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

[Thomas Mailund]

Oh, I see that the make_thunk function is missing in my example. It is just this one

make_thunk <- function(f, ...) f(...)  



On 9 August 2016 at 21:57:05, Thomas Mailund (mailund at birc.au.dk(mailto:mailund at birc.au.dk)) wrote:

> [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  
>