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

[Colin Paul Adams]

>>>>> "Thomas" == Thomas Davie <tom.davie at gmail.com> writes:

    Thomas> On 26 Feb 2009, at 12:07, Colin Paul Adams wrote:

>>>>>> "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?

    Thomas> The tag in the Ints would be included in the structure
    Thomas> created, with UNPACK, they would not.

What is the tag?
-- 
Colin Adams
Preston Lancashire