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RE: Template Haskell question
by Simon Peyton-Jones 12 Jul '03

12 Jul '03

Yes, sorry, as Ian says, type splices just aren't implemented at the moment. The error message is uninformative. This is useful info though -- someone wants type splices! Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Ashley | Yakeley | Sent: 27 June 2003 23:57 | To: haskell-cafe(a)haskell.org | Subject: Template Haskell question | | Can anyone tell me what's wrong with this? | | -- ghc -fglasgow-exts -c TH.hs | module TH where | { | import Language.Haskell.THSyntax; | | class HasZero a where | { | zero :: a; | }; | | aninstance :: TypQ -> Q [Dec]; | aninstance t = [d| | | instance HasZero $t where -- error here | { | zero = 0; | }; | | |]; | | $(aninstance [t|Int|]) | $(aninstance [t|Integer|]) | } | | | $ ghc -fglasgow-exts -c TH.hs | TH.hs:14: Malformed context in instance header | | All I want to do is spin off a number instances for a number of types... | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Hugs Humor
by Steffen Mazanek 12 Jul '03

12 Jul '03

Hello, a quit funny hugs session: Prelude> 0.5::Rational 1 % 2 Prelude> 0.1::Rational 13421773 % 134217728 Prelude> 13421773/134217728 0.1 I do not know how this fraction is calculated, but it does not fit my expectations :-) Ok, ok, it is no bug... Ciao, Steffen -- Steffen Mazanek - www.steffen-mazanek.de - GPG: 791F DCB3 Haskell, that's where I just curry until fail, unwords any error, drop all undefined, maybe break, otherwise in sequence span isControl and take max $, id: (d:[])

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by webmaster 11 Jul '03

11 Jul '03

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Re:Decimal Literals
by jmaessen＠alum.mit.edu 11 Jul '03

11 Jul '03

Ketil Z. Malde <ketil(a)ii.uib.no> writes: > I.e. for 3.14, allow any rational number in [3.14,3.15), or > perhaps (3.135,3.145]? Or restrict it to just the rational number in > range with the least total digits? We can get this effect using a version of (%) adapted specifically for converting decimal floating-point constants. Thus: Data.Ratio> 31415926536%1000000000 3926990817 % 125000000 Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 31415926536 %% 10000000000 312689 % 99532 We can imagine the compiler replacing floating constants by fromRational (n%%d) rather than fromRational (n%d). Of course, then trailing zeros suddenly become significant: Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 314 %% 100 22 % 7 Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 3140 %% 1000 135 % 43 This might be good or bad, depending on your perspective. -Jan-Willem Maessen jmaessen(a)alum.mit.edu

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constructors in O'Haskell
by Peter Padawitz 11 Jul '03

11 Jul '03

How may I hide constructors in O'Haskell 'cause export lists don't work in this language? Peter

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Monad constructors
by Peter Padawitz 11 Jul '03

11 Jul '03

Why must every Monad instance be a datatype or newtype, even if there will be just a single constructor? Peter

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Re: Representing cyclic data structures efficiently in Haskell
by C.Reinke 08 Jul '03

08 Jul '03

> What is the best way to represent cyclic data structures in Haskell? There used to be some work on direct cyclic representations at UCL: Dynamic Cyclic Data Structures in Lazy Functional Languages Chris Clack, Stuart Clayman, David Parrott Department of Computer Science, University College London, October 1995 The link to the project, from http://www.cs.ucl.ac.uk/staff/C.Clack/research/report94.html is broken, but the paper seems to be online at http://www.cs.ucl.ac.uk/teaching/3C11/graph.ps If you want to go for this direct cyclic representation, the interplay with lazy memo functions is also interesting: J. Hughes, Lazy memo-functions, Functional Programming Languages and Computer Architecture, J-P. Jouannaud ed., Springer Verlag, LNCS 201, 129-146, 1985. (lazy memo-functions remember previous input-output pairs based only on pointer-info, which is sufficient to write functions over cyclic structures that, instead of infinitely unrolling the cycles, produce cyclic results) (not online?-( Claus

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by johnmboma＠sify.com 08 Jul '03

08 Jul '03

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Representing cyclic data structures efficiently in Haskell
by Sarah Thompson 08 Jul '03

