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

[Daniel Fischer]

Am Samstag, 29. Dezember 2007 16:00 schrieb Brian Hurt:
> 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
> patterns?
>
> 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
> implementation.
>
> 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?

Oh yes. Imagine how much memory an eager language would need for [1 .. ].
But laziness can also induce space leaks. That's not too uncommon either.
Finding out when which case applies is the art to be learned.
>
> Brian

Cheers,
Daniel