October 2008 - Haskell-Cafe

Simple Table Update
by Paul Keir 09 Oct '08

09 Oct '08

Hi, I'd like to create a new list based on an original list, using information from a second (symbol) list. That second list should be updated as each element in the new list is added. I've been using map a lot, but that's not an option here, and I'm having trouble obtaining a good recursive structure for my function. The minimal "fixpus" function below shows the problem. I have "sts" on both sides of the ":" list construct, but I'd like the second "sts" to be a version modified by the where (singular) function, "fixpu". fixpus :: [ProgUnit] -> [ProgUnitSymbolTable] -> [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = fixpu pu sts : fixpus pus sts where fixpu pu sts = pu (Below) I tried making "fixpu" return a tuple, and then use "fst", "snd" and "let", but I think it looks strange. I know it's quite basic, but I'd like a strong foundation for what's likely to become a medium-scale project. Can anyone offer advice? fixpus :: [ProgUnit] -> [ProgUnitSymbolTable] -> [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = let a = (fixpu pu sts) in fst a : fixpus pus (snd a) where fixpu pu sts = (pu,sts ++ [("",[])]) Regards, Paul

2 4

parsec 3 & parsec 2
by Jason Dusek 09 Oct '08

09 Oct '08

I recently took a look at the new Yi, and it looks neat but I am hesitant to install it -- it require Parsec 3. What will happen to my system, carefully maintained, if I unleash this new Parsec on my programs and the programs of others? Can I expect total breakage, coexistence or...? -- _jsn

4 4

Simplifying a IsFunction type class using type equality constraints
by Corey O'Connor 09 Oct '08

09 Oct '08

I recently had a need to use the IsFunction typeclass described by Oleg here: http://okmij.org/ftp/Haskell/isFunction.lhs and am wondering if the use of the TypeCast class can be correctly replaced by a type equality constraint. The IsFunction and TypeCast classes were defined as: > data HTrue > data HFalse > > class IsFunction a b | a -> b > instance TypeCast f HTrue => IsFunction (x->y) f > instance TypeCast f HFalse => IsFunction a f > > -- literally lifted from the HList library > class TypeCast a b | a -> b, b->a where typeCast :: a -> b > class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b > class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b > instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x > instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' > instance TypeCast'' () a a where typeCast'' _ x = x I found the use of TypeCast in the IsFunction could be replaced by a type family: > class IsFunction a b | a -> b > instance (f ~ TTrue) => IsFunction (x->y) f > instance (f ~ TFalse) => IsFunction a f Which, to me, is easier to understand and appears to function the same. The type equality is a stronger (?) constraint than the TypeCast class, but for the case of IsFunction the use of type equality is correct. Am I correct? Is the second definition of IsFunction actually equivalent to the original? Cheers, -Corey O'Connor

3 2

ghc 6.10 and parsec
by Jason Dusek 08 Oct '08

08 Oct '08

Which Parsec is GHC 6.10 going to ship with -- Parsec 2 or Parsec 3? -- _jsn

3 3

Re: [Haskell-cafe] synchronous channels in STM
by Levi Stephen 08 Oct '08

08 Oct '08

On Thu, Oct 9, 2008 at 9:40 AM, roger peppe <rogpeppe(a)gmail.com> wrote: > I was wondering if it was possible to implement synchronous channels > within STM. In particular, I'd like to have CSP-like send and recv > primitives > on a channel that each block until the other side arrives to complete > the transaction. I did experiment with the actors model (in theory a superset of CSP I believe) in haskell at one stage. Hopefully it will be of some use/interest to you. http://lstephen.wordpress.com/2007/09/08/the-actors-model-and-haskell/ Levi

1 0

ANNOUNCE: SourceGraph-0.1 and Graphalyze-0.3
by Ivan Miljenovic 08 Oct '08

08 Oct '08

I've now uploaded my SourceGraph program to Hackage [1]. It's rather simple at the moment, but if you pass in the .cabal file as a parameter (e.g. run it as "SourceGraph Foo.cabal"), it will create in the same directory as the .cabal file a Directory called "SourceGraph" that contains an html report of some basic graph-theoretic analysis of your code. The output format isn't ideal, but it should serve it's purpose for now (I'll fix it up and actually make it usable once my Thesis has been handed in). What I'd appreciate if people could try it out and tell me if there's any code, etc. that it can't parse. At the moment, it ignores all Data-based functions (e.g. class and instance declarations as well as record functions) and only looks at "stand-alone" functions (i.e. normal functions). SourceGraph requires version 0.3 of my Graphalyze library (version 0.2 added the reports in, but had some bugs that 0.3 fixes). [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SourceGraph -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

