[R] Continuation-parsing / trampoline / infinite recursion problem

Thomas Mailund
Tue, 09 Aug 2016 19:57:05 -0000

[I?m really sorry if you receive this mail twice. I just noticed I had sent it
from a different account that the one I signed up to the mailing list on and I
don?t know if that means it will be filtered; at least I haven?t received it
myself yet.]


I am playing around with continuation-passing style recursions and want to use
the trampoline approach to avoiding too deep recursions. I want to do recursions
on a tree so I cannot simply simulate a tail-recursion with a loop and need
something else, and rather than simulating my own stack I want to use the method
that solves this in general.

I cannot seem to get out of problems with the

  Error: evaluation nested too deeply: infinite recursion /
options(expressions=)?
  Error during wrapup: evaluation nested too deeply: infinite recursion /
options(expressions=)?

error, so I reduced the problem to just computing factorials to see if I could
at least get it to work there, but here I get the problem as well, and in the
example below I am completely stumped as to why.

trampoline <- function(thunk) {
    while (is.function(thunk)) thunk <- thunk()
    thunk
}

thunk_factorial <- function(n, continuation = identity, acc = 1) {
    force(continuation) # if I remove this line I get an error
    cat("call: ", n, " : ", acc, "\n") # same for
this line
    if (n == 1) {
        continuation(acc)
    } else {
        make_thunk(thunk_factorial, n - 1, continuation, n * acc)
    }
}
trampoline(thunk_factorial(10000))

This version works for me. If I remove the ?force(continuation)? it doesn?t ?
even though I never modify the contination in this function (in the tree I want
to do recursion on I will have to). I get all the way down the simulated
recursion to the final call of the continuation and then I get the error. So as
far as I would expect I should just get the identity of the final accumulator at
the end, but instead I get the error.

If I remove the cat-call I also get the error. That *really* puzzles me. What is
cat doing that lets me complete the function when it is involved but not when I
comment out that line?

There is clearly something about this infinite recursion error I am completely
missing. Any help would be greatly appreciated.

Cheers
Thomas


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