Thanks all,
OK, so this definition of fib
fib 0 = 1
fib 1 = 1
fib n = fib (n-1) + fib (n-2)
would involve a lot of recomputation for some large n, which memoization would eliminate?
Michael
--- On Wed, 12/16/09, michael rice wrote:
From: michael rice
Subject: Re: [Haskell-cafe] Haskell and "memoization"
To: "Daniel Fischer" , "Gregory Crosswhite"
Cc: haskell-cafe@haskell.org
Date: Wednesday, December 16, 2009, 12:58 AM
Hi all,
I think this (#3 below) is where I got the idea:
http://en.wikipedia.org/wiki/Lazy_evaluation
Excerpt:
---------------
Lazy evaluation refers to how expressions are evaluated when they are passed as arguments to functions and entails the following three points:[1]
1. The expression is only evaluated if the result is required by the calling function, called delayed evaluation.[2]
2. The expression is only evaluated to the extent that is required by the calling function, called Short-circuit evaluation.
3. the expression is never evaluated more than once, called applicative-order evaluation.[3]
---------------
So, I guess #3 doesn't apply to Haskell, or maybe I just misunderstood the meaning of the statement. I assumed that if f(p) = q (by some
calculation) then that calculation would be replaced by q so the next time the function was called it could just return q, as occurs in memoization.
Michael
--- On Tue, 12/15/09, Gregory Crosswhite wrote:
From: Gregory Crosswhite
Subject: Re: [Haskell-cafe] Haskell and "memoization"
To: "Daniel Fischer"
Cc: haskell-cafe@haskell.org
Date: Tuesday, December 15, 2009, 11:47 PM
Hmm, you raise an
On Dec 15, 2009, at 8:28 PM, Daniel Fischer wrote:
Am Mittwoch 16 Dezember 2009 05:08:39 schrieb Gregory Crosswhite:
Not even then, necessarily. And it's not always a good idea.
f k = [1 .. 20^k]
You raise a really good
point here. One can force sharing, as I understand it, by using a let clause:
n =
let xs = f 20
in length (xs ++ xs)
If I understand correctly, this should cause xs to be first evaluated, and then cached until the full length is computed, which in this case is obviously undesirable behavior.
Cheers,
Greg
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