[Haskell-cafe] memoization

[Tom Ellis]

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