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

On 26 Feb 2009, at 11:42, Erik de Castro Lopo wrote:

Hi all,

I'm looking at some code that defines a record:

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

What does the the exclamation mark mean? And UNPACK?

The ! means "strict" – i.e. don't store a thunk for an Int here, but evaluate it first. The {-# UNPACK #-} tells the compiler that it can unpack the Int – meaning that a Position will be neatly packed into 12 bytes. Bob