[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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