2011/7/26 Siddhartha Gadgil :

      I have been making programs for mathematical applications (low-dimensional topology) in Haskell, which I find a delight to code in. However, the execution is slow, and this seems to be because recursively defined functions seem to be recomputed. For example f(100) needs f(15) which needs f(7) ... The dependencies are not obvious to the compiler.       I was looking for a way to retain the values of a specific function in memory. Is there some way to do this.

It seems you are looking for memoization. Have a look at the comparison between slow/memoized_fib at this page http://www.haskell.org/haskellwiki/Memoization#Memoization_with_recursion Cheers, Thu

Use memoization. Here's an example: cabal-install MemoTrie import Data.MemoTrie fib_fix :: (Integer -> Integer) -> Integer -> Integer fib_fix _ n | n < 0 = error "invalid input" fib_fix _ 0 = 1 fib_fix _ 1 = 1 fib_fix rec n = rec (n-1) + rec (n-2) -- 'tie the knot' on a recusrive function func_fix :: ((a -> b) -> (a -> b)) -> (a -> b) func_fix f = let rec = f rec in rec -- memoized knot tying; 'memo' stops us from recomputing the same value more than once. memo_fix :: HasTrie a => ((a -> b) -> (a -> b)) -> (a -> b) memo_fix f = let rec = memo (f rec) in rec -- try comparing the performance of these two on large arguments fib_slow, fib_fast :: Integer -> Integer fib_slow = func_fix fib_fix fib_fast = memo_fix fib_fix On Tue, Jul 26, 2011 at 2:53 AM, Siddhartha Gadgil < siddhartha.gadgil@gmail.com> wrote:

I have been making programs for mathematical applications (low-dimensional topology) in Haskell, which I find a delight to code in. However, the execution is slow, and this seems to be because recursively defined functions seem to be recomputed. For example f(100) needs f(15) which needs f(7) ... The dependencies are not obvious to the compiler. I was looking for a way to retain the values of a specific function in memory. Is there some way to do this. Thanks, Siddhartha

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