[Haskell-cafe] memoization

[Tom Ellis]

On Mon, Jul 22, 2013 at 04:16:19PM +0200, Andreas Abel wrote:
> In general, I would not trust such compiler magic, but just let-bind
> anything I want memoized myself:
> 
> memoized_fib :: Int -> Integer
> memoized_fib x = fibs !! x
>     where fibs  = map fib [0..]   -- lazily computed infinite list
>           fib 0 = 0
>           fib 1 = 1
>           fib n = memoized_fib (n-2) + memoized_fib (n-1)
> 
> The eta-expansions do not matter.

But this is *not* memoized (run it and see!).  The "eta-expansions" do
indeed matter (although I don't think they are truly eta-expasions because
of the desugaring of the where to a let).

What matters is not the let binding, but where the let binding occurs in
relation to the lambda.  There's no compiler magic here, just operational
semantics.

Tom