Henk-Jan van Tuly is correct. Oops! Sorry I was mis-informed. By the way, there is an excellent post about this topic on StackOverflow.com. It covers, seq, weak-head-normal-form, normal form, and thunks. See here:
http://stackoverflow.com/questions/6872898/haskell-what-is-weak-head-normal-form/6889335#6889335 

Hope that makes up for my earlier comment.

Tim


On Thu, Apr 19, 2012 at 2:31 AM, Henk-Jan van Tuyl <hjgtuyl@chello.nl> wrote:
On Wed, 18 Apr 2012 16:45:05 +0200, Brent Yorgey <byorgey@seas.upenn.edu> wrote:

On Tue, Apr 17, 2012 at 02:41:15PM -0700, Tim Perry wrote:
seq evaluates to Weak Head Normal Form (WHNF). WHNF is the first
contructor. So your use of seq only evaluates the first number and the
cons. I.E., it evaluates to:
s:(Thunk)

Actually, it doesn't even force the first number.  You just get

Thunk : Thunk


A nice way to demonstrate this, is the following GHCi session:
 Prelude> undefined `seq` print "OK"
 *** Exception: Prelude.undefined
 Prelude> [undefined] `seq` print "OK"
 "OK"


Regards,
Henk-Jan van Tuyl


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