[web-devel] Yesod gotcha (kind of, not sure, it got me good though)

Nubis nubis at woobiz.com.ar
Wed Nov 3 15:26:48 CET 2010

Hi guys,
I'm fully getting into haskell thanks to the yesod framework. I've been
pulling my hair with a bug as silly as they come, and for so long that I
cannot tell it without feeling embarrassed. (I'm working on the
haskellers.com website code, just to give you some extra context)

I was getting a 500 in one of my controllers, and I first though there was
some non existing id being referenced from a model, I set up the haskell
interactive debugger and launched my yesod project from ghci, single stepped
and everything looked just fine.
The actual error displayed was:"Internal Server Error" "A LineSingle must
have nested content"

The issue was finally that a cassius file had some trailing garbage (that
went unnoticed by me, probably added accidentally when passing out over my

My main issue with this is that I think of cassius files as just css, I
wouldn't expect to get an error in my page just for a broken css file. I
would expect seeing broken styles, or even better, the program not compiling
at all. I searched for the LineSingle error message and seems to be related
to hamlet, I interpreted that as a pointer but was in the wrong direction.

I know this won't happen to me again, but just wanted to throw it out here
to see if you consider it a gotcha.

cheers :)
----nubis :)
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