[Haskell-cafe] Newbie question: can laziness lead to space compression?

Brian Hurt bhurt at spnz.org
Sat Dec 29 10:00:20 EST 2007

My apologies if this has been beat to death before, I'm still new to 
Haskell.  But I was wondering if it is possible that lazy evaluation could 
lead to space compression, especially under heavily persistant usage 

Here's the argument I'm making.  Say we have a tree-based Set with, say, 
1024 values in it.  For ease of math we'll assume that it's perfectly 
balanced.  Say each node in the tree takes 5 words of memory.  So in an 
eager language (for example, Ocaml), adding a new node to this tree 
requires the allocation of 10 new nodes, or 50 words of memory.  In 
Haskell, what would happen (as I understand it) is that just a new lazy 
thunk would be allocated- say, 10 words of memory.  Conceptually, we could 
think of the returned value as the original tree plus a small delta.  The 
lazy implementation is using 40 fewer words of memory than the eager 

Note that the benefit isn't *big*- we're talking about 40 words of memory 
when the main data structure is taking up 5K plus words of memory- so it's 
less than 1% different.  But there is a (small) upside in memory usage at 
least occassionally, right?


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