On Sunday 13 June 2010 22:12:22, Nathan Huesken wrote:
Hi,
I have often seen in haskell code a "!" in front of variables. In example:
data BrickState = Live | Dying !GLfloat deriving (Eq,Show)
I never read the meaning of the "!", what does it do?
'!' is a strictness annotation. In this case, it means that the field is strict, that is, it can't contain an unevaluated thunk (because GLfloat is a type that is either fully evaluated or not at all). When you call (Dying something), the something is 'seq'ed, in particular Dying undefined === undefined, while without the '!', Dying undefined would be a non-bottom value. You'll also see '!' in pattern matches, fun !x = whatever there it also means that the argument x must be evaluated (to weak head normal form), so if foo !x = 1 and bar x = 1 , we have bar undefined = 1, but foo undefined = Prelude.error undefined However, foo [undefined] = 1.
Thanks! Nathan