CODEWITHSUNDEEP
haskell
softshipper
softshipper

Reputation: 34071

At the top level, results will persist in memory

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?

Upvotes: 1

Views: 69

Answers (1)

Will Ness
Will Ness

Reputation: 71065

"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.

see also: How is this fibonacci-function memoized?

Upvotes: 4

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