On 16 April 2010 15:59, zaxis wrote:

instance (BinaryDefer a, BinaryDefer b) => BinaryDefer (a,b) where    put (a,b) = put2 a b    get = get2 (,)    size x = let ~(a,b) = x in size a + size b    putFixed (a,b) = putFixed2 a b    getFixed = getFixed2 (,)

in `size` function, what does the `~` mean ?

A lazy pattern match: http://en.wikibooks.org/wiki/Haskell/Laziness (there is a better name for it, but I can't remember). -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

On Thu, Apr 15, 2010 at 10:59 PM, zaxis wrote:

instance (BinaryDefer a, BinaryDefer b) => BinaryDefer (a,b) where    put (a,b) = put2 a b    get = get2 (,)    size x = let ~(a,b) = x in size a + size b    putFixed (a,b) = putFixed2 a b    getFixed = getFixed2 (,)

in `size` function, what does the `~` mean ?

This is kind of a funny question, because in this case the ~ doesn't mean anything at all. Pattern matches in let are automatically irrefutable/lazy. A better way to write this is size ~(a,b) = size a + size b which is equivalent to size x = size a + size b where (a,b) = x which is equivalent to size x = let (a,b) = x in size a + size b which is equivalent to size x = let a = case x of (v,_) -> v b = case x of (_,v) -> v in size a + size b If a or b never get evaluated, the case statements (which will fail on bottom values) don't happen.