Daniel Fischer wrote:
On Wednesday 07 July 2010 18:40:07, Ali Razavi wrote:
Thanks a lot for your through and informative response. Your solution is well-structured and pellucid. The only part I don't understand is in the definition of Parens:
data Parens = Mul !Cost Dimension Parens Parens
| Matrix Dimension deriving (Show)
What is the exclamation mark in !Cost, and what does it signify?
It's a strictness annotation and signifies that a Mul-value must always be constructed with an evaluated Int and not with a reference to a computation.
I.e. a small optimization. It is very common to use this for cached values at the branches of a tree. The idea is that we would like to evaluate the cost early; after all, lazy evaluation might build up computations like Mul (((0 + 0 + 10*100*5) + 0 + 10*5*50) + 0 + 10*50*1) ... and only evaluate them at the very end. This might give a stack overflow when the expression is large, but since we know that the cost is needed, we can evaluate it right away. (This is like foldl vs foldl' ) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com