On 12 Jan 2008, at 23:16, Hugh Perkins wrote:
On Jan 12, 2008 10:54 PM, Henning Thielemann
<[EMAIL PROTECTED]> wrote:
On Sat, 12 Jan 2008, Hugh Perkins wrote:
I guess that Haskell's referential transparence means the answers to
the isPerfectSquare will be cached, ie automatically memoized? (not
sure if is correct term?)
http://www.haskell.org/haskellwiki/Memoization
Interesting... but I dont understand... I thought that referential
transparence meant that once the answer to a function has been
calculated once, it will always be the same, and that the interpreter
can, and will, cache this answer?
So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go
away and calculate f(20), but if I call it multiple times, it will
just return the value it already calculated?
No,
Memorisation has it's costs too... Suppose you wanted to computer
map f [1..100000000000000]? Each time f was called, your program
would look up a table of on average 50000000000000 results for f.
That doesn't sound very efficient if f is a simple function. Now
suppose you're running a program for several hours -- imagine how
large your table would become, and how slow your lookup would be.
What you can do however, is introduced constants. Constants are
evaluated once and only once, so using them, you can tell the compiler
exactly what should be memorized.
Bob
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