March 2010 - Haskell-Cafe

Monad laws
by David Sabel 03 Mar '10

03 Mar '10

Hi, when checking the first monad law (left unit) for the IO-monad (and also for the ST monad): return a >>= f ≡ f a I figured out that there is the "distinguishing" context (seq [] True) which falsifies the law for a and f defined below > let a = True > let f = \x -> (undefined::IO Bool) > seq (return a >>= f) True True > seq (f a) True *** Exception: Prelude.undefined Is there a side-condition of the law I missed? Regards, David

5 7

using haskell to serve with apache
by brad clawsie 03 Mar '10

03 Mar '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 hi i'm currently doing some work on a personal site and was considering giving a haskell web tool a spin. i have a fairly complex apache configuration, so i don't want to try to replace it with a native haskell server, but instead use haskell via an interface to apache (i.e. something like wsgi, plack, etc). some searching shows hyena and yesod in various states of development, but i can't figure out if they are appropriate or ready for use. anyone have experiences using haskell behind apache? should i just try out something based on fastcgi? any anecdotes welcome thanks brad -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (FreeBSD) iEYEARECAAYFAkuKoJEACgkQxRg3RkRK91PVJwCdFD4dS47KjJuMcNVZmPkYXKyU ItsAnRoYBE39XtXyXDluh+pHSxzP+Qcd =91Ql -----END PGP SIGNATURE-----

5 4

references for compiler optimizations for functional languages
by Michael Vanier 03 Mar '10

03 Mar '10

Hi everyone, I'm interested in collecting good references for compiler optimizations for functional languages (lazy, strict, statically-typed or not). Any suggestions? Thanks in advance, Mike

4 4

new syntax highlighting library (lexers wanted!)
by John MacFarlane 03 Mar '10

03 Mar '10

Cafe: I wanted to let you know about a new syntax highlighting library I've been working on, called 'illuminate'. The (BSD3 licensed) code is on github: http://github.com/jgm/illuminate illuminate is meant eventually to replace my other highlighting library, highlighting-kate, which is too big, slow, and memory-hungry. In my benchmarks, illuminate is around 70X faster than highlighting-kate and uses half the memory. It can produce output in ANSI, HTML, XHTML, and LaTeX. Highlighting styles can be defined independently of any output format. So far I've written lexers for haskell, literate haskell, cabal, alex, c, c++, c#, python, ruby, java, html, rhtml, xml, javascript, css, tex, and bibtex. Want to highlight a language not on this list? Why not help out by writing a lexer? (Study the lexer for a similar language to get started; if you aren't familiar with Alex, you may want to look at the Alex User Guide too. And let me know if you have any questions.) John

1 0

FFI and lists?
by Kimberly Wallmark 02 Mar '10

02 Mar '10

I'm working with FFI to make a Haskell DLL that's called by C# code. I understand how to share simple types. I've found reasonable documentation for struct-equivalents. Is there a clean way to share lists, or should I make a linked-list struct and do it manually? In other words, if I have adder :: CInt -> CInt -> CInt adder x y = (x+y) then I can use foreign export stdcall adder :: CInt -> CInt -> CInt and on the other side it behaves like int adder(int a, int b); I'd like to know what to do if I have squares :: [CInt] -> [CInt] squares = map (^2) Advice? --Kim

2 1

Evolving Faster Haskell Programs (now with LLVM!)
by Don Stewart 02 Mar '10

02 Mar '10

http://donsbot.wordpress.com/2010/03/01/evolving-faster-haskell-programs-no… … In which I use genetic algorithms to search for optimal LLVM optimizer passes to make Haskell programs faster … LLVM + GHC is a lot of fun. -- Don

2 2

Fwd: [Haskell-cafe] minimal interface
by Sean McLaughlin 02 Mar '10

02 Mar '10

Hi Don, This doesn't tell me what is extraneous in the exports. For instance If I have two modules A and Main module A (f, g) where f :: a -> a f = id g :: a -> a g = id module Main (main) where import qualified A main :: IO () main = do let a = A.f 7 putStrLn $ show a I'd like something to tell me that A.g is not used in the program. Thanks, Sean On Tue, Mar 2, 2010 at 4:28 PM, Don Stewart <dons(a)galois.com> wrote: > seanmcl: > > Hello, > > > > I have a midsize program, and would like to simplify the module > interfaces > > for my own sanity. I know there are tools to check for extraneous > imports, but > > is there something similar for exports? For instance, if module A > exports f, > > but A.f is never used in my compiled program, I would like to be warned > of this > > fact. > > ghc -ddump-minimal-imports > > I think that's the flag. > > -- Dump >

1 0

A few ideas about FRP and arbitrary access in time
by Phil Jones 02 Mar '10

02 Mar '10

...Are hereby presented at: http://www.ee.bgu.ac.il/~noamle/_downloads/gaccum.pdf Comments are more than welcome. (P.S Thanks to a whole bunch of people at #haskell for educating me about this, but most notably Conal Elliott)

1 0

lhs2TeX \eval{} problem
by Tom Lokhorst 02 Mar '10

02 Mar '10

Hello, I'm having a problem with lhs2Tex and \eval{}. It doesn't work. I have the following in a file test.lhs: -------------------------------- %include polycode.fmt One %option ghci Two \eval{4} Three -------------------------------- When I try to run this file through lhs2Tex, it crashes: > lhs2Tex test.lhs ... polycode junk removed ... One %option ghci Two lhs2TeX: fd:7: hGetLine: end of file -------------------------------- Has someone encountered this problem before, or knows how to solve this? - Tom Lokhorst

2 4

Where is HEAD?
by Louis Wasserman 02 Mar '10

02 Mar '10

Where is GHC 6.13 head? I can find sources of 6.13, and darcs for 6.12, but not darcs for 6.13...I'm trying to play with the LLVM backend, and this is the one question it seems to presuppose that you know the answer to. Heh. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 1