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