A simple example to help understand the difference

data NoStrict a = NoStrict a deriving Show data Strict a = Strict !a deriving Show

ns1 = NoStrict () ns2 = NoStrict undefined ns3 = undefined

nf1 (NoStrict ()) = "ok" nf2 (NoStrict _) = "ok"

s1 = Strict () s2 = Strict undefined s3 = undefined

f1 (Strict ()) = "ok" f2 (Strict _) = "ok"

The difference between these: *Strict> nf2 ns2 "ok" *Strict> f2 s2 "*** Exception: Prelude.undefined s2 and s3 are both the same (undefined), while ns2 and ns3 are different. As to why you'd want that behavior? Here are a couple of reasons: 1) You can avoid hiding exceptions in strict data structures; if the top level evaluates, you know the entire structure is valid. 2) With -funbox-strict-fields, the compiler can remove a level of indirection; the performance difference between a structure that contains three machine words and a structure that contains three pointers to boxed integers is pretty significant. On the other hand, you lose the benefits of laziness; I find most "structure-like" data structures work better strict, but that "list-like" data structures definitely gain an advantage from laziness. -- ryan 2008/7/8 Daryoush Mehrtash <dmehrtash@gmail.com>:

Can some one explain what the !a does in this:

data Color3 a = Color3 !a !a !a

http://cvs.haskell.org/Hugs/pages/libraries/OpenGL/Graphics-Rendering-OpenGL...

Thanks,

Daryoush

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