stupid strictness question

[Hal Daume III]

i know i know, this has been asked a million times, but i was searching
through the archives with no success, so i figured i'd bug you guys.

according to the report:

  A declaration of the form 

    data cx => T u1 ... uk = ... | K s1 ... sn | ... 

  where each si is either of the form ! ti or ti, replaces every 
  occurance of K in an expression by 

    (\x1 ... xn -> ( ((K op1 x1) op2 x2) ... ) opn xn) 

  where opi is the lazy apply function $ if si is of the form ti, 
  and opi is the strict apply function $! (see Section 6.2) if si 
  is of the form !ti. Pattern matching on K is not affected by
  strictness flags. 

so.  we define:

> data L = L  Int deriving Show
> data S = S !Int deriving Show

and, as expected, we get:

*Strict> L undefined
L *** Exception: Prelude.undefined
*Strict> L $! undefined
*** Exception: Prelude.undefined
*Strict> S undefined
*** Exception: Prelude.undefined

Now, we define:

> data SMaybe a = SNothing | SJust !a  deriving Show

Now, we run:

*Strict> Just (undefined::Int)
Just *** Exception: Prelude.undefined
*Strict> Just $! (undefined::Int)
*** Exception: Prelude.undefined
*Strict> SJust $! (undefined::Int)
*** Exception: Prelude.undefined
*Strict> SJust (undefined::Int)
SJust *** Exception: Prelude.undefined

I can't figure out why this last one is different from the one before it,
or the one before that.  Interestingly, Hugs disagrees (the previous was
with ghc 5.04.1):

Strict> Just (undefined::Int)
Just 
Program error: {undefined}
Strict> Just $! (undefined::Int)
Program error: {undefined}
Strict> SJust $! (undefined::Int)
Program error: {undefined}
Strict> SJust (undefined::Int)
Program error: {undefined}

Which is what I expected.  Can someone clarify here?

 - Hal