Whereas most memo combinators memoize based on equality, stable-memo
does it based on whether the exact same argument has been passed to
the function before (that is, is the same argument in memory).
stable-memo only evaluates keys to WHNF.
This can be more suitable for recursive functions over graphs with
cycles.
stable-memo doesn't retain the keys it has seen so far, which
allows them to be garbage collected if they will no longer be
used. Finalizers are put in place to remove the corresponding
entries from the memo table if this happens.
Data.StableMemo.Weak provides an alternative set of combinators
that also avoid retaining the results of the function, only
reusing results if they have not yet been garbage collected.
There is no type class constraint on the function's argument.
For motivation, here is an implementation of map that preserves
sharing of the spine for cyclic lists. It should even be safe to use
this on arbitrarily long, acyclic lists since as long as the garbage
collector is chasing you, the size of the memo table should stay
under control, too.
map :: (a -> b) -> [a] -> [b]
map f = go
where go = memo map'
map' [] = []
map' (x:xs) = f x : go xs
This library is largely based on the implementation of memo found in
"Stretching the storage manager: weak pointers and stable names in
Haskell", from Simon Peyton Jones, Simon Marlow, and Conal Elliott
(http://community.haskell.org/~simonmar/papers/weak.pdf).
