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

[Felipe Lessa]

On Thu, Feb 26, 2009 at 8:07 AM, Colin Paul Adams
<colin at colina.demon.co.uk> 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?

For example, if you write

valid :: Position -> Bool
valid (Position o r c) = o < r && c < 80

It is just an example, not necessarily meaningful :). If you use the
UNPACK pragmas, then internally the compiler will create something
like

valid :: Int# -> Int# -> Int# -> Bool
valid o r c = o <# r && c <# 80

Basically the three integers will be passed on registers. Without the
UNPACK, they would be passed on the heap. Without the strictness
annotation, a pointer on the heap would be passed to them (i.e. a
pointer on the register would point to the Position structure which
would have a pointer to the integer, not very nice).

HTH,

PS: I'm not sure if the unboxed operations I wrote are correct, but
you get the idea :).

-- 
Felipe.