- Haskell-Cafe - Haskell.org

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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haskell emacs mode
by Chris Wright 21 Sep '05

21 Sep '05

Hi, When I download and install "Latest" haskell-mode from http://www.iro.umontreal.ca/~monnier/elisp/ it seems as if (though the dir is named haskell-mode-2.0) that version 1.43 is loaded doing $ grep version haskell-mode.el gives: ;; the Free Software Foundation; either version 2, or (at your option) ;; any later version. ;; In some versions of (X)Emacs it may only support Latin-1, not Unicode. ;; thorn(a)irisa.fr quoting the version of the mode you are using, the ;; version of Emacs you are using, and a small example of the problem (defconst haskell-version "1.43" "`haskell-mode' version number.") (defun haskell-version () "Echo the current version of `haskell-mode' in the minibuffer." (message "Using haskell-mode version %s" haskell-version)) Use `haskell-version' to find out what version this is. So, I'd be grateful for any help to get version 2.0 d/l and installed. 1.43 seems to have a bug in its handling of strings. If a string has the world "of" in it, the next line doesn't indent properly (I think the mode is expecting a case statement syntax). thanks Chris _________________________________________________________________ SEEK: Over 80,000 jobs across all industries at Australia's #1 job site. http://ninemsn.seek.com.au?hotmail

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Constructor constraints
by Rich Neswold 20 Sep '05

20 Sep '05

Hello, I've looked through the two tutorials and the Report, but couldn't find help on this topic. My question is whether you can place constraints on new data types. For instance, I want to make a new type that is a 4 element tuple where each element is greater than or equal to the previous entry. Is this possible? i.e. data Category = Membership a a a a I'd like to be able to prevent invalid Category data from being created. Any information would be appreciated. -- Rich AIM : rnezzy ICQ : 174908475

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case of (was: [Haskell] Mixing monadic and non-monadic functions)
by Andrew Pimlott 20 Sep '05

20 Sep '05

On Sat, Sep 17, 2005 at 06:56:36PM +0100, Ben Rudiak-Gould wrote: > * The new syntax is really nice as a replacement for the annoyingly > common "x <- foo ; case x of..." idiom that I've always disliked. I might wish for "case of" to mean "\x -> case x of": foo >>= case of ... Useful outside of monads, eg to write anonymous functions for map. Andrew

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Trapped by the Monads
by Mark Carter 20 Sep '05

20 Sep '05

I'm puzzling out how to get a Bool from am IO Bool. I know I'm not supposed to, but I don't see any way around my predicament. The basic setup is: I have an edit box, and a panel. If you click the LMB on the panel when the edit box is checked, this means you want to move a graphical object around the panel. If it is unchecked, then clicking the LMB means you want to add a graphical object. The relevant bits I've managed to put together so far are: mainFrame = do -- main application frame streams <- varCreate [] ... cbEdit <- checkBox p1 [text := "Edit Mode", on command ::= onCbEdit textlog] -- p1 is the panel, ignore textlog let isEditing = get cbEdit checked -- returns type IO Bool windowOnMouse p False {- no motion events -} (onMouse p streams isEditing) ... where onMouse w streams isEditChecked mouse = case mouse of MouseLeftDown pt mods -> if isEditChecked then findStream w streams pt else addStream w streams pt other -> skipCurrentEvent -- unprocessed event: send up the window chain where -- define findStream and addStream The problem is that isEditChecked is of type IO Bool, not Bool. I presume that I should actually be taking a different (non-imperative) approach, and I'm wondering if anyone could suggest what that approach should be? Many apologies for being a clueless n00b. ___________________________________________________________ Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com

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How to locate a <<loop>>?
by Jørgen Hermanrud Fjeld 20 Sep '05

20 Sep '05

Hi. I have a generated Haskell program that emits a "Fail: <<loop>>" at runtime. Are there some tools or standard manner for identifying the lines that participate in the loop? The use of "-debug" and "-prof -auto-all" only provides a call stack, and the loop is, is I understand it, due to a circular definition, so the call stack is of little help. -- Sincerely | Homepage: Jørgen | http://www.hex.no/jhf | Public GPG key: | http://www.hex.no/jhf/key.txt

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