October 2004 - Haskell-Cafe

Strict evaluation not working?
by Christian Hofer 12 Oct '04

12 Oct '04

Hi, having found a bit of time to play with Haskell, I am trying to figure out how to enforce strict evaluation. I wrote the following program: main = let x = zipWith (+) [5..] [6..] in putStrLn $ show $ x `seq` head x I expected this program not to terminate - because of the seq-Operator, but it happily returns 11 in ghc as well as in ghci. What do I make wrong? Chris

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Re: Language extension idea (was Re: [Haskell-cafe] Re: OCaml list sees...)
by J.Garrett.Morris＠Dartmouth.EDU 12 Oct '04

12 Oct '04

--- Tom Pledger wrote: It could get more convenient, though, if we have data-constructors-as-class-members and move the familiar list constructors into the List class. (The following might need to be built into the compiler, because of the special [] syntax.) --- end of quote --- Changing the subject very slightly, this brings up two questions: 1) Wouldn't implementing the Views proposal allow this? and, 2) Why haven't views been implemented (and why are they a "never" in the HaskellTwo discussion in HaWiki)? /gXm

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Re: [Haskell-cafe] List implementation.
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

>class List l where > head :: IArray UArray a => l a -> Maybe a > tail :: IArray UArray a => l a -> Maybe (l a) changed my mind about this... you cannot do: tail $ tail $ tail x so added a 'null' test instead.

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Re: [Haskell-cafe] List implementation.
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

Hows this: ... tail (Many i a l) | i < (e-s) = (Many (i+1) a l) | otherwise = l where (s,e) = bounds a ... a ++: l | e >= s = Many s a l | otherwise = l where (s,e) = bounds a A futher though is that with constructors you can do: f (a:as) = f [] = As you cannot use the constructors to access the list elements here, maybe the following would be more useful: class List l where head :: IArray UArray a => l a -> Maybe a tail :: IArray UArray a => l a -> Maybe (l a) then you can write: f x | Just h <- head x = ... | otherwise = ... Keean.

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List implementation.
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

With reference to the discussion a couple of days ago about list implementations, here is some code showing the idea I was talking about... Its a list that you can write either single elements or blocks (UArrays) to, but it always reads like a list of elements, so blocks can be read in, but you can recurse over individual elements. There is obviously some overhead with this in-haskell implementation, but if this were the default list implementation in the RTS, you could use the encoding trick I mentioned before to get practically no overhead for its use. -------------------------------------------------------------- {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} module List where import Data.Array.Unboxed data AList a = One !a (AList a) | Many !Int !(UArray Int a) (AList a) | Nil class List l where head :: IArray UArray a => l a -> a tail :: IArray UArray a => l a -> l a (+:) :: IArray UArray a => a -> l a -> l a (++:) :: IArray UArray a => (UArray Int a) -> l a -> l a infixr 9 +: infixr 9 ++: instance List AList where head (One a _) = a head (Many i a _) = a!i tail (One _ l) = l tail (Many i a l) | i < la = (Many (i+1) a l) | otherwise = l where (_,la) = bounds a a +: l = One a l a ++: l = Many 0 a l --------------------------------------------------------------- Keean.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

One to add to your list, string edit distance - as its hard, and useful. Keean.

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RE: [Haskell-cafe] strictness and the simple continued fraction
by Simon Peyton-Jones 12 Oct '04

12 Oct '04

If you are interested in arbitrary precision arithmetic using continued fractions, you may want to check out the work of David Lester. And Peter Potts et al. Just type "exact real arithmetic" into Google. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | William Lee Irwin III | Sent: 12 October 2004 04:53 | To: Scott Turner | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] strictness and the simple continued fraction | | On Mon, Oct 11, 2004 at 09:53:16PM -0400, Scott Turner wrote: | > I tried using continued fractions in a "spiffy lazy list" implementation a | > while ago. Never got them working as well as expected. | > Evenutally I realized that calculating with lazy lists is not as | > smooth as you might expect. | > For example, the square root of 2 has a simple representation | > as a lazy continued fraction, but if you multiply the square root of 2 by | > itself, your result lazy list will never get anywhere. The calculation will | > keep trying to determine whether or not the result is less than 2, this being | > necessary to find the first number in the representation. But every finite | > prefix of the square root of 2 leaves uncertainty both below and above, so | > the determination will never be made. | > Your problems may have some other basis, but I hope this helps. | | I hit that one, too. That's nasty enough it may be best to give up on | the infinite case, at least. I can't think of a way to salvage all this. | | | -- wli | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 11 Oct '04

11 Oct '04

I though clean was always strict, and that was the major difference between clean and haskell (that and the fact clean is a proprietry language) Keean.

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Being Compatible
by John Goerzen 11 Oct '04

11 Oct '04

Hi, I have appreciated some of the comments here about what exactly is standardized. My next question is how to work at the greatest number of places. I am working on a library of Haskell-related utilities. There is some code in there that works with binary I/O, and some code that works with rawSystem(), etc. Some of these are not supported on Hugs, nhc98, or ghc5 to varying degrees. My initial thought was to use the cpp-style ifdefs I've seen elsewhere to mask those unsupported features on those particular systems. But Hugs at least doesn't support that, and I've found it extremely difficult to find a list of predefined macros for the other systems anyway. What is the suggested way to work around these problems?

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Re: [Haskell-cafe] Re: OCaml list sees abysmalLanguage Shootoutresults
by MR K P SCHUPKE 11 Oct '04

11 Oct '04

>No, you still have to copy "aaaa" so you can change the tail pointer Do you? If "aaaa" is treated as a single buffer contents (IE implemented as a UArray Int Char for example) then as our 'new' list implementation# can have cells which are single elements of buffers of elements, we simply 'cons' the buffer onto the beginning of the list (IE the next pointer is not in the constants string but the cell containing it) Keean.

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