Adding metadata to an object produces a new object, rather than
altering the existing object. Every time you increment the counter for
a function in p it becomes different, and the memoize treats it as
new. That is, p-apply-memoized takes parameters p and v. Your
post-walk over p replacing functions with copies with changed metadata
changes p:
user=> (def q (fn [x] (inc x)))
#'user/q
user=> (= q (with-meta q {:foo true}))
false
Since functions have object identity for =, and adding metadata
changes object identity, the function is not = to its meta-added self
-- and that means a collection containing the function also is not =
to a copy produced using post-walk and meta-izing functions in the
collection, and that means a data structure built of collections and
containing functions and post-walked to meta-ize the functions is not
= to its un-metaized self.
So, each time p-apply-memoized gets called with "the same" p, because
all the counters have been bumped, it's not actually the same p as far
as memoize is concerned, and so you don't get the benefits of
memoization anymore.
Solution: don't mess with the function metadata. Instead, use a global
map from fns to counts, or something.
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