davidleothomas at gmail.com
Tue Jul 23 01:41:22 CEST 2013
I, for one, would love to have a compiler do (a) based on (b), my
specification of (c), and the ability to pin particular things...
On Mon, Jul 22, 2013 at 4:04 PM, wren ng thornton <wren at freegeek.org> wrote:
> On 7/22/13 9:06 AM, Tom Ellis wrote:
> > On Mon, Jul 22, 2013 at 07:52:06PM +1200, Chris Wong wrote:
> >> A binding is memoized if, ignoring everything after the equals sign,
> >> it looks like a constant.
> >> In other words, these are memoized:
> > [...]
> >> f = \x -> x + 1
> > [...]
> >> and these are not:
> >> f x = x + 1
> > In what sense is the former memoised? I'm not aware of any difference
> > between these two definitions.
> 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.
> 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.
> Live well,
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> Haskell-Cafe at haskell.org
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