February 2001 - Haskell-Cafe

Re: Primitive types and Prelude shenanigans
by Ketil Malde 16 Feb '01

16 Feb '01

William Lee Irwin III <wli(a)holomorphy.com> writes: > Some analysis of the value of the literal would need to be > incorporated so that something like the following happens: > literal "0" gets mapped to zero :: AdditiveMonoid t => t > literal "1" gets mapped to one :: MultiplicativeMonoid t => t Indeed. Is it a reasonable assumption that all values of literal "0" are intended to be the additive identity element? How about "1", might we not have it as a successor element in a group, where multiplication isn't defined? I guess other behaviour (e.g. using implicit fromInteger) assumes even more about the classes that can be represented by literal numbers, so it appears this would be an improvement, if it is at all workable. > literal "5" gets mapped to (fromPositiveInteger 5) Is something like (fromInteger 5) *> one (where *> is Module scalar multiplication from the left? Could we avoid having lots and lots of from* functions? I guess having to declare any datatype as Module is as bad as the explicit conversion functions... Anyway, I like it, especially the zero and one case. > I think Dylan Thurston's proposal is probably the best starting point > for something that should really get support. Indeed. How far are we from being able to import it, e.g. with "import DTlude as Prelude" or whatever mechanism, so that we can start to play with it, and see how it works out in practice? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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RE: need help w/ monad comprehension syntax
by Konst Sushenko 16 Feb '01

16 Feb '01

thanks, did not know that. the articles that i read are outdated in that respect... using the "do" notation is just fine. with the list notation not working i thought that i musunderstand something about monads. ;-) konst -----Original Message----- From: Tom Pledger [mailto:Tom.Pledger@peace.com] Sent: Thursday, February 15, 2001 5:22 PM To: haskell-cafe(a)haskell.org Subject: need help w/ monad comprehension syntax Konst Sushenko writes: | what am i missing? : | > --g :: State String Char | > g = [ x | x <- return 'a' ] Hi. The comprehension syntax used to be for monads in general (in Haskell 1.4-ish), but is now (Haskell 98) back to being specific to lists. Does it help if you use do-notation instead? Regards, Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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need help w/ monad comprehension syntax
by Konst Sushenko 16 Feb '01

16 Feb '01

hello, i am having trouble getting my program below to work. i think i implemented the monad methods correctly, but the function 'g' does not type as i would expect. Hugs thinks that it is just a list (if i remove the explicit typing). i want it to be functionally identical to the function 'h'. what am i missing? thanks konst > newtype State s a = ST (s -> (a,s)) > > unST (ST m) = m > > instance Functor (State s) where > fmap f m = ST (\s -> let (a,s') = unST m s in (f a, s')) > > instance Monad (State s) where > return a = ST (\s -> (a,s)) > m >>= f = ST (\s -> let (a,s') = unST m s in unST (f a) s') > > --g :: State String Char > g = [ x | x <- return 'a' ] > > h :: State String Char > h = return 'a'

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Downloading Hugs
by zulf jafferi 15 Feb '01

15 Feb '01

hi, I tried to download the Hugs 98.after downloading Hugs 98,when i try to click on the Hugs icon it gives me an error saying COULD NOT LOAD PRELUDE.i am using Windows 2000. I would be much obliged if you could help me solve the problem. cheers!! _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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Re: framework for composing monads?
by Jan Kort 15 Feb '01

15 Feb '01

Andy Gill's Monad Template Library is good for that, but the link from the Haskell library page is broken: http://www.cse.ogi.edu/~andy/monads/doc.htm Jan

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RE: Primitive types and Prelude shenanigans
by Simon Marlow 15 Feb '01

15 Feb '01

> (Of course, this is not Haskell 98 behaviour.) I think I > managed to forget > to tell anyone of this flag. And to my surprise I can't find > it any more! It's lumped in with -fno-implicit-prelude, but the extra functionality isn't supported in 4.08.2 (but hopefully will be in 5.00). Cheers, Simon

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RE: Typing units correctly
by Andrew Kennedy 14 Feb '01