6 9

[hmatrix] build error
by Xiao-Yong Jin 08 Oct '08

08 Oct '08

Hi, looks like I hit a bug, but I'm not sure which software it belongs to, gcc, ghc or atlas? ---------- error doing "runhaskell Setup build" ---------- Preprocessing library hmatrix-0.4.1.0... /usr/bin/ld: dist/build/Numeric/GSL/Special/Internal_hsc_make: hidden symbol `__powidf2' in /usr/lib/gcc/x86_64-unknown-linux-gnu/4.3.2/libgcc.a(_powidf2.o) isreferenced by DSO /usr/bin/ld: final link failed: Nonrepresentable section on output collect2: ld returned 1 exit status linking dist/build/Numeric/GSL/Special/Internal_hsc_make.o failed command was: /usr/bin/ghc -optl-lgsl -optl-llapack dist/build/Numeric/GSL/Special/Internal_hsc_make.o -o dist/build/Numeric/GSL/Special/Internal_hsc_make ---------- error doing "runhaskell Setup build" ---------- I believe `__powidf2' is a internal symbol in gcc and it is hidden. The system is archlinux x86_64 and the software I used are atlas-lapack 3.8.2-1 gcc 4.3.2-1 Using built-in specs. Target: x86_64-unknown-linux-gnu Configured with: ../configure --prefix=/usr --enable-shared --enable-languages=c,c++,fortran,objc,obj-c++,treelang --enable-threads=posix --mandir=/usr/share/man --infodir=/usr/share/info --enable-__cxa_atexit --disable-multilib --libdir=/usr/lib --libexecdir=/usr/lib --enable-clocale=gnu --disable-libstdcxx-pch --with-tune=generic Thread model: posix gcc version 4.3.2 (GCC) ghc 6.8.2-2 [("Project name","The Glorious Glasgow Haskell Compilation System") ,("Project version","6.8.2") ,("Booter version","6.8.2") ,("Stage","2") ,("Interface file version","6") ,("Have interpreter","YES") ,("Object splitting","YES") ,("Have native code generator","YES") ,("Support SMP","YES") ,("Unregisterised","NO") ,("Tables next to code","YES") ,("Win32 DLLs","") ,("RTS ways"," debug thr thr_p thr_debug") ,("Leading underscore","NO") ] I googled but no useful information. Best, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

6 8

Bluetooth libraries
by Creighton Hogg 08 Oct '08

08 Oct '08

Hello, In the interest of not duplicating any effort, I wanted to ask if anyone was aware of/working on libraries for doing bluetooth programming from Haskell. I didn't find any on hackage nor did The Google indicate that this was a filled niche. Cheers, Creighton

2 1

Re: [Haskell] Catching error / making library functions monadic (in failure)
by Duncan Coutts 08 Oct '08

08 Oct '08

On Wed, 2008-10-08 at 18:07 +0200, Philip K.F. Hölzenspies wrote: > I think this is the core problem I have. Really, error should be implemented > as fail, rather than vice versa. I see why this doesn't work, because it > would type > > error :: Monad m => String -> m a > > even though the whole idea is that it's typed 'a'. Right, that's exactly why it would not work to give error that type. However error should only be being used for programming errors, not for expected runtime errors. > Nonetheless, I would like > it if either IO wasn't the only monad allowing catching, or standard > functions not only being available with 'error' stuff (like !!, head, etc.). Those functions do have safe counterparts for when you cannot guarantee the pre-condition. It typically involves pattern matching, eg instead of head use: case foo of [] -> (x:xs) -> Or for !! you could use take or splitAt and pattern matching. If you want more exact analogues of head, !! etc then see Neil's Safe library. > > If you can report them we can fix them. > > I think I spoke too soon. The error occur in other libraries' functions, so I > unjustly smudged your wonderful gtk2hs work (for which, btw: thanks very, > very much). ok :-) > To be honest; when I write a library, I also don't assume monadic > contexts for stuff that I do in purely functional terms. Sometimes errors > occur only in deeply nested code and "just putting in error" is the easy way > out. Especially, libraries that are not intended for "industrial strength" > production code seem to be prone to this type of coding, when they're not > already monadic. I think it's not as bad as you suggest. In haskell, error is like unchecked exceptions or assertions in Java. They are for cases that really indicate a programmer error, not for bad input or IO failures. Even these kinds of errors can be caught and contained in a sensible way using exception handlers in IO. For example supposing some web server page handler had some programming error and called a function in a way that violated the pre-condition and called error. That need not kill the entire server. The page handler just needs to catch all exceptions and return a 500 internal server error and write the error out to the logs. So it's actually a reasonably robust way of dealing with the inevitable programmer errors. Duncan

1 0

monadic parser with Happy and Alex
by Manlio Perillo 08 Oct '08

08 Oct '08

Hi. I have completed a draft of a CSS lexer, using Alex. http://hg.mperillo.ath.cx/haskell/webtools/file/tip/src/CSS/Lexer.x The lexer use the posn wrapper. Now I'm starting to write the parser with Happy, however for the final product I would like to: 1) Be able to do I/O in the lexer, for stylesheets inclusion (@import rule) 2) be able to keep state in the parser (or lexer?), for character transcoding (@charset rule) This should be possible with Happy (and there are some example), however I don't find examples that make use of a lexer written with Alex. Should I write a lexer using only the Alex basic interface (without wrappers)? Thanks Manlio Perillo

3 4