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RE: recip for Ratio
by Simon Peyton-Jones 03 Apr '01

03 Apr '01

Folks, I'm finally getting back to revising the Haskell 98 report, you'll be glad to hear. So I'll be emitting a few consultative messages. This is the first. Scott Turner points out: | In the Haskell 98 Library Report, recip is defined for the | Ratio type as: | recip (x:%y) = if x < 0 then (-y) :% (-x) else y :% x | | Is it intentional that | recip (0 % 1) | is not an error? Everywhere else the denominator is forced | to be positive. Infinity can be a useful number, if the type | is consistent about it. He's right. Everywhere else in the Ratio library we use '%' to construct rationals, a smart constructor that enforces the invariant that the denominator is positive and nonzero, and the numerator and denominator have no common divisor. It seems that the above specification for recip is guilty of premature optimisation. We should instead define it as recip (x :% y) = y % x leaving the smart constructor to enforce the invariant. An implementation might optimise this, knowing that x and y have no common divisor, but that is not the role of a specification. Any objections to this change? Simon

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A Hugs bytecode interpreter
by Tore Lund 03 Apr '01

03 Apr '01

Hugs is a neat system, but the very idea of compiling from source code each time you run a program makes Hugs unsuited for many purposes. So my question is, has anyone tried to rearrange the components of Hugs into a bytecode compiler plus runtime? And if not, maybe the Hugs maintainers had a word of advice on how to go about this? There is nch98, of course, but it is somewhat bulky for the purposes I have in mind. I am looking for something small that would compile to 16-bit code that would run in 64K code + 64K data. The raw material for such a system is probably present in Hugs, but what's the best way to extract it with a minimum of fuss? Grateful for any hint. -- Tore

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Follow up question
by MikeKn22＠aol.com 03 Apr '01

03 Apr '01

I am a bit confused by your response Mieszko. What I am looking to do is have a function say called ConvertString :: IO String -> String or something like that. Not sure if your examples achieve this. I want to take the IO String and be able to use it as a String somehow. My problem is I have a function for instance getWord :: String -> String getWord [] = [] getWord (x:xs) | elem x whitespace = [] | otherwise = x : getWord xs If I change the header to getWord :: IO String -> IO String I get errors of types not matching. Sorry if these are stupid questions but I am very confused by the difference of IO String and String, and want to know how you can use the IO String like a String. Thank you. Rob

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Hello List Have an IO String question
by MikeKn22＠aol.com 03 Apr '01

03 Apr '01

I am new to the list and to Haskell, and would appreciate any help anyone can give me on a problem I am having. I am using Hugs98 in case that matters. My question involved reading from a file. I used the line: readFile "c:/myfile.txt" To read my file and this works fine. I now want to take the contents of the file and use it in a function that takes in a String, but am getting an error since readFile returns an IO String. Is there a way to convert an IO String to a normal String? Thank you all. Rob

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Multiple modules in a file
by andrew＠andrewcooke.free-online.co.uk 02 Apr '01

02 Apr '01

Hi, I don't seem to be able have multiple modules in one file (at least for hugs and ghc). The gentle intro says that there is no restriction and I can't find any mention in the ghc jobs (apart from a recommendation to name a file after the module it contains to help make). It's not a desperate problem - I can split files - but I'd like to know if I'm doing something stupid, or if this is a real restriction. (I can see arguments why from an engineering/maintenance point of view, but one module is small and fits nicely with another; it also makes literate programming slightly more complicated). Thanks, Andrew -- http://www.andrewcooke.free-online.co.uk/index.html

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question class tree
by Maldonado Castiblanco Camilo Ernesto 02 Apr '01

02 Apr '01

I need a class tree for haskell. where can i find it?

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ffi & recursive data types
by dario.bahena＠correo.unam.mx 02 Apr '01

02 Apr '01

hi ... I'm new in ffi stuff and tools like greencard. Everything was working ok, until I wanted to: translate a haskell recursive data type(list,trees,etc) into a corresponding C type and back again. How can I do this? Thanks in advance saludos dario estepario ... ------------------------------------------------- Obtén tu correo en www.correo.unam.mx UNAMonos Comunicándonos

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modules Eigensystem and LinearAlgorithms
by Jan Skibinski 02 Apr '01

02 Apr '01

Experimenting recently with some quantum problems I've decided that it's time to add some long overdue algorithmic support for module QuantumVector. So here are two new modules added to our collection: http://www.numeric-quest.com/haskell/LinearAlgorithms.hs http://www.numeric-quest.com/haskell/Eigensystem.hs The former partially overlaps with module Orthogonals, but their most important algorithms are different, although both modules use lists for representation of matrices. This is still work in progress, but usable as it is for Hermitian matrices. Both modules are thoroughly documented, so here are just few buzzwords: Housholder reduction, complex reflection, Hessenberg matrix, similarity, triangularization, tridiagonalization, operators and maps. Jan

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[Fwd: beginners silly question : apologies and lib question]
by luc 01 Apr '01

01 Apr '01

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Re: Welcome to the "Haskell" mailing list
by luc 31 Mar '01

31 Mar '01

haskell-request(a)haskell.org wrote: > Welcome to the Haskell(a)haskell.org mailing list! > > To post to this list, send your email to: > > c > hi. im just a newcomer to haskell (www.taesch.com) quite impressed till now ! im trying to have haxml lib running with hugs and had the following problem : 1) under win, quite an issue to tun the cpp stuff. did it under linux, (for info) 2) then i had a "maybeA" which is unknown. some lib i had to add ? ("maybe" doesnt seems to work) 3) a module declare a "with" function, which hugs doenst seem to like . renamed it withlt for now. this make me thinks that this lib is probably not tested under hugs98. could someone confirm that ? as im too newbies, im maybe missing something obvious. Id better asking, i thought. thanks Luc

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