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Re: Writing a counter function
by Jon Cast 02 Jul '02

02 Jul '02

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > On Sat, 29 Jun 2002, Samuel E. Moelius III wrote: > (snip) > > Here's another not-exactly-what-you-wanted solution. :) > (snip) > Do any of the experimental extensions to Haskell allow a > what-he-wanted solution? I couldn't arrange one in H98 without > something having an infinitely-recursive type signature. That's because the problem requires an infinitely-recursive type. > I'm sure it would have been easy in Lisp, and he already gave a Perl > equivalent, That's because both Lisp and Perl are weakly typed. Haskell is strongly typed. Consider Lisp: (defun self-apply (f) "Apply F to itself." (f f)) No problem (and I'm sure you can pull the same trick off in Perl). Consider Haskell: self-apply f = f f The *typing algorithm* (the thing that didn't complain in Lisp) proceeds roughly as follows: f is applied to at least one argument, so f must have type a -> b. Therefore, f's argument (f) must have type a. So, we conclude: f :: a -> b f :: a But f can only have one type, so we set its types equal: a = a -> b This is clearly recursive, right? > so I'm wondering if it could be at all sane for Haskell to allow > such stuff Sure. All it has to do is: 1. Create its own newtype in response to such things as `self-apply' above. 2. Ensure that self-apply f = f f and self-apply' g = g g have the same type. I would *love* to hear ideas on how to do that, but it's difficult. > and if Haskell is somehow keeping us on the straight and narrow by > disallowing the exact counter that was originally requested. Well, it's a more general problem than the `exact counter'. And, yes, Haskell is erring on the side of safety here; there's a reason it's called a ``statically typed language''. > The beauty of his request was that it was so simple and seemed to > make sense; I went ahead and tried to fulfill it before realising I > couldn't do it either. Well, it makes sense, yes. But I think learning to manually `unify' these things (add a newtype) is not hard, and probably useful --- Haskell is not the only statically-typed language out there. Learn how to solve this problem, and you understand the problem, the solution, typing, and life in general :) > -- Mark Jon Cast

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Writing a counter function
by Shlomi Fish 01 Jul '02

01 Jul '02

I'm trying to write a counter function that would return a tuple whose first element is the current value and whose second element is a new counter. The following line: counter n = (n,(counter (n+1))) Generates the following error on Hugs and a similar one with ghci: ERROR "counter.hs":6 - Type error in function binding *** Term : counter *** Type : a -> (a,b) *** Does not match : a -> b *** Because : unification would give infinite type Is there any way to do it? I tried using data, type and newtype and none of them worked. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ Home E-mail: shlomif(a)iglu.org.il He who re-invents the wheel, understands much better how a wheel works.

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Defining instances of superclasses
by Zdenek Dvorak 30 Jun '02

30 Jun '02

Hello. I've got the following problem: I have class Structure, that says that its members have some internal parts that can be read from/written to Strings: >data StrItem a=forall b c.(Show b,Parse c)=>StrItem String (a->b) (a->c->a) > >class Structure a where > strName::a->String > strEmpty::a > strItems::[StrItem a] Now I would like to use it to create instances of Show and Parse (and perhaps others (for input/output from various formats, ...)). The approaches I have tried: 1) >instance Structure a => Show a where > showsPrec _ w = > showsStruct (strName w) (map showsPair strItems) > where > showsPair (StrItem fName fGet _)=(fName,shows $ fGet w) This is nice, but it only works in ghc with allowed overlapping and undecidable instances. It may not seem too important, when I use existential types anyway, but existential types are supported by at least one more implementation (hugs) and they seem to me to be a well defined extension that has good chances to become part of standard Haskell later; handling of overlapping and undecidable instances in ghc seems quite too much ad-hoc and experimental for me. 2) >newtype Str a=Str a >instance Structure a => Show (Str a) where > showsPrec _ (Str w) = > showsStruct (strName w) (map showsPair strItems) > where > showsPair (StrItem fName fGet _)=(fName,shows $ fGet w) This seems to be a standard way how to do it. The problem is, that I don't get Show instance for a, but for isomorphic type Str a; this is a bit annoying when I want to use it; also I may no longer use deriving Show on types containing such structure. Is there some better way how to do this? Zdenek Dvorak _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com

