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Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> How can I test that partial order in Haskell ? You can't. It's kinda internal. More specifically, if you try to compute a value, which is not maximal in this order, you'll get an error or loop forever. But you still can use such values in your programs if you don't try to compute them. The point is, maximal values are the only interesting ones, and all others are just approximations. In fact, you only have to worry about non-maximal values when you use recursion. The fundamental property of Haskell is that it computes the least fixed point. For example, if f maps (_|_) to (_|_), then "let x = f x in x" will evaluate to (_|_) - and you loop forever, see above.

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Re: [Haskell-cafe] Wikipedia on first-class object
by Cristian Baboi 28 Dec '07

28 Dec '07

On Thu, 27 Dec 2007 14:02:36 +0200, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > Comparing functions is certainly possible in Haskell, but there's no > standard function that does it. > If course, it might not terminate, but the same is true for many other > comparable objects in Haskell, e.g., infinite lists (which are > isomorphic to > Nat->T). The list [1 .. ] is a single value in Haskell ? ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

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Function Stores
by Cristian Baboi 28 Dec '07

28 Dec '07

Here is what I was able to come up with: module Store where data FStore1 a b = Empty1 | FS1 ( a->b , FStore1 a b ) store1 :: (a->b) -> (FStore1 a b) store1 f = let x = FS1 (f, x) in x data FStore2 a b = Empty2 | FS2 ( a->b , FStore2 a b, FStore2 a b ) store2 :: (a->b) -> (FStore2 a b) store2 f = let x = FS2 (f,x,x) in x First I tryed store1, but after a while it went out of business. Second I tryed other business model with store2. Same result. Then I tryed to make a street of stores. After a while, they all went out of business. Then I tryed to make several streets of stores. After a while they all went out of business. Then I tryed to make an infinite street of stores. After a while they all went out of business. Then, following some rumors that there is a store on that street with some function in stock that might be willing to give it away, I noticed that the street disappeared, and I cannot go to that store. Can you help me find it ?

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OT: ghc 6.8 in Debian?
by Magnus Therning 28 Dec '07

28 Dec '07

It seems my emails to the Debian Haskell list (CC'd on this email as well) are silently dropped :-( Since I suspect the readers of /that/ list are likely to be on this one as well I thought I'd give this a try. What's happening with GHC 6.8 in Debian? There was an email about a month ago announcing the availability of the core package. Since then it seems very little has happened. What can I do to help, if anything? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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nice simple problem for someone struggling....
by Nicholls, Mark 28 Dec '07

28 Dec '07

I'm just trying to pick up the basics....and I've managed to write this code...which remarkably works...... module Main where data SquareType = SquareConstructor Int class ShapeInterface shape where area :: shape->Int data ShapeType = forall a. ShapeInterface a => ShapeType a instance ShapeInterface SquareType where area (SquareConstructor sideLength) = sideLength * sideLength main = do putStrLn (show (area (SquareConstructor 4))) name <- getLine putStrLn "" But my next iteration was to try to parametise SquareType.... So something like data SquareType a = Num a => SquareConstructor a but of course doing this breaks everything.......sepecifically the instance declaration `SquareType' is not applied to enough type arguments Expected kind `*', but `SquareType' has kind `* -> *' In the instance declaration for `ShapeInterface SquareType' And I can't seem to get it to work.....

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FFI"pointer" data types question
by Galchin Vasili 28 Dec '07

28 Dec '07

Hi, If I am calling a ANSI function that requires a pointer to a C struct, which FFI pointer type should use? Kind regards, Vasya

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Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> >> take 5 [1,2,3,4,5,undefined] > (What would GHCi print? [1,2,3,4,5] > Then please square that with your comment that we don't care how much of > the list gets evaluated.) I don't care if it evaluates it, reads it's answer from some internal table, or consult it's lawyer. I just want it to agree with my reasoning about complete partial orders; that's all I need. I don't know how to explain this clearer.

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Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> I'll bite. Please, don't. > So what do you expect: > take 5 [1,2,3,4,5,undefined] > to do? Nothing! It's a value, not an instruction! > So it does seem to matter how much of the list it evaluates... No, it's implementation details. I'm reasoning about values, not computations, and I use complete orders for reasoning. What really matters is that implementation gives me results that agree with this reasoning - until we consider performance issues. > > Well, that depends of your notion of interesting. > I defined it - I'm interested in things that are observationally > equivalent. That's your choice, and I certainly can't argue, but I can't help mentioning that this is a bit weird.

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Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> Right, so when I say to GHCi: > Prelude> take 5 [1..] > and it says: > [1,2,3,4,5] > then it really has computed the entirety of the infinite sequence > [1..], and not some approximation? Of course not! In fact, it doesn't even compute the value "1". It just juggles with the bytes somehow, so that it seems that it computes the whole sequence and then applies "take 5". But does it really matter if it computes 5 integers, or 10, or the whole sequence, at least if we are not concerned about performance? > Perhaps it is better to say that in a lazy language both > approximations and maximal values (least upper bounds of a chain) are > interesting. Well, that depends of your notion of interesting.

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Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> >> It sounds like a limit. xn --> x for n --> :-) > > > > Yeah, that's right. > If xn is monotone, then I only have to worry when it is not bounded, right > ? Sorry, don't understand what you mean. Yes, the monotone sequence has a limit. No, it's not necessarily maximal. > >> [1 .. ] and x=1:x are maximal ? > Can you give me some examples of non maximal ones ? let x = x+1 in x Or even let x = x in x In both cases, (_|_) is the least fixed point, and is non-maximal.

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