[web-devel] Strange HAppS problem (tutorial)

Marc Weber marco-oweber at gmx.de
Mon May 7 21:45:16 EDT 2007

On Mon, May 07, 2007 at 03:33:39PM -0500, John Melesky wrote:
> So, i'm working my way through the HAppS tutorial ( http:// 
> www.haskell.org/haskellwiki/HAppS_tutorial ), and i get to section 3.2.
> I type in the code, it builds fine, but when i run it, i get the  
> following error:
>     hello: user error (decodeStringM: parsing length field failed @ "")
> However (and this is the strange part), if i just runghc the .hs  
> file, it all works fine.
> This is running ghc 6.6, darwinports version, on intel OSX 10.4.9,  
> however i've verified the same behavior on ghc, Audrey Tang's bundle,  
> G4 OSX 10.4.9 (for whatever that's worth).
> Anyone have any ideas? Anyone run into this before?
> -johnnnnnnn

Hi John.

I'm not sure what's happening in your case.
The HAppS applications try to read some state when starting. If there is
a syntax error (which shouldn't occur because read/ show isntances are
fine in most cases).. you'll get a error message like that.

You can try investigating wether this is the cause by getting to know
which files are actually beeing read and parsed.
This can be done by using strace ./yourexecutable &> out and some
This may also help you getting to know wether there are some differences
when invoking ghci/ghc..

The last time I did try HAppS those state files where located somewhere
near the current directory. Thus I propose moving your executable to
/tmp and try again ?

You may also try the HAppS mailing list http://groups.google.com/group/happs


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