[Haskell-beginners] Meaning of '!' in record defintion? UNPACK?

[Daniel Fischer]

Am Donnerstag, 26. Februar 2009 12:07 schrieb Colin Paul Adams:
> >>>>> "Thomas" == Thomas Davie <tom.davie at gmail.com> writes:
>
>     Thomas> The {-# UNPACK #-} tells the compiler that it can unpack
>     Thomas> the Int – meaning that a Position will be neatly packed
>     Thomas> into 12 bytes.
>
> What would be the difference if there was no UNPACK pragma?

Section 8.12.10 of the users' guide says:
"The UNPACK indicates to the compiler that it should unpack the contents of a 
constructor field into the constructor itself, removing a level of 
indirection."

It has more, and also says when it's not a good idea to use it.

Int is defined as

data Int = I# Int#

where Int# is a raw machine int. If you use the {-# UNPACK #-} pragma, you 
tell GHC that you'd very much like Position to be stored as constructor + 
three contiguous raw machine integers. Mostly, it will do so.
If you don't use the pragma, i.e. have

    data Position = 
           Position { posOffset :: !Int
                    , posRow :: !Int
                    , posColumn :: !Int
                    }

, GHC may or may not decide to store it thus, with -O2 it's not too unlikely, 
I think. But it's also not unlikely that it will be stored as constructor + 
three pointers to three evaluated Ints, which is much better than pointers to 
thunks, but not as good as having the raw values directly by the constructor.

Cheers,
Daniel