingIoType > > > Thanks. I'll take a look at it. I also need to take a look at the basic Haskell syntax. An interesting-looking web page which discusses monads is: http://www.nomaware.com/monads/html/analogy.html What struck me was this bit of code: assemblyLine w = (return w) >>= makeChopsticks >>= polishChopsticks >>= wrapChopsticks Interestingly, this looks like Forth (!), where you put a value on the stack, and successive operations fiddle with the stack as a series of transformations. Not that I know Forth, you understand. Hmm, so Haskell can be a concatenative language if you want it to be. ___________________________________________________________ Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com

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