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Greencard-Canada-Aupair
by Greencard 28 Jun '02

28 Jun '02

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Biz Kurduk Ordan Biliyoruz
by �leti... 22 Jun '02

22 Jun '02

1 0

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by ��ȷ� 22 Jun '02

22 Jun '02

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Question about GUM
by Changming Ma 20 Jun '02

20 Jun '02

Hi there, I'm getting confused of the run-time system behavior of GUM. When I ran a quicksort program in GUM with profiling flag, I got different "task" distributions in the PS file generated everytime, e.g., the peak of task number may be 19 at the first time and 48 at the second. While it's quite reasonable that the clock cycles vary dramatically, it's somewhat strange to me that the task distributions are so different. Any hints? I do appreaciate it. BTW, seems the program does not stop if the input list has more than 350 elements, althought it works fine otherwise. Will it get better if I use an SMP system? Thanks in advance. PS: I'm using GUM-4.06, PVM3, RH LINUX6.2 on a single processor system. Program and RTS flags quicksort +RTS -qP -sstderr grs2gr *quicksort.???.gr > quicksort.gr gr2ps -o quicksort.ps quicksort.gr --source code module Main(main) where import System(getArgs) import Parallel forceList :: [a] -> () forceList [] = () forceList (x:xs) = x `seq` forceList xs quicksortF::[Int]->[Int] quicksortF [] = [] quicksortF [x] = [x] quicksortF (x:xs) = (forceList losort) `par` (forceList hisort) `par` losort ++ (x:hisort) where losort = quicksortF [y|y <- xs, y < x] hisort = quicksortF [y|y <- xs, y >= x] args_to_IntList :: [String] -> [Int] args_to_IntList a = if length a < 1 then error "Parallel Quick Sort: no enough args \n" else map read a main = putStr ("get " ++ (show (quicksortF l))++"\n") l = [ 658986,123141,737929,681645,329651, ... 925104]

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% 6,95
by finans400 19 Jun '02

19 Jun '02

1 0

Data Tracing in GPH
by Ma Changming 16 Jun '02

16 Jun '02

Hi there, Does anyone know how to trace the GPH program to see the data processed at specific processors? I'm using gum-4.06, PVM3, RedHat Linux6.2. When I execute the following QuickSort program, I want to make sure what happens with different CPUs, i.e., how the data are partitioned between processors? Is there any debugging tools to trace the data? Thanks in advance. Changming Ma PS: QuickSort module Main(main) where import System(getArgs) import Parallel forceList :: [a] -> () forceList [] = () forceList (x:xs) = x `seq` forceList xs quicksortF::[Int]->[Int] quicksortF [] = [] quicksortF [x] = [x] quicksortF (x:xs) = (forceList losort) `par` (forceList hisort) `par` losort ++ (x:hisort) where losort = quicksortF [y|y <- xs, y < x] hisort = quicksortF [y|y <- xs, y >= x] args_to_IntList :: [String] -> [Int] args_to_IntList a = if length a < 1 then error "Parallel Quick Sort: no enough args \n" else map read a main = getArgs >>= \ a -> let l = args_to_IntList a in putStr ("get " ++ (show (quicksortF l))++"\n") __________________________________________________ Do You Yahoo!? Yahoo! - Official partner of 2002 FIFA World Cup http://fifaworldcup.yahoo.com

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reversing a list
by Matthias 15 Jun '02

15 Jun '02

Hi there, I thought i can reverse a list with reverse a list easily with the reverse function in order to achieve following: I wanted to check for a certain suffix (then reversed a prefix) and substitute that. So i did -- let dstfile = case tmpfile of -- ('a':'s':'p':'.':rest) -> rest ++ ".psx" -- anything -> anything ++ ".psx" Guess what: I did not work. dstfile was reversed (tmpfile ++ ".psx"). i guess this deals with lazy evalutation, but would be glad about some definit ideas. Matthias

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