May 2002 - Haskell-Cafe

Segmentation fault
by ck15 22 May '02

22 May '02

Hi, I am getting a segmentation fault , when I execute a code in hugs. (only some times) What are the conditions that give rise to a segmentation fault , when executing a Haskell code in hugs? I would appreciate very much if some could spare some time to help me. Thanks Chitra

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by web 22 May '02

22 May '02

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RE: Re: Fixed point of n-mutually recursive functors
by gonflores＠guay.com 22 May '02

22 May '02

> I don't think this is possible in plain Haskell 98. > It is possible in Generic Haskell, see: > http://www.generic-haskell.org/ > [...] > Generic Haskell's type system is sufficient. > > Is it possible using dependent types? > > I would expect so. I have been looking to Generic Haskell and it may be possible to define a generic map function like: mapN :: (a1 -> b1) -> ... -> (aN -> bN) -> f a1 ... aN -> f b1 ... bN However, I still don't know how could I define something like: > data FixN1 f1 ... fN = InN1 (f1 (FixN1 f1 ... fN) ... (FixNN f1 ... fN)) > ... > data FixNN f1 ... fN = InNN (fN (FixN1 f1 ... fN) ... (FixNN f1 ... fN)) I think that I would need dependent types, because I want to define a number of data types that depend on a given natural...any suggestions? Best regards, Gonzalo Flores

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convering Fds to Handles
by Abraham Egnor 22 May '02

22 May '02

Is there a way to convert an Fd variable to a Handle variable? Specifically, I'd like to use "pipe" from the Posix library to create a pipe and then pass one end of it to runProcess as its stdin. Abe

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Fixed point of n-mutually recursive functors
by gonflores＠guay.com 22 May '02

22 May '02

I am able to define the fixed point of a functor as > data Fix f = In (f (Fix f)) where I can also define the type class > class Functor1 f > where map1 :: (a -> b) -> f a -> f b I can also define the fixed-point of 2 mutually recursive bifunctors as: > data Fix21 f g = In21 (f (Fix21 f g) (Fix22 f g)) > data Fix22 f g = In22 (g (Fix21 f g) (Fix22 f g)) where bifunctors can be captured in the type class > class Functor2 f > where map2 :: (a1 -> b1) -> (a2 -> b2) -> f a1 a2 -> f b1 b2 Now, I would like to generalize those definitions to n-functors. Is it possible? If it isn't, what kind of type system would I need? Is it possible using dependent types? I would also appreciate any points to literature using n-mutually recursive functors. Best regards, Gonzalo Flores

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derivative (S f g) ?
by Conal Elliott 21 May '02

21 May '02

This isn't really a Haskell question, but I'm hoping a fellow Haskeller might have some helpful pointers. Has anyone seen a generalization of the chain rule for derivatives that applies to applications of the S combinator? The conventional chain rule applies to the more restricted composition combinator: D (f . g) = (D f . g) * D g Where D is the differentiation higher-order function, "*" is multiply lifted pointwise to functions (\ a b x -> a x * b x), and "." is function composition. Thanks, - Conal

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(no subject)
by Andy Fugard 21 May '02

21 May '02

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Re: SUGGESTION: haskell-announce mailing list
by Jens Petersen 21 May '02

21 May '02

"Simon Marlow" <simonmar(a)microsoft.com> writes: > I'm happy to set up and moderate > haskell-announce(a)haskell.org if there's a general > consensus that this would be a useful thing. I think there are enough announcements on the haskell list, to warrant having a separate announce list. It would make filtering announcements to a separate folder much easier too. > As to whether we should merge haskell and haskell-café - > personally I wasn't in favour of the split at the time, > but I recall that support was roughly 50/50 in favour. > Unless there's an overwhelming majority in favour of a > merge I suggest we leave things as they are. I don't have a strong opinion on this either way, however I agree with some of the earlier comments that it only really makes sense to have two different lists if their agenda are more clearly distinct. (IMHO "cafe" is too vague.) Jens

