On Mon, Jul 22, 2013 at 04:04:33PM -0700, wren ng thornton wrote:
Consider rather,
f1 = let y = blah blah in \x -> x + y
f2 x = let y = blah blah in x + y
The former will memoize y and share it across all invocations of f1; whereas f2 will recompute y for each invocation.
Indeed.
In principle, we could translate between these two forms (the f2 ==> f1 direction requires detecting that y does not depend on x). However, in practice, the compiler has no way to decide which one is better since it involves a space/time tradeoff which: (a) requires the language to keep track of space and time costs, (b) would require whole-program analysis to determine the total space/time costs, and (c) requires the user's objective function to know how to weight the tradeoff ratio.
This is called the full laziness transformation http://foldoc.org/full+laziness and indeed with optimization on GHC (sometimes) does it, even when not appropriate http://www.haskell.org/pipermail/haskell-cafe/2013-February/105201.html Tom