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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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classes and instances
by Russo, Tom 04 Oct '01

04 Oct '01

Hello, I'm learning haskell and I'm having some trouble with type classes. I understand them conceptually, but when I try to create an instance I run into problems. As a simple example, I create a class called ToInt: > class ToCh a where > int :: a -> Int Then I create an instance: > instance ToInt Int where: > int n = n However, when I try to use the function int :: Int -> Int at the hugs prompt, I get the following error message: 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? thanks _t

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Re: Small confusion with monads
by Ashley Yakeley 04 Oct '01

04 Oct '01

At 2001-10-04 02:02, Dmitry Olyenyov wrote: >I have the list ["file1","file2","file3"] and I need the >function to return me the list of contents of these files. > >contents :: [String] -> [String] That cannot be safely done with that type. A "pure" function like that is not allowed to do "imperative" stuff such as reading files. -- Ashley Yakeley, Seattle WA

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