- Haskell-Cafe - Haskell.org

Constructing Cases
by Carl McTague 27 May '02

27 May '02

Hi there, is there a simple way to carry out the following type of conversion? Suppose I have a list (finite) of (value,image) pairs such as: list = [(0,1),(1,0)] >From this I want to generate a function f 0 = 1 f 1 = 0 Is there a way to do this? Note, I don't want to define a function that searches through the list each time it is invoked, I want to generate the function once and have it be as fast as the pattern-matcher can make it. So, I'm looking for a function g :: [(a,b)] -> a -> b Is this simple to do? Thanks, Carl

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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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