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Re: Hashtable ADT
by John Hughes 06 Oct '01

06 Oct '01

> As I understand from the concepts of Functional Programming, it is not > possible to implement a Hashtable ADT in Haskell language, where one can > insert, and access values in O(1) complexity. It has to be implemented > with an external language. I don't know if it can be done in standard Haskell 98, but it can certainly be done using extensions provided by most or all implementations (IORef, IOArray). There is no need of using an external language, although it will not fit well the functional style. Better is to use the ST monad (which provides creation, reading, and writing of arrays and references in constant time). Side-effects in the ST monad can be encapsulated using runST :: (forall s. ST s a) -> a You give it a computation using updateable state as an argument, a "state thread", and it gives you the result as a pure value. The "forall s" is a typing trick which prevents an encapsulated state thread from referring to references belonging to a different state thread. Using this you can, for example, create a function share :: Hashable a => [a] -> [a] which takes a list of hashable values, and returns an equal list, in which equal values are always represented by the same pointer. Internally, there's a hash table which lets you map each element of the input to a unique representative. You could compose share with a lexical analyser, for example, to share all the copies of the same identifier. Encapsulated stateful operations like this fit very nicely with the functional style! Take a look at "State in Haskell", John Launchbury and Simon Peyton Jones, http://www.cse.ogi.edu/~jl/Papers/stateThreads.ps which explains all of this at length. John Hughes

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Re: Hashtable ADT
by Marcin 'Qrczak' Kowalczyk 06 Oct '01

06 Oct '01

Sat, 6 Oct 2001 14:10:05 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > As I understand from the concepts of Functional Programming, it is not > possible to implement a Hashtable ADT in Haskell language, where one can > insert, and access values in O(1) complexity. It has to be implemented > with an external language. I don't know if it can be done in standard Haskell 98, but it can certainly be done using extensions provided by most or all implementations (IORef, IOArray). There is no need of using an external language, although it will not fit well the functional style. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Hashtable ADT
by Cagdas Ozgenc 06 Oct '01

06 Oct '01

Hi, As I understand from the concepts of Functional Programming, it is not possible to implement a Hashtable ADT in Haskell language, where one can insert, and access values in O(1) complexity. It has to be implemented with an external language. Is my argument correct? Thanks for taking time.

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Re: solutions to excercise
by S.J.Thompson 06 Oct '01

06 Oct '01

Sorry about this, but I have no online solutions. Simon

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solutions to excercise
by riz er 06 Oct '01

06 Oct '01

hi iam a Computer Science student who is currently studying Haskell programming using the "Craft of Functional Programming second edition" by Simon Thompson. And i was wondering if you have the solutions to the excersise to the book. Becasue the book does not have them..a few students are having difficulty using the book because the that reason. If possible to send a url site where i can download them or a alternative way .. Thank you _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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Re: newtype | data
by Marcin 'Qrczak' Kowalczyk 05 Oct '01

05 Oct '01

05 Oct 2001 11:53:05 -0700, Carl R. Witty <cwitty(a)newtonlabs.com> pisze: > Data> (T2 undefined) `seq` () > *** Exception: Prelude.undefined > Data> (T3 undefined) `seq` () > *** Exception: Prelude.undefined > > I can't think of a semantic difference between newtype and data > with a single unary strict constructor; It could indeed be represented in the same way, but they behave differently in pattern matching: case undefined of T _ -> () is () in the case of newtype and undefined in the case of strict data. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: newtype | data
by cwitty＠newtonlabs.com 05 Oct '01

05 Oct '01

Mark Carroll <mark(a)chaos.x-philes.com> writes: > Why does "newtype" exist, instead of letting people always use "data" and > still get maximum efficiency? After all, surely the implementation is an > implementation detail - a compiler could see the use of "data" with a > unary constructor and implement it as it does "newtype", instead of making > the programmer worry about how things are actually represented? > > I'm obviously missing something obvious here; I'm hoping to learn what. > (-: newtype is strict; data is not. Given > data T1 = T1 Int > newtype T2 = T2 Int > data T3 = T3 !Int you get the following results. Data> (T1 undefined) `seq` () () Data> (T2 undefined) `seq` () *** Exception: Prelude.undefined Data> (T3 undefined) `seq` () *** Exception: Prelude.undefined I can't think of a semantic difference between newtype and data with a single unary strict constructor; I suppose it would be possible to remove newtype from the language and make people declare things like T3 instead. Carl Witty

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newtype | data
by Mark Carroll 05 Oct '01

05 Oct '01

Why does "newtype" exist, instead of letting people always use "data" and still get maximum efficiency? After all, surely the implementation is an implementation detail - a compiler could see the use of "data" with a unary constructor and implement it as it does "newtype", instead of making the programmer worry about how things are actually represented? I'm obviously missing something obvious here; I'm hoping to learn what. (-: -- Mark

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broken link
by Jorge Adriano 05 Oct '01

05 Oct '01

Just thought I'd report that the PolyGP link: http://www.cs.ucl.ac.uk/staff/T.Yu/research.html at the 'Haskell in Practice' section (http://www.haskell.org/practice.html) seems to be broken. J.A.

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Re: classes and instances
by Ashley Yakeley 04 Oct '01

04 Oct '01

At 2001-10-04 08:31, Russo, Tom wrote: >Main> int 5 >ERROR - Unresolved overloading >*** Type : (Num a, ToInt a) => Int >*** Expression : int 5 > >Can someone explain what the problem is, and how to interpret the error >message I'm getting? Oh, 5 isn't necessarily an Int. It's of type "Num a => a", which is more general. You can restrict it to Int with "(5 :: Int)". -- Ashley Yakeley, Seattle WA

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