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by giftcaravan＠usa.com 19 May '02

19 May '02

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Re: Q: Resolving ambiguous type variable
by tty＠SAFe-mail.net 17 May '02

17 May '02

Would it be correct to conclude from your reasoning that the following would be properly typed by the compiler ? first_h :: (Show a) => [a] -> Result a first_h [] = Ex "No list" first_h (x:xs) = Value x since we are explicitly stating first_h can only take a list containing elements deriving Show. My expectations would be that this would be unambiguous, unfortunately ghc-5.02.3 still complains with the reported error. Regards Tee -------- Original Message -------- From: hdaume(a)ISI.EDU To: tty(a)SAFe-mail.net Cc: haskell-cafe(a)haskell.org Subject: Re: Q: Resolving ambiguous type variable Date: Fri, 17 May 2002 10:24:29 -0700 (PDT) > Erm, I think I said something stupid (or at least implied it). > > main = getArgs >>= print . first_h > > *will* work, because it knows that the result of a getArgs is a list of > Strings. > > The problem you originally had was that when you say: > > first_h [] > > The "[]" could refer to a list of any type of element. Since not every > element type is an instance of Show, it doesn't know that it can apply > show to the list. > > By saying, for instance, (first_h ([] :: [Char])) you're saying "okay, > this is an empty list, but it's a list of Characters and you know that a > character can be shown." If you backtrack what the type checker is doing, > it says: > > print (first_h []) > > okay, the argument to print must be an instance of Show (print has type > Show a => a -> IO ()). therefore, > > first_h [] > > must be an instance of Show. well, what's the type of first_h []? Well, > the type of first_h is [a] -> Result a. is "Result a" an instance of > show? well, let's see...you said data Result a = ... deriving > (Show). this basically means you get an instance declaration: > > instance Show a => Show (Result a) where > ...some stuf... > > so, we want to know if Result a is an instance of show. Well, it is > whenever a is an instance of show. so, if the elements of your list are > an instance of show, then (first_h []) is an instance of Show, as we > require. > > however, since you just say "[]" it knows this is of type [a], but it > *doesn't* know what a is. therefore, a could either be or not be an > instance of show. however, if you explicitly specify what a is and that > explicit type is an instance of show (like Char in my example), then > everything is fine. > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > On Fri, 17 May 2002, Hal Daume III wrote: > > > In short, you cannot. > > > > What if your main were: > > > > main = getArgs >>= print . first_h > > > > The compiler doesn't know the difference and so it needs a type. > > > > Simple fix: > > > > > main = print (first_h ([] :: [Char])) > > > > > > -- > > Hal Daume III > > > > "Computer science is no more about computers | hdaume(a)isi.edu > > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > > > On Fri, 17 May 2002 tty(a)SAFe-mail.net wrote: > > > > > Hello, > > > I am writing a function which returns an exception/error string should an unexpected parameter is passed in. Borrowing from the Maybe module I have the following: > > > ------------------------------------------------------------ > > > data Result a = Ex String | Value a deriving Show > > > > > > -- Testing the Result a type > > > first_h :: [a] -> Result a > > > first_h [] = Ex "No list" > > > first_h (x:xs) = Value x > > > > > > -- Trying out the first_h > > > main :: IO() > > > main = print (first_h []) > > > ------------------------------------------------------------ > > > > > > Which the compiler complains: > > > > > > Ambiguous type variable(s) `a' in the constraint `Show a' > > > arising from use of `print' at atype.hs:8 > > > In the definition of `main': print (first_h []) > > > > > > This is understandable since it is unable to unify the empty list with a concrete list of type 'a' i.e. there are an infinite types which would match. My question is how can I indicate to the compiler that it does not matter which type the empty list is since the return result is of a concreate type. > > > > > > Thanks > > > > > > Tee > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe(a)haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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