14 Feb '01

To be frank, the poster that you cite doesn't know what he's talking about. He makes two elementary mistakes: (a) attempting to encode dimension/unit checking in an existing type system; (b) not appreciating the need for parametric polymorphism over dimensions/units. As others have pointed out, (a) doesn't work because the algebra of units of measure is not free - units form an Abelian group (if integer exponents are used) or a vector space over the rationals (if rational exponents are used) and so it's not possible to do unit-checking by equality-on-syntax or unit-inference by ordinary syntactic unification. Furthermore, parametric polymorphism is essential for code reuse - one can't even write a generic squaring function (say) without it. Best to ignore the poster and instead read the papers that contributors to this thread have cited :-) To turn to the original question, I did once give a moment's thought to the combination of type classes and types for units-of-measure. I don't think there's any particular problem: units (or dimensions) are a new "sort" or "kind", just as "row" is in various proposals for record polymorphism in Haskell. As long as this is tracked through the type system, everything should work out fine. Of course, I may have missed something, in which case I'd be very interested to know about it. - Andrew Kennedy. > -----Original Message----- > From: andrew(a)andrewcooke.free-online.co.uk > [mailto:andrew@andrewcooke.free-online.co.uk] > Sent: Wednesday, February 14, 2001 5:02 PM > To: haskell-cafe(a)haskell.org > Subject: Re: Typing units correctly > > > > Hi, > > I don't know if this is useful, but in response to a link to that > article that I posted on Lambda, someone posted a link arguing that > such an approach (at least in Ada) was impractical. To be honest, I > don't find it very convincing, but I haven't been following this > discussion in detail. It might raise some problems you have not > considered. > > Anyway, if you are interested, it's all at > http://lambda.weblogs.com/discuss/msgReader$818 > > Apologies if it's irrelevant or you've already seen it, > Andrew > > On Mon, Feb 12, 2001 at 01:51:54PM -0500, Dylan Thurston wrote: > [...] > > The papers I could find (e.g., > > http://citeseer.nj.nec.com/kennedy94dimension.html, > "Dimension Types") > > mention extensions to ML. I wonder if it is possible to work within > > the Haskell type system, which is richer than ML's type system. > [...] > > -- > http://www.andrewcooke.free-online.co.uk/index.html >

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A sample revised prelude for numeric classes
by Dylan Thurston 14 Feb '01

14 Feb '01

I've started writing up a more concrete proposal for what I'd like the Prelude to look like in terms of numeric classes. Please find it attached below. It's still a draft and rather incomplete, but please let me know any comments, questions, or suggestions. Best, Dylan Thurston

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Primitive types and Prelude shenanigans
by William Lee Irwin III 14 Feb '01

14 Feb '01

It seems to me that some additional primitive types would be useful, most of all a natural number type corresponding to an arbitrary- precision unsigned integer. Apparently Integer is defined in terms of something else in the GHC Prelude, what else might be needed to define it? Some of my other natural thoughts along these lines are positive reals and rationals, and nonzero integers. Rationals I have an idea that they might be similar to natural numbers and nonzero integers, though the nonzero and positive reals pose some nasty problems (try underflow). Would such machinations be useful to anyone else? Further down this line, I've gone off and toyed with Bool, and discovered GHC doesn't like it much. Is there a particular place within GHC I should look to see how the primitive Boolean type, and perhaps other types are handled? I'd also like to see where some of the magic behind the typing of various other built-in constructs happens, like list comprehensions, tuples, and derived classes. Cheers, Bill

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Re: Revamping the numeric classes
by Dylan Thurston 13 Feb '01

13 Feb '01

Other people have been making great points for me. (I particularly liked the example of Dollars as a type with addition but not multiplication.) One point that has not been made: given a class setup like class Additive a where (+) :: a -> a -> a (-) :: a -> a -> a negate :: a -> a zero :: a class Multiplicative a where (*) :: a -> a -> a one :: a class (Additive a, Multiplicative a) => Num a where fromInteger :: Integer -> a then naive users can continue to use (Num a) in contexts, and the same programs will continue to work.[1] (A question in the above context is whether the literal '0' should be interpreted as 'fromInteger (0::Integer)' or as 'zero'. Opinions?) On Wed, Feb 07, 2001 at 06:27:02PM +1300, Brian Boutel wrote: > * Haskell equality is a defined operation, not a primitive, and may not > be decidable. It does not always define equivalence classes, because > a==a may be Bottom, so what's the problem? It would be a problem, > though, to have to explain to a beginner why they can't print the result > of a computation. Why doesn't your argument show that all types should by instances of Eq and Show? Why are numeric types special? Best, Dylan Thurston Footnotes: [1] Except for the lack of abs and signum, which should be in some other class. I have to think about their semantics before I can say where they belong.

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