At the top level, results will persist in memory

Question

I am working through the book https://www.packtpub.com/application-development/haskell-high-performance-programming and at moment, I am trying to understand the memoization.

It shows the following memoization function:

fib_mem :: Int -> Integer
fib_mem = (map fib [0..] !!)
where fib 0 = 1
      fib 1 = 1
      fib n = fib_mem (n-2) + fib_mem (n-1)

In the book, it says:

So if fib_mem is defined at the top level, the results will persist in memory over the lifetime of the programm itself!

What does it mean, define at the top level?

I believe it means it's in global scope, not inside a `let` or `where` clause or similar local expression — Robin Zigmond, Feb 13 '19 at 19:19

score 4 · Accepted Answer · answered Feb 13 '19 at 19:51

4

"Top-level" is a synonym for "REPL", or if you use a source code file, it's its most outer scope -- it's global scope.

Traditionally, in Lisp, loading source code file was as if you executed its top level statements by typing them at the REPL one by one.

answered Feb 13 '19 at 19:51

Will Ness

1

FWIW I believe that for `fib_mem` to be memoized, it must be a CAF in addition to being a top-level declaration. So the book was not as precise as perhaps it could have been. https://wiki.haskell.org/Memoization#Memoising_CAFS – Rein Henrichs Feb 14 '19 at 02:54
@ReinHenrichs you are absolutely right. with these kinds of what looks like almost self-published books, it's always a gamble. --- It must also have a monomorhic type, I believe, as is discussed on the entry I linked IIRC. – Will Ness Feb 14 '19 at 06:43
Packt is basically a book mill. – Rein Henrichs Feb 14 '19 at 07:19

1 Answers1