08 Jul '03

What is the best way to represent cyclic data structures in Haskell? Lists are trivial. Trees are trivial. But what if the thing you're trying to represent is shaped like an electronic circuit, with a (possibly large) number of nodes connected by multiple arrows to other nodes? In an imperative language like C++, I'd probably just model the circuit directly, with objects representing nodes and pointers representing links between nodes. A good way to model this in Haskell is less obvious, however. A simplistic approach would be to represent such a structure as a list of nodes, where each node contained a list of other connected nodes. Containing the actual nodes within the sub-lists probably isn't feasible -- some kind of reference would be necessary in order to get around chicken-and-egg problems with instantiating the data structure. But, in this case, it's not so easy to see how one could 'walk' the structure efficiently, because moving from node to node would involve quite horrible (possibly slow) lookups. In C++, I'd simply follow a pointer, which would be an extremely fast operation. I would also need to implement efficient indexes into the data structure -- flat searching will be too slow for non-trivial amounts of data. In C++, I'd implement one or more external (probably STL-based) indexes that point into the main data structure. How do people typically solve this problem at the moment? I know a few people out there have written hardware compilers in Haskell and similar languages, so there should be some experience of this in the community. I can probably work something out soon enough, but reinventing the wheel is never a great idea. Thank you in advance, -- ---------------------------------------------- / __ + / Sarah Thompson **** / / (_ _ _ _ |_ / * / / __)(_|| (_|| ) / sarah(a)telergy.com * / / + / http://findatlantis.com/ / ----------------------------------------------

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RE: Representing cyclic data structures efficiently in Haskell
by Hal Daume 07 Jul '03

07 Jul '03

>From reading your message, it seems you want a graph library of sorts. I've used FGL extensively and found it to be very nice. It even provides both a purely functional interface as well as an imperative-esque interface. It also comes with a bunch of standard algorithms built in, though I don't know enough about circuitry stuff to know whether those would be useful or not. If FGL seems like overkill to you, I have my own little mini graph library which basically looks like: - An ADT, Node: newtype Node = Node Int deriving ... - The 'Graph n e' data structure, containing: - a FiniteMap from n -> Node - an (growable?) array of type IOArray (Node,Node) e the array represents the edges in the top half (i do some index fiddling to get this efficient) and the finitemap represents the node labellings. It's a pretty stupid interface and works a lot better when the graphs are fixed size (that way you don't need to grow the array). But it works. -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-cafe-admin(a)haskell.org > [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Sarah Thompson > Sent: Monday, July 07, 2003 8:04 AM > To: haskell-cafe(a)haskell.org > Subject: Representing cyclic data structures efficiently in Haskell > > > What is the best way to represent cyclic data structures in Haskell? > > Lists are trivial. Trees are trivial. But what if the thing you're > trying to represent is shaped like an electronic circuit, with a > (possibly large) number of nodes connected by multiple arrows > to other > nodes? In an imperative language like C++, I'd probably just > model the > circuit directly, with objects representing nodes and pointers > representing links between nodes. A good way to model this in > Haskell is > less obvious, however. > > A simplistic approach would be to represent such a structure > as a list > of nodes, where each node contained a list of other connected nodes. > Containing the actual nodes within the sub-lists probably > isn't feasible > -- some kind of reference would be necessary in order to get around > chicken-and-egg problems with instantiating the data > structure. But, in > this case, it's not so easy to see how one could 'walk' the structure > efficiently, because moving from node to node would involve quite > horrible (possibly slow) lookups. In C++, I'd simply follow a > pointer, > which would be an extremely fast operation. > > I would also need to implement efficient indexes into the > data structure > -- flat searching will be too slow for non-trivial amounts of > data. In > C++, I'd implement one or more external (probably STL-based) indexes > that point into the main data structure. > > How do people typically solve this problem at the moment? I > know a few > people out there have written hardware compilers in Haskell > and similar > languages, so there should be some experience of this in the > community. > I can probably work something out soon enough, but > reinventing the wheel > is never a great idea. > > Thank you in advance, > > -- > ---------------------------------------------- > / __ + / Sarah Thompson **** / > / (_ _ _ _ |_ / * / > / __)(_|| (_|| ) / sarah(a)telergy.com * / > / + / http://findatlantis.com/ / > ---------------------------------------------- > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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