[Haskell-beginners] Meaning of the ! operator

[Daniel Fischer]

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