June 2007 - Haskell-Cafe

Re: Good article on why implementing a functional
by Monique Monteiro 07 Jun '07

07 Jun '07

I think that using an existing back end of an optimizing compiler such as GHC is more promising than targetting Java, C# or other imperative/OO language. GHC already does a lot of work to achieve a good performance. So, if you generate Java code, you'd have to do a lot of work to get good performance in the final code, 'cause of the huge semantic gap between Haskell and Java. Also, you don't have any control on the JVM internals, such as GC and other things. However, if you want portability or easy access to object-oriented APIs, generating Java code (or .NET code, for interoperability with other languages) should be your choice. Cheers, Monique > Gentlefolk: > > Help. I need support for a technical argument: why going to an intermediate > form for an existing functional back end like Haskell really, truly is > better for implementing a functional language than is going to an > intermediate form like the Java intermediate form and re-doing all the > various specialized mechanism needed to support a true lazy functional > language. In other words, I need a pithy paper or book chapter that will > convince someone unfamiliar with functional languages that Functional Back > Ends Really Are Different --- no, Really Truly Different. Something on the > order of Why Functional Programming Matters, but for the implementor of the > language, not the programmer. > > Any pointers to a good article or book chapter that might help? I keep > saying that this bookkeeping is a BIG task, but they keep saying they know > they can "handle" it. Pithy comments? Staff year estimates? Horror > stories? > > Thanks, anyone. > > Dave Barton > -- __________________________________________________________ Monique Monteiro, MSc MCP .NET Framework 2.0 / SCJP / IBM OOAD Project Manager Recife Microsoft Innovation Center +55 81 34198137 http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br MSN: monique_louise(a)msn.com

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faster factorial function via FFI?
by Dan Drake 07 Jun '07

07 Jun '07

Hello everyone, I have some code in which the bottleneck is the factorial function. I began by using a naive fac n = product [1..n] but it looks like there are faster ways to do it. I could try to look up those faster algorithms and implement them, but I'm guessing that using libgmp's factorial function is the best way to do it. I'm a poor programmer, so I don't trust myself to implement those algorithms properly. Hence I need to use FFI. ...but ghc already uses libgmp for Integers, so shouldn't I be able to somehow call libgmp's factorial function? Using regular FFI stuff, it looks like this might get complicated, since the function doesn't return one of the usual foreign types, so I'd need to mess around to get the result into an Integer, which is what I need. Is there an easy way to do this? It might also be faster to use a lookup table, since most of the time I'll be taking factorials of relatively small numbers. Thanks, Dan -- Ceci n'est pas une .signature.

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Finding points contained within a convex hull.
by Daniel McAllansmith 07 Jun '07

07 Jun '07

Hello. I've got a system of linear inequalities representing half-spaces. The half-spaces may or may not form a convex hull. I need to find the integral coordinates that are contained within the convex hull, if there is one. For example, given 0 <= x <= 4 0 <= y <= 3 0 <= 2x - y 0 <= 1.2y - x I want the following (x,y) coordinates [(0,0),(1,1),(1,2),(2,2),(2,3),(3,3)] Anybody have any suggestions, or libraries, for solving this in many dimensions and equations? Thanks Daniel

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Compiling packages for GHCi
by Grzegorz 07 Jun '07

07 Jun '07

Hi, I'm having problems using a package which links in foreign libraries from GHCi. I use a .cabal file to build the package and have the following option there: extra-libraries: stdc++ maxent z m gfortran m gcc_s After installation, it works fine when I compile code using that package. However, when I try it from GHCi I get: Loading package binary-0.3 ... linking ... done. Loading package filepath-1.0 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package mtl-1.0.1 ... linking ... done. Loading package parsec-2.0 ... linking ... done. Loading package gramlab-lib-0.1 ... linking ... done. Loading package maxent-0.0 ... can't load .so/.DLL for: maxent (libmaxent.so: cannot open shared object file: No such file or directory) I don't have a libmaxent.so, the maxent library in at /usr/local/lib/libmaxent.a Can I somhow use my package with GHCi? Thanks!, Grzegorz

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Newbie Q: Monad 'fail' and 'error'
by Dmitri O.Kondratiev 06 Jun '07

06 Jun '07

Monad class contains declaration *fail* :: String -> m a and provides default implementation for 'fail' as: fail s = error s On the other hand Prelude defines: * error* :: String -> a which stops execution and displays an error message. Questions: 1) What value and type 'error' actually returns in: error "some message" ? 2) How declaration String -> m a matches with String -> a ? 3) In Maybe monad: fail = Nothing When and how 'fail' is used in Maybe monad? Thanks! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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Parsec problem
by Neil Mitchell 06 Jun '07

06 Jun '07

Hi, I'm using parsec to parse something which is either a "name" or a "type". The particular test string I'm using is a type, but isn't a name. I want things to default to "name" before "type". Some examples of the parsec function, and the result when applied to a test string: > parsecQuery = do spaces ; types Right Query {scope = [], names = [], typeSig = Just [a], items = [], flags = []} > parsecQuery = do spaces ; names Left (line 1, column 1): unexpected "[" expecting white space, "--", "/", "(", letter, "::" or end of input > parsecQuery = do spaces ; try names <|> types Left (line 1, column 1): unexpected "[" expecting white space, "--", "/", "(", letter, "::" or end of input I would have expected try names <|> types to return a Right (since types matches), but it doesn't. I assumed this would be a property of Parsec, but it doesn't appear to hold. Can anyone shed any light? Thanks Neil

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FP v. OOP
by PR Stanley 06 Jun '07

06 Jun '07

Hi BAE Systems which specialises in military technology is looking for programmers who have experience in C, C++ and Java and UML. Where soundness and completeness but especially soundness is of the essence the OOP framework is far from ideal. The FP paradigm, on the other hand, offers the perfect alternative. Yet, its application is for the most part limited to the realms of academic research. Discuss. Regards, Paul

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Use of Data.Generics.everywhere'
by Neil Mitchell 06 Jun '07

06 Jun '07

Hi, I'm looking into SYB, and I'm trying to find examples where people have used everywhere' (note: not everywhere, but with a \prime) - unfortunately I can't find any. I've tried Google Code Search, and it comes up with one rather trivial example which could equally have been everywhere. The original SYB paper doesn't give an example of everywhere' in use. So is everywhere' useful for anything? Has anyone used everywhere' in practice? Thanks Neil

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HList questions
by Marc Weber 06 Jun '07

06 Jun '07

Do you know what a type indexed coproduct is ? (TIC.hs from HList) What is the purpose of this module? Why the Proxy type has been introduced? Can you think of a short application? Marc

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standard function
by Steffen Mazanek 06 Jun '07

06 Jun '07

Hello, is there a function f::[a->b]->a->[b] in the libraries? Couldn't find one using hoogle although this seems to be quite a common thing... Steffen

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